Bearhawk STOL mods

schu

schu

Well-known member
I remember reading somewhere where Battson got the thing stalling down in the 30KIAS range (I think it was indicated).... That seems pretty slow for a 4 place, so I wanted to keep a list of what is needed to make that happen. Battson (and others) can you update my list and fill in any blanks about how to get the airplane to fly slow?

1. Build light.

2. CG in the right place. How much difference does CG make in regards to minimal controllable airspeed on the BH?

3. Hoerner wingtips to make the effective wing length longer. How long on the Hoerners? I seem to recall 16"

3. Gap seals? Elevator gap seals? Aileron?

4. Rigging. Does rigging the aileron differently affect slow flight much?

5. Flap settings. I'm guessing the first notch or two of flaps add lift and everything past that is drag. What is the best flap setting for slow short landings?

6. Airfoil horizontal stabs. To make the tail fly and take some incidence out of the stab.

7. VG's. Some airplanes really like VG's others not so much. Seems like the bearahwk does..... where were they installed (wing setback, and if on the H-stab).

8. Anything I'm missing?


The goal is to get the airplane to operate slow so that I can operate in 400-500ft when light and 700-800ft when heavier.

Thanks,
schu
 
Schu:

1. Every flight plan I have flown in the last 20+ years has a "delta" fuel consumption per unit of weight increase. Increased empty weight doesn't just reduce you useful load, you would be surprised how much extra fuel you burn over the life of the aircraft if it is 10% heavier. And for the same load carried, you WILL stall at a faster speed, and need to adjust your approach speed. An ultru light or 787 are exactly the same in this regard.

2. The tail is always producing negative lift unless you fly with a CG well aft of the rear limit. It doesn't matter if it is airfoil shaped or not. For a given CG, it requires a given negative load. I think the airfoil tail has a better L/D, which lets it cruise a little faster and/or more efficiently. I believe that was its' purpose. Aft CG is more efficient, but unless you have Fly-by-wire, it is difficult and/or unpleasant to fly.

The rest of the list? I want to try it all after my plane is flying, one at a time. Right now the only thing I can do is get-er-done, with #1 on your list being the only thing I can improve on right now.

Some planes will "droop" the ailerons a bit with the flaps to help generate lift. The BH ailerons are small compared to the flaps. I am not sure how much it would help. I like the idea of fowler flaps, and with the huge flaps area the BH has, I think they would improve the stall speed a lot, but you might run out of elevator authority as you get slower (power on would help). They also add weight and complexity. But I like the idea of them.
 
Keller flaps on a BH would be really nice. I want to say someone asked about it, but Keller wanted the wing in cad so that he could do a proper load analysis.
 
Would Hoerner tips have the same effect on the Bravo model as on the original model ? Or is the net effect of them already designed into the Bravo wing ?

Regarding ailerons and raising the neutral position to assist with lightening control forces. Could a similar result be achieved by offsetting the aileron bell-crank to create differential ? If this works to lighten control forces as well, would it then enable the aileron neutral position to then be lower and in line with the raised flap position, and create slightly more lift in all flight modes ? It would effectively reduce washout slightly.
 
How about Slats? I've been looking at the Just Super Stol and also the Q2. Very impressive. The SCub manufacturer will be selling them. They work like a hinge instead of poping out. I don't know if Just Aircraft has tried VGs behind the slats yet. Troy said they were going to try it.
 
At this point I'm looking for a catalog of what Battson has done along with easy as you build or bolt on stuff. Slats or changing the flaps is way down the road, if never....
 
schu said:
Keller flaps on a BH would be really nice. I want to say someone asked about it, but Keller wanted the wing in cad so that he could do a proper load analysis.
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We are working on this with a friend in the US taking the lead. The latest was the tail-wings need to be enlarged to handle the increased pitching moment created by Keller flaps.
 
Ooohhh, rad! I wonder if that means the stab carry through needs to be enlarged or if the tail needs to be braced more....

Can you comment on the rest of my list? That look like what you have done to get down in the 30KTAS range?
 
It all depends how STOL you need to be. The distances you are talking about should be achievable in a stock plane. "so that I can operate in 400-500ft when light and 700-800ft when heavier"

- Feedback:

1. Build light.
- Appears to be pretty important, but if you do a linear extrapolation it's only a couple of knots difference between 1500 and 1700lbs. A couple of knots isn't the whole world for those distances.

2. CG in the right place. How much difference does CG make in regards to minimal controllable airspeed on the BH?
- Basically irrelevant at light loads provided you are within limits and have total control.

3. Hoerner wingtips to make the effective wing length longer. How long on the Hoerners? I seem to recall 16"
- Correct. They make a considerable difference like VGs.

3. Gap seals? Elevator gap seals? Aileron?
- I've not gone there, would help accomplish better control but I use power instead. You can also use VGs under the tail but that equals drag.

4. Rigging. Does rigging the aileron differently affect slow flight much?
- Not a great deal for our airfoil I am told, see comment below about ailerons.

5. Flap settings. I'm guessing the first notch or two of flaps add lift and everything past that is drag. What is the best flap setting for slow short landings?
- No builder mods really necessary to the flap settings. Full span bravo model would be best, but I have alpha and stock flaps, for now.

6. Airfoil horizontal stabs. To make the tail fly and take some incidence out of the stab.
- Yes they help. Again just control and efficiency I suppose.

