Bearhawk STOL mods

[Hewko]

Would a sportsman style LE cuff make any difference on the Ribblet airfoil? Does anyone know if this has been tried on a Bearhawk?

[Battson]

It would certainly make a difference (not necessarily better) - but it depends, what are you trying to achieve? If you only care about slow flight, adding camber and increasing angle of incidence are no-brainers. If you also care about cruise speed, maybe not such good ideas...

[jaredyates]

Would a sportsman style LE cuff make any difference on the Ribblet airfoil? Does anyone know if this has been tried on a Bearhawk?

When I put on my science hat, my answer is I don't know, and I don't know of anyone who has tried it. When I put on my intuition hat, I have to wonder of it's like putting ketchup on a high dollar steak. As with any proposed mod, the key question is what is the shortcoming that the cuff would be addressing?

[rodsmith]

Would a sportsman style LE cuff make any difference on the Ribblet airfoil? Does anyone know if this has been tried on a Bearhawk?

Having read Harry Ribblet's book I would say no. The sportsmans cuff is a bandaid on a poor airfoil. Harry Ribblets airfoils corrected the design deficiencies of the NACA series like the 24012 used on a lot of Cessnas. The sportsmans cuff is adding leading edge droop to the 24012 airfoil. This is already designed into the Ribblet 30 series airfoils. Adding additional droop on a Ribblet airfoil would have unpredictable results, might improve one end of the speed envelope with major detriment on the other end. So how about the 4412 airfoil on the A model? Bob modified the 4412 adding leading edge droop. There are two methods of calculating this change. According to Harry, Bob did it the correct way.

[whee]

In essence I agree with Rod.

Both the A and B wings were designed with slow flight in mind, the Cessna was not designed for that specifically. While I think the Cessna wing is pretty good on it’s own the Sportsman cuff offers better slow flight for the guys that want it.

[Hewko]

So would it be possible during the build to make the standard Bearhawk flap into a fowler flap by attaching a second lower flap pivot point, about 2 inches lower similar to a Maule? And then moving the flap actuating rod attach point correspondingly lower on the flap? The roughly hand drawn in extension point on the attached image shows it better than I can explain it. Is this worth trying or will it introduce problems?

[Battson]

Yes that is the Maule flap idea discussed on previous pages, but no that is not a fowler flap - that is a slotted flap.

[Helidesigner]

Has anyone tried flap seals? I know the gap is small but air is smaller. Gap seal on my elevator and rudder worked quite well. Tape along the bottom like my elevator and rudder would be easy to do.

[rodsmith]

I have also wondered if this would be beneficial. You could try it and let us know.

[schu]

Has anyone tried flap seals? I know the gap is small but air is smaller. Gap seal on my elevator and rudder worked quite well. Tape along the bottom like my elevator and rudder would be easy to do.

It for sure couldn't hurt, but I'm not flying yet, so I don't know.

I wonder how much difference lowering the flap hinge point below the wing will help, like a Husky or Maule. I suspect a bit, but then we would have to figure out if the wing is strong enough to handle the change in forces.

I also wonder how well your drooping ailerons work. Would you do it again? Any details you can share?

schu

[zkelley2]

Has anyone tried flap seals? I know the gap is small but air is smaller. Gap seal on my elevator and rudder worked quite well. Tape along the bottom like my elevator and rudder would be easy to do.

With the plain flap on the bearhawk you're not really counting on any air up and over to re-energize the boundary layer. I don't think it'd do anything.

[Battson]

My thoughts exactly.

[Bcone1381]

I'll expand on curiosity of flap effectiveness.....

What about Cessna Style flap tracks. You get a true "fowler" flap action and miss out on bumping your head on the hinge that hangs down.

Screen Shot 2021-10-12 at 7.33.34 AM.png

[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.

[Battson]

I wonder how much difference lowering the flap hinge point below the wing will help, like a Husky or Maule. I suspect a bit, but then we would have to figure out if the wing is strong enough to handle the change in forces.

The wing has been modelled by the guys at Javron, it was found to be strong enough to hang double slotted AA flaps on it. I am sure a lower hinge point would not be a lot different. Maybe cause to be more careful with flap speeds.

This is my personal favourite to improve Bearhawk performance, in terms of bang for buck.

[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

[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 !

[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

[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]

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.

[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.

[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 ?