Bearhawk STOL mods

[Hewko]

I checked the flaps on a friends Husky yesterday, the hinge point is moved forward as well, which lowers the flap as it is deployed, to bring the airflow over top. I'm not sure if the Maule hinge point is moved forward or just down.

[schu]

Not only that, I have a full autopilot setup which can control the trim servos. This means my AP can trim my airplane. Also, there is a bearhawk flying like this already, it works just fine.

[Nev]

The B model still has a servoing trim system, there is no reason to believe it will act any different than an A model. Here is the low hanging fruit: https://www.youtube.com/watch?v=mFQ1VcCiBJo

Woah, that's brilliant ! Removing the servo action and adding electric trim all in one go. Very keen to see the flying result.

[schu]

I'm about ready to cover (which for me is the last part of the fuse), but it's getting to be winter, so I have to wait until spring. Now it's on to firewall forward or wings. I think wings because I don't want to do firewall forward then remove it for the rotisserie. Maybe in the next few weeks I can mock up some lowered flap hinges.

[Battson]

The high nose angle is reduced by about 50% by a trailing edge lift device, this has been proven by a Utah pilot with a 4 place.
Stall speeds about 2 kts slower.
Stable approach speeds 4 to 5 kts slower.

The nose angle is the main thing - you understand this Nev

[whee]

I enjoy nerding out on just about anything. Fun discussion. I basically agree with Nick on my reason to install slotted flaps. Tweaking things is a lot of fun.

[nborer]

Nev, in my case, it's tinkering with a well-established design because it's fun to try to eek out a little more. From every account I've read, the stock machine performs wonderfully. But, think of the change from Model A to Model B - reportedly a few knots faster speed in cruise (likely owing to the reduction in tail incidence), and a favorable change in already good stall characteristics. There's nothing wrong (and plenty good!) with the Model A, but the B is "slightly better." Why not try? It's Experimental, it's fun to try something new if it doesn't kill you in the process. But, if you have a flying plane and don't want the down time, or you just want to get your build in the air, keep with what's been proven.

I think the improvement in stall speed with a slotted flap would be small, but noticeable, and would also help with the deck angle when flying very slow. It would also improve roll control slightly, but again, with the current slotted aileron design, there may little, if any, noticeable improvement. I estimate that the Bearhawk plain flap improves maximum section lift coefficient by about 20-25%; a good slotted flap design could probably double that. It may sound like a lot, but stall speed will go with the square root of the ratio of lift coefficients. So, if I WAG our unflapped airfoil as having a Cl of 1.7 (about what you'd see with a NACA 4412 at this Reynolds number range), a plain flap may be about 2.1, and a slotted flap would be about 2.5, giving the plain flap a stall speed improvement of sqrt(2.1/1.7) = 1.11 (~11% reduction in Vstall) for that section; the slotted flap is then sqrt(2.5/1.7) = 1.21 (~21% reduction). That sounds like a lot, but only about 55% of the Bearhawk span is flapped. So if the outer section stalls at a Cl of 1.7, and the flapped section at 2.1 or 2.5, then, from a 2D flow perspective, the plain flap reduces stall speed by (0.55*1.11 + 0.45*1 = 1.06) about 6%, and the slotted flap by about 12% (or, conversely, a 6% reduction in stall speed from the plain flap). If your unflapped stall speed is 50 knots, that logic says your plain flap stall speed is 47 knots, and your slotted flap stall speed is 44 knots. (All speeds are notional, I don't want to get into a stall speed war at this time. Point is, you're talking *at most* a 6% and 12% reduction in stall speed vs. unflapped for plain and slotted, respectively.)

Reality won't even be that kind, because that assumes you are hitting maximum lift on both the unflapped and flapped portions of your wing at the same time, and have no 3D losses (vortex at the wingtips or from the flap tip, uneven downwash distribution, etc.). Since the slotted flap will hit maximum lift at an angle of attack that is lower than the unflapped portion with the aileron (which is also slotted, but it won't be deflected as much as the flap unless you're in quite a crosswind!), the aileron portion of the wing won't be at maximum lift when the flapped portion is. However, what you will notice is the potential for a lower deck angle and slightly better aileron effectiveness, since the aileron portion of the wing won't be as close to stall.

I'm attaching the following picture (not copyrighted) from Aerodynamics for Naval Aviators; a great, practical reference for aerodynamics if you haven't seen it: https://www.faa.gov/regulations_poli.../00-80t-80.pdf. I think it under-represents what you can get from a thicker basic section, but the trends are all good.

flap_effectiveness.png

[schu]

The B model still has a servoing trim system, there is no reason to believe it will act any different than an A model. Here is the low hanging fruit: https://www.youtube.com/watch?v=mFQ1VcCiBJo

[zkelley2]

That all makes sense.

Out of interest, do you guys normally land with full available flap ? I'm wondering what effect it would have if you flew an approach a few kts faster, with full flap so that the drag took care of the extra speed on round-out. Clearly a work-around. But would the few extra kts lower the approach attitude and increase over-the-nose visibility ?

Yes, and that's generally what you have to do. The faster you fly, the lower AoA you can fly for a given weight. But energy is mass * velocity^2.

[Nev]

That all makes sense.