7. VG's. Some airplanes really like VG's others not so much. Seems like the bearahwk does..... where were they installed (wing setback, and if on the H-stab).
- Definitely yes and apparently the location is absolutely critical. Some people say they noticed no difference, "you're doing it wrong". See comment above about H-stab.

8. Anything I'm missing?

- Big tires OR long legs (if that's even possible), or both, to increase the native angle of attack to allow shorter take-off and steeper AoA at touchdown. The landing part is an issue with small tires on the plane, you need to land tail first to get everything the wing has to offer.

- A slotted flap would be a big step forward, if the Keller flaps are a step too far. Slotted flaps would be possible just with a minor adjustment to the hinge design and installing a false spar, but as yet it's an unproven mod in the publicly available domain.

- One of our active members here Helidesigner has also installed Flaperons which fit into the major modifications category. But certainly buys a couple of knots slower stall. More than offsets the weight and complexity, but then again you need to be dead serious about STOL. They deserve a separate thread though, as they do come with some small downsides.

- There are probably a tonne more things would could be done - slats etc, but I need to stress that for the distances you are talking about they are NOT NECESSARY.

The plane can do (all without obstacle clearance) with 350ft with these mods, light. 600ft at gross, including an allowance for pilot error. Required distances are 250ft and 500ft.
 
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Thanks, that helps. If I can get down into 400-500ft heavy, that would be even better, but 700-800ft gets you in and out of most backcountry strips.

Are you looking at slotted flaps? Or just going straight to the Keller design if that gets figured out?

The flaperons are like the ones in the Robinson STOL kit? I've heard about those.... I'll do some reading.

What flap setting do you use for shortest landings? 2 notches? Full flaps? I'm asking because I doubt that the BH gives you much lift past 2 notches, just more drag.

I'll be running 31" desser tires, so I'll have some AOA on rotation.

Thanks for the helpful replies!
 
schu said:
I remember reading somewhere where Battson got the thing stalling down in the 30KIAS range (I think it was indicated).... That seems pretty slow for a 4 place, so I wanted to keep a list of what is needed to make that happen. Battson (and others) can you update my list and fill in any blanks about how to get the airplane to fly slow?

1. Build light.

2. CG in the right place. How much difference does CG make in regards to minimal controllable airspeed on the BH?

3. Hoerner wingtips to make the effective wing length longer. How long on the Hoerners? I seem to recall 16"

3. Gap seals? Elevator gap seals? Aileron?

4. Rigging. Does rigging the aileron differently affect slow flight much?

5. Flap settings. I'm guessing the first notch or two of flaps add lift and everything past that is drag. What is the best flap setting for slow short landings?

6. Airfoil horizontal stabs. To make the tail fly and take some incidence out of the stab.

7. VG's. Some airplanes really like VG's others not so much. Seems like the bearahwk does..... where were they installed (wing setback, and if on the H-stab).

8. Anything I'm missing?


The goal is to get the airplane to operate slow so that I can operate in 400-500ft when light and 700-800ft when heavier.

Thanks,
schu
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I know I shouldn't respond but I can't help it. My measly 30ish hrs of BH time doesn't amount to much. Maybe someone else will chime in.

1. Yeah, you know that lighter means better STOL. What's the says "Light as possible, heavy as necessary."

2. During flight testing I didn't notice CG making any measurable difference in lowest controllable airspeed. Except when way aft, then all airspeed are marginally controllable.

3. No personal experience here but my go to BH guy says elevator gap seals make a measurable difference and should be added.

4. No thoughts.

5. Full flaps worked the best for me. It's mostly about pilot technique IMO. That last notch of flaps adds a ton of drag allowing for a steeper approach thus a smaller horizontal velocity vector.

6. No personal but I don't think it will a measurable difference in STOL ops.

7. My go to BH guy said VGs are a must for STOL ops. I measured the location of his but lost that notebook.

Maybe reread the "long wing bearhawk" thread. We are still going with Hoerner tips. 10" of additional airfoil and 8" for the Hoerner tip shape.

Even a small engine BH will benefit from more AOA during the takeoff roll. Your 31" tires should accomplish this.

Honestly, I think a stock lightweight BH can operate within your set parameters with a proficient pilot.
 
Trying to get the landing speed down would be good for what I want, but the airplane is designed for a certain minimum speed, including controllability at that speed. Going too much slower might have some serious issues. The fowler flaps will move the center of pressure aft, requiring more nose up authority out of the tail/elevator. That combined with the lower speed might not be a good combination for control authority.

I like the SuperStol and other aircraft like it, but I would rather own a BH that cruises a lot faster. I think for now I will stick with all the little improvements, and keep the cruising speed. I think all the little stuff will add up to at least a couple of knots, and increase controllability.
 
I'm not familiar with Keller flaps. Harry Ribblett says his airfoils do not benefit from VGs. Would like to see someone with the "B" airfoil try them to see if that is true. I originally planned to build slotted flaps, but ended up buying quick build wings.
 
Battson Where did you decide was the best location for the VGs? In a previous post you said "I placed them so the back of the VGs aligned to the fourth (4th) rivet from the front." Is that what you settled on?

Thanks!
 
schu said:
Are you looking at slotted flaps? Or just going straight to the Keller design if that gets figured out?

The flaperons are like the ones in the Robinson STOL kit? I've heard about those.... I'll do some reading.

