Bearhawk STOL mods

[svyolo]

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.

[Rollie]

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.

[svyolo]

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.

[Battson]

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.

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.

[whee]

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.

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.

[Battson]

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.

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.

[PaulSA]

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.

[Bdflies]

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!

Bill

[Battson]

I'm asking because I doubt that the BH gives you much lift past 2 notches, just more drag.

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.

[Battson]

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?

[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.

[Battson]

Yes that is where they stayed.

[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!

[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.

[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.