Ways to improve slow flight performance

[Battson]

Sorry for my absence, working overtime!

Mark Scott put drooping ailerons on his BH. He said it did decrease stall speed by a few knots but increased stick forces more than he would like. They are operated independently by a switch on the panel.

This is very, very interesting. I figured on doing the same - seems to be the easiest mod, although it clearly loads up the direct control cables...

Do you know how he did it?
I don't think I know Mark, is he on the forum here or happy to be contacted otherwise?

[Battson]

Battson,
My dream is to do a lot of STOL and I read of your adventures with envy, but i will also be travelling in it. My goal is to build this airplane light and usable. Reading about your build helps a lot.

The extra flap span by itself may increase L/D by a non-measurable amount, but I have a theory that something similar to a wingtip vortex may form around the inboard (and outboard) end of the huge flap and extending it to the fuselage may help negate it. Either way, Bob seemed to think this design was better since he built the Patrol that way. I wish there was a simple way to flight test it.

With the extension, my flaps barely fit into our minivan.

Nic
BH1217

Thanks Nic,

Would you be able to share photos of how you extended your flaps?

[Battson]

I'll throw this out for your consideration. How about sealing up your elevator and rudder gap seals? Jared mentioned losing elevator effectiveness at high AOA. I experienced the same problem on a my previous plane which was much slower and lighter than the Bearhawk. I was working on the shortest possible landing distances. I got to the point where the elevator just didn't work as I got slower and slower. I was making steep approaches over obstacles to a short field. It was fun but a lot of work and concentration. Timing was the key factor and I likened it to a bird landing on a wire. Sometimes I missed the wire. I kinda remember a discussion on this on the old list. I think Pat Fagan and others use a Pitts style rubber gap seal on their elevator/HS tubes.
I don't know if this would hold up to the higher Bearhawk speeds but it illustrates the concept. It is on my list when I reach the place you are with flying the Bearhawk.

http://eaavideo.org/video.aspx?v=3801322456001

Thanks for the suggestion John.
I have considered this before from the Yahoo group times. Mainly in the hopes of increasing speed at cruise and elevator authority at the stall.
I find the slowest landings are accomplished with some power on, well behind the drag curve, and I haven't been suffering from the lack of elevator authority since I started using that technique.
Incidentally, I don't want to make the elevators more twitchy at cruise speed, which is another side effect.

[Battson]

If you were willing to make the airplane useless for anything but slow flight, it would be easy- you could just do the things that Knapp does. As you've found the hard part is not compromising your overall utility.
Generally, I would think that wing loading would be one of the most important factors in determining stall speed. Reducing weight can be easier than increasing wing area. You could lose close to 100 pounds by changing engine size, and that would allow you to fly approximately 2 knots slower, but then it would approximately double your takeoff roll. I gather that going from 2500 pounds to 1700 pounds gross weight would give you something on the order of a 10-knot reduction. Going from 2000 pounds gross weight (a heavy BH plus one pilot and min fuel) to 1400 pounds (a very light BH with the same load) would account for a reduction on the order of 8 knots. But if the goal is to fly slow and be able to transport stuff, then it's hard to find gains there. If a mod increases weight significantly, then it may well provide a net gain in stall speed rather than a loss.
To the degree that a slower speed comes from a higher AOA, you might experiment with incrementally moving the CG aft. I find that the elevator effectiveness is a limiting factor in reaching a high AOA if I don't have some ballast in the baggage area. My tuft testing at forward CG revealed that the air is separating at the hinge line, I suspect because the corner is just too sharp when the elevator is deflected enough to approach a stall at forward CG. At a further aft CG, the elevator deflection required to reach a stall would be less, and you'd be able to get the wing to a higher AOA before losing elevator effectiveness. This would also allow you to more readily stall and/or spin, which may be an operational concern that will vary in severity based on pilot ability and air stillness.
Have you been able to quantify any improvements and/or losses for the mods you have already made? My goal for this month is to do some before and after testing with big tires to see if I can quantify the performance loss. It's more of a hassle than just putting the tires on and going to fly, but I'd like to have an idea of what the overall operational cost is, so that I can weigh that against the overall operational gain.
You might also try tufting the wing to see how much of it you are able to get stalled before you feel like you don't have enough control authority. Such testing may show a weak link if there is one. All it would take is a Gopro, a few packs of latchook yarn, a roll of shipping tape, and a day or two of effort.

Thanks Jared - I think you've summarised the challenges and opportunities well.

My main goal is certainly to install "deployable" lift surfaces / devices, which don't add cruise drag or significant weight.

I detailed my findings in the VGs thread, and possibly mentioned the effect of the larger tires there too in terms of cruise performance. I didn't notice much loss with either individual mod, but combined they started to add up to a measurable loss of a handful of knots. I have knowledge of the effects of larger tires (than 24") from friends and it appears the drag increases sharply - 29's maybe cost 5kts, 31's maybe 8kts or more.

[N942VT]

My 31's and Baby Bushwheel cost me an honest 10 knots over 26 goodyears. Slow, but I look good doing it.
With big tires, take off performance suffers and I don't like the way she flies as well. Landings are great but overall performance is not increased with big tires. I've always thought the STOL demo crowd would be better off with long gear and small tires.

[Battson]

Very interesting - thanks for sharing your experience there!

[patrickh99]

Have you considered changing wingtips? Longer tips with drooping ends will increase wing area and help hold the hi-pressure air under the the wing. Stall fences are also reportedly helpful. Check out this website for some pictures and possible inspiration: http://www.wingxstol.com/html/gallery.html I have no experience with their products, just find their pictures helpful.

[Battson]

Have you considered changing wingtips? Longer tips with drooping ends will increase wing area and help hold the hi-pressure air under the the wing. Stall fences are also reportedly helpful. Check out this website for some pictures and possible inspiration: http://www.wingxstol.com/html/gallery.html I have no experience with their products, just find their pictures helpful.

Funny you should ask - I have looked into this at length years ago. Here is what I found, from memory:

Drooped wing tips were originally developed and tested by Cessna, in the 70's or 80's if I recall correctly. In a letter which you can find online, the test pilot recounts the testing process, and how there were "no measurable benefits" in terms of cruise or stall speed, although control was somewhat different.
See page -7- for the "fraud of the drooped tip" https://dl.dropboxusercontent.com/u/...ing%201979.pdf

Nonetheless, the marketing guys got one look at them and decided to put them into production them out anyway because their "visual appeal" would sell aircraft.
I agree that longer wing tips would reduce wing loading slightly. The issue is they also increase induced drag and hurt cruise speed quite a bit. So I am undecided whether the potential benefits outweigh the possible downsides.

Stall fences are useful to avoid span-wise flow during a stall, and prevent the stall progressing from the wing root out to the wing tip and creating a wing-drop stall. Typically they are located at the outboard edge of the flaps, to separate them from the aileron.
The Bearhawk doesn't have any washout in the wings, so I think that makes stall fences somewhat more attractive. I have put a higher concentration of VGs on the outboard parts of the wing, to achieve the same effect of ensuring the wing tips don't stall when the wing root does.