What flap setting do you use for shortest landings? 2 notches? Full flaps? I'm asking because I doubt that the BH gives you much lift past 2 notches, just more drag.
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I am not going to try a "bleeding edge" design with our plane, we can't tolerate long downtime or repeat downtime if the modification doesn't work. We'll be a fast follower if someone else figures it out. But I'll consider slotted flaps once I have more free time (years away).

The flaperons which Helidesigner did are quite different in their install to a Robertson STOL. The aerodynamic effect is different too, because our ailerons are the Frise design.

Flap for landing is always the maximum allowance given the wind. Drag is important. Best take-off distance appears to be 3 notches, the extra drag is not a problem in the 540 machine.
 
Oh - Full flaps definitely gets the plan flying slower, and popping full flaps gets it off the ground a little sooner.

When people talk about "best lift flap setting", I think they mean best lift:drag ratio for a given flight regime?
 
schu said:
I'm asking because I doubt that the BH gives you much lift past 2 notches, just more drag.
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Oh - I think I missed this part.

Full flaps definitely gets the plane flying slower, and popping full flaps gets it off the ground a little sooner too. The drag is much higher of course, but lift still increases considerably. The 540 machine just eats the drag.

Importantly, flap also lowers the deck angle and helps you see what you are doing. This pilot-element is key to STOL performance.

When people talk about "best lift flap setting", I think they mean best lift:drag ratio for a given flight regime?

The one time drag worries the plane is go-around / climb out.
 
Soon after beginning work on my Patrol project, I started thinking about the Keller double slotted flaps. The Patrol flaps are SO much larger than PA-18 flaps, it wouldn’t be a modification of the Kellers, but fabrication of a system, based on the Kellers. I continue to consider such. In my many hours, spent flying Huskies, I’ve grown quite fond of the single slotted flaps. If I ventured in that direction, I’d go with a single slotted flap, rather than the complexity of the double slot system.
Just this past week, I measured movement of the flap actuator pushrod. Full stroke is about 2 1/4”. The Husky has about 6” of travel. The Husky has a horizontal bellcrank in the wing, actuated by a cable, routed to a 'Y' harness. I think pushrod stroke is the key to a slotted flap system. I can build a set of fiberglass flaps. I can build a 'dropped hinge' system, similar to the Husky, to swing the flaps down and out. To build a bellcrank that would achieve 6” of pushrod travel, would require me to pull the wings and do massive surgery. I think the actuation system would require a complete re-design.
The Patrol flaps work fine. My experience with slotted flaps, on a similar airframe, makes me really curious about how the Patrol could benefit from such. The force required to deploy the hinged flaps is very high. I’d estimate flap deployment forces in Huskies to be around half of those in the Patrol. I think slotted flaps would be worthwhile, but I can’t see myself taking the plane apart to do the mod. If, on the other hand, anyone has a set of Patrol wings they'd feel like volunteering for development of such, I think it would be a great project! :cool:

Bill
 
Full flap may technically give a shorter TO roll but I suspect that most short strips are going to have hedges or trees at the end (that's why they are short!). The extra drag is going to severely degrade the initial climb performance. Sure, an IO540 powered aircraft is going to climb well anyway but it'll climb better with half flap. So, it's always going to be a compromise between TO roll and initial climb performance.

The generally accepted theory (certainly amongst the VANS forums) is that a flap setting equal to the down aileron deflection is optimum.
 
PaulSA said:
Full flap may technically give a shorter TO roll but I suspect that most short strips are going to have hedges or trees at the end (that's why they are short!). The extra drag is going to severely degrade the initial climb performance. Sure, an IO540 powered aircraft is going to climb well anyway but it'll climb better with half flap. So, it's always going to be a compromise between TO roll and initial climb performance.

The generally accepted theory (certainly amongst the VANS forums) is that a flap setting equal to the down aileron deflection is optimum.
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A really interesting discussion point!

That aileron theory you're referring to is certainly a well known 'rule of thumb' for best lift:drag ratio. It does not work so well for high power:weight ratio aircraft. Refer to this backcountry flying source.

The Bearhawk 540 can pull itself off the ground fastest with three notches of flap set, or by pulling full flap at the right moment. Naturally you have to ease off the flap ASAP to climb efficiently. Best angle of climb is with half flap, and best rate of climb is with flap retracted. We can't really generalise and say half flap gives best climb, it depends why you are climbing... and according to this other backcountry flying source its a wash between Vx and Vy. Splitting hairs.

At minimum weight with flap retracted at 57kts, my Bearhawk climbs at greater than 45 degrees nose to the horizon, which is about 25 degrees climb angle. Obstacle clearance is almost never an issue for the 540 Bearhawk in my experience. If the strip is *that* tight for a 540 Bearhawk, then other planes are not going there. Think of situations like a gravel bar, tundra, mountain valley, or beach. Those situations probably cover 90% of STOL flying. In those places there is often a shortage of usable ground, but normally no immediate obstacles to clear. In fact, I can only think of four or five situations in the last 650hrs backcountry / STOL flying where immediate obstacles were an issue for me. These were places where few other planes short of a Carbon Cub could go.

In the STOL community it is widely accepted that popping flaps and lifting off as soon as possible is an important skill. It does depend on who you talk to, some prefer to set and forget. Refer to videos of any STOL contest to confirm this statement. In my view, optimising the wing shape for each phase of flight gives better performance overall, like birds do. However the margins are slim and it takes a lot of currency in the plane to get it exactly right. Done wrong, it adds workload and distracts the pilot for absolutely no benefit, and 50hrs a year simply won't get the pilot enough practice to ever get those skills. For intermediate skill levels, it's best to set and forget. Same goes for dumping flaps on landing.
 
