Bearhawk STOL mods

[schu]

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

[svyolo]

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.

[schu]

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.

[Nev]

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.

[yateselden]

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.

[schu]

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....

[Battson]

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.

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.

[schu]

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?

[Battson]

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.

[schu]

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!

[whee]

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

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.

[svyolo]

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.

[rodsmith]

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.

[swpilot3]

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!

[Battson]

Yes that is where they stayed.

[Battson]

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 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.