Out of interest, do you guys normally land with full available flap ? I'm wondering what effect it would have if you flew an approach a few kts faster, with full flap so that the drag took care of the extra speed on round-out. Clearly a work-around. But would the few extra kts lower the approach attitude and increase over-the-nose visibility ?

[zkelley2]

This is a very interesting discussion.

I'm curious - what issue would be solved by changing the flap design ? Where is the Bearhawk falling short ? Does it need to land slower ? Shorter ? Fly the same approach speeds with more control ? Better "over the nose" visibility ? I've yet to fly a B model and mine will probably be the first that I get to fly, but I'm interested to see if the pitch sensitivity (and pitch trim sensitivity) from the servo tab is still present in the B model.

My point being is that I wonder if there are lower hanging fruit on what is already well proven aircraft before getting into more complex areas such as the flap design. Examples that come to mind are reducing the pitch sensitivity, and increasing pitch control at forward CG/low speeds, ensuring that adverse yaw is limited through correct rigging during test flying, lowering flap bar forces etc.

On the other hand, if it's a case of doing it because it's an experimental aircraft and we need a project, then bring it on !

The plain flap doesn't make much lift. It's the least effective of all flaps designs, and it's also the simplest. It also doesn't make much more lift at a reduced AoA. Which is the entire purpose of flaps. To be able to make X lift at a lower AoA. If we didn't care how high the AoA was on landing, you'd see slats on everything instead of flaps. Plain flaps do make a lot of drag, which is sometimes desired.

There wouldn't be a massive decrease in stall speed with any of the improved designs. There would be a rather large decrease in ref speed. There would be better over the nose visibility and the ability to fly at slower speeds and touch in the 3 point or 2 point attitude. A max performance landing right now is always a tailwheel first landing.

That all said if you're not using all the performance your bearhawk is capable of and landing off airport at short strips, there's no realistic gain to be had. If you're flying even fully loaded out of 1000ft of grass, there's nothing to be gained. You can just come in at 55 or 60kts and have all the visibility and 2 point landings you want and still not come close to using that runway up.

[whee]

In my option the most significant item to improve is visibility over the nose when doing true short field work. The vast majority of us have no need to change anything but there’s a few who would benefit from being able to fly slow and a lower AOA.

We had a conversation about this last year I think. I nerded pretty heavily over it and came up with what I felt like was a pretty simple solution to install a dropped hinge slotted flap..
It’s been a year and I kept bad notes so I’d have to do an in-depth review to recall specifics. At this point I can’t accept taking the plane out of service for something I don’t need.

[Nev]

This is a very interesting discussion.

I'm curious - what issue would be solved by changing the flap design ? Where is the Bearhawk falling short ? Does it need to land slower ? Shorter ? Fly the same approach speeds with more control ? Better "over the nose" visibility ? I've yet to fly a B model and mine will probably be the first that I get to fly, but I'm interested to see if the pitch sensitivity (and pitch trim sensitivity) from the servo tab is still present in the B model.

My point being is that I wonder if there are lower hanging fruit on what is already well proven aircraft before getting into more complex areas such as the flap design. Examples that come to mind are reducing the pitch sensitivity, and increasing pitch control at forward CG/low speeds, ensuring that adverse yaw is limited through correct rigging during test flying, lowering flap bar forces etc.

On the other hand, if it's a case of doing it because it's an experimental aircraft and we need a project, then bring it on !

[nborer]

Completely agree, so much so that I've delayed forming my flap nose ribs while I explore a few options (I otherwise finished all the ribs and was moving on to spars prior to entering remodeling hell). Torsional strength of the wing box was one of my concerns; I don't do structural analysis for my day job and haven't done shear flow calculations since college (now over 20 years ago). A better lifting flap is going to increase the lift on the aft part of the wing as well as the associated pitching moment, so it'll increase rear spar bending loads and overall skin/wing box shear loads. It's good to know that the analysis has already been done for a different flap system that is probably more effective (I'd think a double-slotted flap would have higher loads than even a well-designed single-slotted drooped hinge design). Don't worry, if I do something crazy like actually try to build it, I'll talk to Bob first.

A redesigned tail may get rotten tomatoes thrown my way, but that *may* also be a good "bang for the buck" improvement, particularly if the high-lift capabilities of airfoil/flap system are improved. The Bearhawk tail is beautiful, and the ribs introduced with the Model B makes it even more functional. But, I've seen a lot of discussions on this forum about handling qualities at various CGs - fore vs. aft, and the ability to use the useful load. Even with the profiled tail, I think the thickness/chord ratio is somewhere around 6% for horizontal stab, if I've done it right. My hypothesis is that doubling that (something like the NACA 0012 profile) could yield a little more performance, or conversely require slightly less elevator deflection for trim. It could be more of a problem, particularly at forward CG, if one were to design a flap with higher lift (which would inevitably increase nose-down pitching loads). I'll throw in my usual caution that I have no reasonable analysis result or Bearhawk flight experience to back this up; just what I've tried to read between the lines in the forums and other reports online.

Nick

[Battson]

I have looked at this in detail. Its fraught with issues and you end up having to do major modifications. The flaps also have to get a lot smaller (shorter cord). The double slotted flaps are a much simpler bolt on solution, less work and cost overall.