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Battson said:
At minimum weight with flap retracted at 57kts, my Bearhawk climbs at greater than 45 degrees nose to the horizon, which is about 25 degrees climb angle.
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This seems to indicate that using 19deg for fuel flow testing is not steep enough for big engine BHs. Are you saying that your level flight datum is at a angle great than 45deg during max climb? 45deg is crazy steep and far greater than the 19deg Russ Erb calculated for his flow testing.
 
whee said:
This seems to indicate that using 19deg for fuel flow testing is not steep enough for big engine BHs. Are you saying that your level flight datum is at a angle great than 45deg during max climb? 45deg is crazy steep and far greater than the 19deg Russ Erb calculated for his flow testing.
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Yes, the angle of the deck to the horizon can exceed 45 degrees at max performance. I should video it from the cockpit one time, it's impressive. Bearhawk 540's are just the best fun.

I suggest testing whatever works for you and your inspector, 45 degrees is not practical for a ground-based flow test. We did three point.

My system is pumped twice in those circumstances. However, we can take-off / climb that steeply with only one pump and with very low fuel pressure (like 12 psi), we've tested it.

That angle is for Vx, but I can go steeper just for the fun of it. The plane will climb steady state at over 50 degrees to the horizon "hanging on the prop" at about 50 knots. I have seen 55 degrees for short periods. You'd have to see it to believe it. This is practically useless though, just for fun!! Not many planes can do that kind of trick.
I wait for perfect weather to ensure safety, despite a huge margin over the stall showing on the AOA display, call me chicken.
 
Wow. That is impressive. I have only flown one "overpowered" plane. By that I mean the engine can put out more power than the airframe can actually handle, and can actually drag it through the air at speeds greater than it can comfortably fly (or control). But the some of the descriptions of the 540 BH remind me of the same thing. Especially going fast.

I think it is a good generalization that more flaps is better for the shortest TO roll, but at the expense of climb. Higher flap settings on bigger planes also works, but at a large penalty in Max TO weight. The loss of performance is just to great. It does work well when fairly light.
 
I thought I would go ahead and weigh in on this discussion on flap usage. Flaps for takeoff, for me, is situationally dependent. I don't like buying tires so I always want to get off the runway fairly short so I always start my takeoff roll with two notches of flaps. Sometimes I leave it there for takeoff, sometimes I pull it to the 3rd notch as I rotate to get off the ground a bit earlier. I haven't gone to the 4th on takeoff yet. The takeoff roll is short, but it is also quick. It starts rolling, the tail comes up, about a second later I can lower the tail and pull more flaps and I'm flying. A few seconds later I'm going back to 2 notches of flaps so I don't overspeed the flaps. I could just climb really steep to keep the airspeed down but after a few seconds I'll be above any nearby obstacles and then there isn't really a reason to stay in an attitude that would be difficult to recover from in the event of an engine failure that close to the ground so its time to accelerate which means its time to raise the flaps a bit. What I'm getting at is that no matter how much flaps you apply for a shorter takeoff roll, the extra drag from more flaps isn't really an issue for climb because you won't be leaving them down for more than a few seconds. I'm just flying behind about 200hp in my Patrol. In the 250 hp four place, you really have to be paying attention to keep the climb steep enough or reduce power to keep your airspeed down until you raise the flaps. Its a lot different than the Pacer I used to have. On my Pacer, which had two notches of flaps, I would start the roll with one notch of flaps, add the second as I rotated, get off the ground, climb slowly while waiting for the speed to build enough to raise the flaps back to one notch without stalling, then climb out with either one notch or flaps up. The drag of full flaps on the Pacer definitely hurt climb performance, but it was still the only way I could get out of short, tight strips.

As for landing, I either use 3 or 4 notches of flaps. I'm still trying to work out the best way to land with 4 notches. With three I can glide in, flare to a nice touch down. With four, if I'm not right at the max speed for four notches, I don't get much of a flare, it will kind of drop through, which I attribute to all that extra drag. Now if I carry a bit of power with 4 notches and drag it in slow, and reduce power as I flare and reach idle about the time I touch down, I can get a nice landing and not use much runway. I haven't got to practice this much yet but I had it down pretty good in my Pacer and this plane is so much more capable.
 
I guess if I was planning on doing really short field work (I do), I would try to validate how the plane climbs at high flap settings and high weights before I was doing it for real. Load the plane up, get up to 2-3000 feet, and see how the plane climbs at 3 or 4 notches flaps. If it won't climb at gross weight or close, I would rather find that out with some altitude to recover.

Obviously density altitude is going to have a huge effect as well.
 
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Battson said:
That angle is for Vx, but I can go steeper just for the fun of it. The plane will climb steady state at over 50 degrees to the horizon "hanging on the prop" at about 50 knots. I have seen 55 degrees for short periods. You'd have to see it to believe it. This is practically useless though, just for fun!! Not many planes can do that kind of trick.
I wait for perfect weather to ensure safety, despite a huge margin over the stall showing on the AOA display, call me chicken.
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Boy, that is interesting.... Sounds like your basic power on stall. Have you continued to the stall to ensure that your AOA is accurate in that situation?

I think Jon is right, makes me think differently about testing fuel flow.
 
schu said:
Boy, that is interesting.... Sounds like your basic power on stall. Have you continued to the stall to ensure that your AOA is accurate in that situation?

I think Jon is right, makes me think differently about testing fuel flow.
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Yes we have stalled it like that, call it "advanced stalls". We can fly around at maximum AoA doing turns up to 30 degrees bank left and right, well out of balance, and it just wont stall. You really need to mistreat it to make it stall.

When you are 50 plus degrees nose up and 30 degrees in a bank, out of balance, and the Bearhawk is holding everything together for you - you develop a deep sense of trust in your aircraft.

Of course all the discussion above is predicated on the plane being at minimum weight. With a load on it's a totally different ball game.

We did some stalls with a new pilot the other day, it was his first flight ever in a plane anything like this - after five minutes of turns we did the stalls, he was easily able to control / recover all but the deepest of wing-drop stalls. He is used to high wing loadings and nasty stall behaviour, so was really impressed. The Bearhawk really is an exceptionally safe plane for pilots.
 
I only use the 4th notch on short final. The 3rd notch is just as you describe. 4th is like air brakes! Great isn't it? I don't pull 4 above 65mph...then it's Game On!
Rollie, did you extend your flap handle? Or can you get the 4th notch with the standard handle?

Bill
 
I didn't extend my flap handle. I have been able to get to the fourth notch one handed a couple times by slowing down to below 45 knots. Usually I just use two hands at just over 50 knots. My wife was having trouble getting to the third notch but once she started slowing down an extra 10 kts instead of trying to lower them right at the max speed she has been able to do it. The fun part is learning to pull that hard without pushing on a rudder pedal. I had done it a couple times during my test flying so I expected it when she did it. Not the best time to get way out of trim, especially if you slowed down close to stall to make it easier to lower the flaps.
 
I built a new extended handle, as long as possible without busting my thumb on the cross bar. The 4th notch isn’t easy, but way easier than before.

Bill
 
For information, I am told the double-slotted Bearhawk flaps are now under production ("Keller Flaps").

For those who are unfamiliar with these flaps, see below for an install on a Maule.

full_flaps.jpg
 
I wonder if airframes will sell a kit to keep the price down a little.... I think this with the other things mentioned would make the bearhawk into a much better bush rig.
 
schu said:
I wonder if airframes will sell a kit to keep the price down a little....
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I think they must. Once you've done all the engineering and tooling, you've done it all. The only thing remaining is to recoup your investment.
I expect a kit would be over $10,000 delivered, and it would not be "plug and play". You'll need to modify some of the wing parts.
 
Battson said:
I think they must. Once you've done all the engineering and tooling, you've done it all. The only thing remaining is to recoup your investment.
I expect a kit would be over $10,000 plus S&H.
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Grrrr, they are so expensive....
 
That is a known issue. The cub needs a different tail as well when using keller flaps.
 
For what it's worth, I was stalling clean at 48 KIAS. I put some STOLSPEED vortex generators on my wings and tail. It dropped the clean stall speed to 41KIAS. My full flap stall speed went from 44 to 41 also. I'm certain that my tail is stalling first... I'm sure my forward CG with just me is contributing to this. I'm gonna play with gap seals and ballast for aft CG next.

Another observation is that my current 3-point attitude doesn't allow me to take advantage of the slower speed. I need bigger tires to help take advantage of it.
 
Bobby, which wingtips are you running? If you aren't using the ones that Battson recommends, might be money ahead to get those going first before you look at bigger tires.
 
swpilot3 said:
Another observation is that my current 3-point attitude doesn't allow me to take advantage of the slower speed. I need bigger tires to help take advantage of it.
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This is a relevant observation for practical STOL work.

- Leading edge devices like slats and VGs don't improve practical STOL performance at the touchdown or take-off. They do improve the safety margin during climb-out and approach, allowing for slower flight in those regimes. They increase stall angle and stall aggression.

- Trailing edge devices like slotted flaps or fowler flaps will improve practical touchdown speed and shorten take-off. This is the most practical STOL outcome.
They lower the deck angle, reduce Vso, and improve visibility by generating lift near the trailing edge and moving the centre of pressure (CP) aft.

- Wingtip devices can produce more lift, but don't move the CP much, so the stall angle of attack remains unchanged.

The most practical additions to any STOL Bearhawk will be some form of blown flap (slotted or fowler), followed by the wingtip.
 
swpilot3 said:
For what it's worth, I was stalling clean at 48 KIAS. I put some STOLSPEED vortex generators on my wings and tail. It dropped the clean stall speed to 41KIAS. My full flap stall speed went from 44 to 41 also. I'm certain that my tail is stalling first... I'm sure my forward CG with just me is contributing to this. I'm gonna play with gap seals and ballast for aft CG next.

Another observation is that my current 3-point attitude doesn't allow me to take advantage of the slower speed. I need bigger tires to help take advantage of it.
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Interesting observation that your tail is stalling first. I hadn't heard that one before, but again, I don't have a lot of light aircraft time, and zero bush plane time.
 
When it stalls in a "nose up" elevator situation, the flow that was causing the tail to go down separates and the tail now goes less down, which is the same as the nose going more down. That may be the most confusing sentence I've ever written. But what I mean to say is that in this case the nose either drops or stays about where it is as the plane just mushes along. Another way to put it is at forward CG, if we try a power-off stall, we can get the plane to a point where it steadily descends under control, but the stick is all the way back and the wing is not fully stalled yet (see the video link in the previous post).
 
Paraphrasing Jared here – when the tail stalls in a nose-high angle-of-attack, the down-force being provided by the tail is reduced, causing a forward-pitching moment. This tends to reduce the pitch of the tail, resulting in it regaining aerodynamic effectiveness (and thus the down-force). The result can be a "nodding" action as the tail stalls and un-stalls. In some aircraft, and especially if allowed to fully develop, this may result in a quite rapid "mushing" descent (not specific to Bearhawks - just in general).
 
Battson said:
The most practical additions to any STOL Bearhawk will be some form of blown flap (slotted or fowler), followed by the wingtip.
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Now there's an idea! Blown flaps, but like on the C-17 and US-2. Something like how Cessna experimented with it in the 50s on the 170.
 
Aviat describes the Husky flaps as "Slotted semi Fowler". The inboard portion of those flaps is in the prop blast, thus 'Blown flaps'. They are extremely effective!

Bill
 
Bill, what I'm talking about is mass air injection from an motor or the like. The C-17 and US-2 use bleed air from their APU for takeoff. The Cessna 170/O-1 design from the 50's was using a hydraulic air pump getting power off the accessory case of the engine. Holes on the top side of the wing where boundry layer separation occurs first are made and air is injected to re-attached the airflow.
https://en.wikipedia.org/wiki/Blown_flap

Its is extremely effective.
 
zkelley2 said:
Bill, what I'm talking about is mass air injection from an motor or the like. The C-17 and US-2 use bleed air from their APU for takeoff. The Cessna 170/O-1 design from the 50's was using a hydraulic air pump getting power off the accessory case of the engine. Holes on the top side of the wing where boundry layer separation occurs first are made and air is injected to re-attached the airflow.
https://en.wikipedia.org/wiki/Blown_flap

Its is extremely effective.
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I wasn't referring to that impractically heavy experimental technology. I meant to summarise the family of flaps including slotted, fowler, semi-fowler, etc. which work like an aerofoil at the back of the wing. Sorry for the confusion.
 
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Any of the flaps that move aft as well as down and increase the wing are also move the center of pressure aft. That means you need more tail downforce to compensate, including in the flare. I can definitely see running out of flare authority, especially at forward CG.

I like the idea of this type of flap, but you would definitely need a test program to see not just how much slower you stall, but controllability when you are doing it, at various CG's.

But, I like it.
 
When Battson brought up the topic of flap changes a couple years ago I began to wonder if using a exterior flap track like used on the Quest Kodiak would be an effective way to go.

KODIAK-100-sn-100-0155-exterior-right-762x456.jpg

Jump to 12:00 min and you get a decent view of the Kodiak flaps deploying:
 
Fowler types always have some external actuators/support/support. If you look at the bottom of any airliner you will see multiple fairings sticking out the bottom of the wing. Some planes they are huge. I was surprised how little hangs down in the pictures of the Keller flaps that I have seen.

They do add drag, as well as weight and complexity.
 
whee said:
When Battson brought up the topic of flap changes a couple years ago I began to wonder if using a exterior flap track like used on the Quest Kodiak would be an effective way to go.
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I've gone as far as designing flap tracks for the Bearhawk, for a true Fowler system. It was based on the tracks used by the Sherpa Turbine. They are expensive to have machined, so I started looking deeper before progressing.
The problem I found was the bending moment / torque on the rear spar. This would need more steel braces between spars to avoid cracking the wing skin from fatigue. That work would require opening the wing skin. Too invasive for a "working" plane, it would be out of action for many months!

The beauty of the slotted flap or double slotted flap is they can be retrofitted with just a few simply mods to the trailing edge of the wing (non-structural), plus a new hinge which can be bolted on. They also create less torque on the rear spar.

The price of the Airframes Alaska flaps is high, but that's just aviation. I hope the tailwings are large enough with VGs and gap seals installed. Alternatively, they could easily be replaced.
 
Where do you put the VG's on the tail? Top on the leading edge, bottom just forward of the hinge line?
 
Bottom side of the horizontal stabilizer forward of the hinge...about 4”. Since experimental you can also use VG’s on the vertical stabilizer just forward of the rudder hinge line...at some distance...maybe 4” as well?
 
Dave Roberts is a promoter of VG’s. Flew his Bearhawk with an O540 without VG’s....didn’t like how it flared...loves VG’s now that he’s installed them. And if you ever get a chance to do short field landings with him...flat eff’n impressive!

A friend of mine with a 207 installed VG’s on his Horton STOL equipped airplane..left off the vertical stabilizer VG’s. Could get air borne before having good directional control...after vertical stabilizer VG’s he had great control. He also feels that if light with an engine out he can keep the yoke in his lap all the way to the ground at 700 feet per minute decent rate...says in emergency at 700 feet per minute hitting the ground is something he can walk away from.

I have them on my Pacer and Marv has them on his...improved elevator authority during flare. Shorter take off...maybe better aileron control when hanging on the prop during slow speed maneuvers. Almost eliminates moose stall...
 
Schu,

You could add a light weight prop to your list perhaps. It is rotational weight, not sure if it has a greater effect than “fixed” weight.
 
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I thought of that, but for CG and cost reasons I’ll probably stick with a metal prop unless I find that I can land significantly shorter than I can take off and I’m looking for more thrust.

My airplane has extensive modification so I’m waiting to see where the CG ends up.
 
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Mark Moyle said:
Dave Roberts is a promoter of VG’s. Flew his Bearhawk with an O540 without VG’s....didn’t like how it flared...loves VG’s now that he’s installed them. And if you ever get a chance to do short field landings with him...flat eff’n impressive!

A friend of mine with a 207 installed VG’s on his Horton STOL equipped airplane..left off the vertical stabilizer VG’s. Could get air borne before having good directional control...after vertical stabilizer VG’s he had great control. He also feels that if light with an engine out he can keep the yoke in his lap all the way to the ground at 700 feet per minute decent rate...says in emergency at 700 feet per minute hitting the ground is something he can walk away from.

I have them on my Pacer and Marv has them on his...improved elevator authority during flare. Shorter take off...maybe better aileron control when hanging on the prop during slow speed maneuvers. Almost eliminates moose stall...
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Really good info.
 
Well, 700 FPM sounds like a lot, but 700 FMP is only 7.95 MPH...

The Cirrus web site says following a CAPS deployment, the airplane will be descending at 17 mph (or roughly 2X the 700 FPM described above. So far, there have been zero fatalities from ground impact after a Cirrus parachute deployment.

Of course, the Cirrus' seats are designed to absorb a good bit of the G-forces from that vertical impact. That "vertical g-force absorption" is a big part of why I'm using multiple layers of different density foam in my seat cushion.

Frankly, I'm more concerned about the 30-40 mph forward velocity at impact – another good reason to land into the wind, aim between trees, etc...
 
700fpm is a normal landing with something like TK1's. You still don't bounce. People not used to such things think you'll bounce hard, but any airframe should be able to handle it.
I've certainly put large jets on the runway at 700fpm. It's an arrival and everyone asks if the runway is now 10ft lower, but it works.
 
Yes I mixed up acceleration and velocity. My bad. I was thinking same as dropping a plane 10 ft off the ground, but that give the right average velocity - not the right instantaneous velocity at touchdown.
The correct analogy is dropping a plane from 5 ft off the ground. Still a hard impact, but not a "serious crash".
 
Battson said:
I've gone as far as designing flap tracks for the Bearhawk, for a true Fowler system. It was based on the tracks used by the Sherpa Turbine. They are expensive to have machined, so I started looking deeper before progressing.
The problem I found was the bending moment / torque on the rear spar. This would need more steel braces between spars to avoid cracking the wing skin from fatigue. That work would require opening the wing skin. Too invasive for a "working" plane, it would be out of action for many months!

The beauty of the slotted flap or double slotted flap is they can be retrofitted with just a few simply mods to the trailing edge of the wing (non-structural), plus a new hinge which can be bolted on. They also create less torque on the rear spar.

The price of the Airframes Alaska flaps is high, but that's just aviation. I hope the tailwings are large enough with VGs and gap seals installed. Alternatively, they could easily be replaced.
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The current price of the double slotted experimental flap from Performance STOL/Airframes is between $6400 and $8400 depending on the number of hangars required. Has anyone taken the leap and built or modified a Bearhawk wing to accept this system? Is it possible to mount these on a QB wing kit?
 
Shopperly said:
The current price of the double slotted experimental flap from Performance STOL/Airframes is between $6400 and $8400 depending on the number of hangars required. Has anyone taken the leap and built or modified a Bearhawk wing to accept this system? Is it possible to mount these on a QB wing kit?
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Someone is doing this at the moment.

The design will allow Airframes AK to offer a standard bolt-on solution for Bearhawk aircraft wings, hopefully they do.

The issue (big unknown) is the pitching moment. Bob has advised that the tail may not be large enough to overcome the increased nose-up pitching forces created by the double slotted flaps. The ultimate solution will be larger tail wings, if gap seals / VGs / greater control deflection don't solve it first.
 
The speed range of the BH range is the equal of anything. But that comes with limitations at both ends of the speed spectrum.Twitchy at the high end, run out of control authority at the low end. I bought the kit because of the compromise.

How much would I pay for a 4 mph reduction in landing speed? If I can't control it, zero. If it works, I will get in line, price dependent.
 
Battson said:
The issue (big unknown) is the pitching moment. Bob has advised that the tail may not be large enough to overcome the increased nose-up pitching forces created by the double slotted flaps. The ultimate solution will be larger tail wings, if gap seals / VGs / greater control deflection don't solve it first.
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I agree this is a concern. However, a Bearhawk with plain flaps pitches down when flaps are deployed while a Cessna with slotted flaps pitches up. The plain flap produces less down-wash over the tail than a slotted flap. There is a chance that the increased angle of flow over the tail will counter balance the increased pitching moment of the wing and the problem will correct itself. I can't wait to see results of a test as we won't know until that happens.
 
Question: Will slotted flaps allow for a more nose down approach and touchdown when flying at minimum airspeed?

I’m beginning to understand why others have wanted a Fowler flap or a semi-Fowler like a Cessna. When practicing max performance landings I touchdown in a very nose high attitude which someday will result in something broken in the tail.

Second question: Will VGs allow the plane to fly at a slower speed for a given AOA when in the slow flight regime?
 
whee said:
Question: Will slotted flaps allow for a more nose down approach and touchdown when flying at minimum airspeed?

I’m beginning to understand why others have wanted a Fowler flap or a semi-Fowler like a Cessna. When practicing max performance landings I touchdown in a very nose high attitude which someday will result in something broken in the tail.

Second question: Will VGs allow the plane to fly at a slower speed for a given AOA when in the slow flight regime?
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Trailing edge devices facilitate a lower nose on approach - the more efficient they are (ie: slotted vs straight), the lower the nose. Have a look at the video here https://youtu.be/NZ9I61UGwcg for a comparison of deck angles on a Super Cub with stock flaps and Keller flaps.

To me, safety is the most important benefit to slotted flaps. While it is dramatic to see a slow approach at a high deck angle and slow airspeed, the pilot usually is blind to the landing zone in the final stages of the approach. In the Airframes PSTOL video, it is clear that the deck angle is substantially lower with the slotted flaps compared to straight flaps.

A nice additional benefit is a slower approach speed leading to a shorter landing roll.

VG’s (and other leading edge devices like slats and slots) will lower the stall speed at high AOA by energizing the flow over the leading edge and keeping it attached for more of the wing chord.
 
I am really hoping these flaps work out on a BH because I am interested. Controllability might be a huge issue, and not just running out of elevator authority at low speed. The BH has comparatively large area flaps, so they will be very effective, but that might include a lot more unpleasantness as well. Huge pitch changes can happen in only 5-15 knots of speed change.

A go around could get interesting really quickly, and not in a good way. Most big airplanes have huge and very effective Trimmable Horizontal Stabilizers that can handle the change in CP quickly. With the flaps down some of them make the trim motors run much more quickly to help deal with it. But it is always an issue. The second step of every go around is raising the flaps (to less than half). Part of the reason is to reduce drag. A big part is to reduce the pitch change from when accelerating.

I hope they work out great.
 
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whee said:
Question: Will slotted flaps allow for a more nose down approach and touchdown when flying at minimum airspeed?
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Most likely yes. More flap usually results in a higher max lift coefficient, but at a lower angle of attack.

<img src=http://www.ballyshannon.com/air/aoaliftgraph.jpg>

Second question: Will VGs allow the plane to fly at a slower speed for a given AOA when in the slow flight regime?
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No. VG's allow flying to a higher angle of attack but normally produce the same lift for the same AoA.
 
kestrel said:
http://www.ballyshannon.com/aoaflapbias.html
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Regarding AoA flap bias, for simplicity sake, in GA, just set it to stall with flaps full, which will be a lower AoA than with flaps retracted. If you approach the calibrated critical angle of attack while clean, you'll then have a healthy buffer before an actual stall.
The only thing with the fowlers is you'll have a very large difference in the critical angle of attack vs a plain or slotted. Where it's 2-4 degrees for the latter, the former looks like it could be 5-7 degrees according to the graph from the naval aviators book. 7 degrees is massive(considering an aircraft usually has a longitudinal flight envelope of 0ish to 15ish degrees AoA), and might result in stall warnings at rather high airspeeds. Might be worth contacting the manufacturer of your AoA system to see if flap bias is programmable.... if you have fowlers.
 
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Q1: About 8 to 10 degrees improvement could be expected, based on the results for other aircraft.
Q2: VGs make a big difference throughout the whole approach. They transform the wing when installed properly. I have the benefit of flying two sister-ship Bearhawk 4 places, to compare the differences with and without.
 
The Dynon system accounts for this possibility and has had a calibration process dealing with this, for about the last 10 years.
 
Battson said:
Someone is doing this at the moment.

The design will allow Airframes AK to offer a standard bolt-on solution for Bearhawk aircraft wings, hopefully they do.

The issue (big unknown) is the pitching moment. Bob has advised that the tail may not be large enough to overcome the increased nose-up pitching forces created by the double slotted flaps. The ultimate solution will be larger tail wings, if gap seals / VGs / greater control deflection don't solve it first.
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Can you share any more info regarding progress on this development?
 
Battson said:
The plane should be finished and delivered in a week from today. Then they'll be doing paint and probably flying within another week.

98.5" of double slotted fowler flaps. I can hardly wait to hear the reports.
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Where is the airplane? In Alaska? Oregon?

This is an A model or a B model?

Does the pilot know how to fly slow well enough to properly evaluate?

VERY interested in this!
 
Is he installing a true Fowler that actually moves aft or the STOL Performance flap from Airframes Alaska?

Mark or Jono, If you thinks it’s appropriate I’d appreciate the owners contact info. I’d sure love to go see this plane and get a ride when it’s done. He’s likely only an hour or two flight from me.
 
I have seen the flaps installed and functioning.
They look great and function well with the Bearhawk drive modification, to convert the Bearhawk flap handle through to a cub throw.
He has indicated they just bolted straight on, and were easily installed. Of course there are some simple wing skin modifications around the flap pocket too.

One opportunity missed, the modifications to the wing pocket have not created the 'ideal' pocket for the first stage of the flaps. The first stage will probably be un-blown. Though this could be improved at a later stage.

The aircraft is not flight-ready yet. All builders will understand this, 90% finished and 90% still to be done.
It will be interesting to see how they perform.


IMG_2163.JPG - Click image for larger version Name: IMG_2163.JPG Views: 0 Size: 1.17 MB ID: 50728

Yes that is a Bearhawk 4-place, and those flaps are 98.5" long.... They increase the wing area by several square feet when extended, not to mention the camber!!
 
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I assume (hope?) that we will learn what the "ideal" pocket for the flaps is? Maybe I'll just plan on building for these off the bat? Looking forward to flight test impressions.
 
This is really interesting news - thanks for updating us!

Are these flaps a 3 or 4 hanger design?

Can you post some more pictures please?!
 
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