Ways to improve slow flight performance

[Battson]

I have been contemplating ways to improve slow flight performance of the Bearhawk for some months now. My objective is to expand the slow flight envelope without losing lots of cruise speed or adding an undue amount of weight. A little weight wouldn't hurt if the resultant lift increase was great enough.

I am looking for sensible / realistic ideas, like minded people, and particularly any examples of work in progress or flying modifications. If anybody has knowledge of any of those, please post.

For background, we have already done the obvious / easy stuff including: VGs installed, lightest build we could manage, and lots of PIC practice.

• I have worked out a conceptual design for slotted flaps which would be easily done, but possibly not add much performance per cost / weight / effort / complexity. I guess this is why Bob changed his mind about building the slotted flaps he originally designed on the Bearhawk (so I am told).
• I am still musing a true fowler flap, which would be the ideal solution. It would need significant re-engineering including opening the wing to tie the flap rails to both spars. I'd rather start a new set of wings than open the existing set.
• Similarly, self-retracting leading edge slats would be great - although hard to retrofit. The main problem with them is, they need much steeper angles of attack to produce their lifting benefits - which results in unusable performance in real world backcountry situations. The Bearhawk tailwheel won't take it.
• Flaperons would be a fairly easy mod by comparison to all of those, although they would need to be limited to perhaps 10 degrees for safety and control reasons, but would have almost no penalties in cruise drag and weight. They would increase the maximum lift - but I have no idea how much of a difference only 10 degrees would make. May not be worth the hassle? I would not slave them to the flap controls if I installed such a mod, you would want to use them selectively, because of the impact they have on the stall characteristic of the wing.

[Chewie]

What about power and prop considerations?

Mark
Scratch building Patrol #275

[Battson]

What about power and prop considerations?

Mark
Scratch building Patrol #275

What would you suggest?

[Chewie]

I was hoping you would answer that. Depending on the slow flight goal you may be able to optimize your prop efficiency at that point, but it would require all the engine data you could get your hands on. I could ask my boss on Monday, he's an aerodynamics guy. A simpler approach might be to discuss with Bob and see if his prop testing sheds any light on slow flight characteristics.

Mark
Scratch building Patrol #275

[Battson]

I was hoping you would answer that. Depending on the slow flight goal you may be able to optimize your prop efficiency at that point, but it would require all the engine data you could get your hands on. I could ask my boss on Monday, he's an aerodynamics guy. A simpler approach might be to discuss with Bob and see if his prop testing sheds any light on slow flight characteristics.

Mark
Scratch building Patrol #275

I guess the main thing I am interested in is flying slower, reducing the stall speed in other words. Using a lighter engine and prop would help that a little I guess.

[Bcone1381]

http://wingsandwheels.com/tapes-seals.html

Gliders improve their performance significantly by sealing up the wing. They seal gaps at the ailerons, flaps, elevator, and rudder. In addition, they seal up push rods both at the aileron (and flap) and at the wing root where push rods pass through the area. They want the wind totally sealed up. I think the theory is uncontrolled airflow causes turbulence which increases drag.

In the very low speed arena, the pressure differential is max. Would sealing up your wing provide some improvement? My expereince in a glider with no tape sealing the wing root - fuselage joint resulted in additional noise, turbulence and decreases glide performance. Before that experience I thought those guys were a little anal. Now maybe I am bit anal....My Patrol will have a sealed wing.

[nichzimmerman]

On my 4 place I am building the flap to extend inboard to the root like the Patrol. Specifics were approved by Bob. I hope for better slow flight and no significant weight change.

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[Battson]

http://wingsandwheels.com/tapes-seals.html

Gliders improve their performance significantly by sealing up the wing. They seal gaps at the ailerons, flaps, elevator, and rudder. In addition, they seal up push rods both at the aileron (and flap) and at the wing root where push rods pass through the area. They want the wind totally sealed up. I think the theory is uncontrolled airflow causes turbulence which increases drag.

In the very low speed arena, the pressure differential is max. Would sealing up your wing provide some improvement? My expereince in a glider with no tape sealing the wing root - fuselage joint resulted in additional noise, turbulence and decreases glide performance. Before that experience I thought those guys were a little anal. Now maybe I am bit anal....My Patrol will have a sealed wing.

Good thinking. Thanks for posting that.

My only question is, are those gliders using laminar flow wings? In other words, do you think the benefits are the largest in a laminar flow situation?

The vortex generators are already making a mess of the air over the top, so perhaps I should be sealing the wing on the underside. The main place to do this would be the flap.

[Bcone1381]

The glider that I experienced this on was of the 1970's vintage....a Schliecher ASK-13. Tube and fabric fuselage with a wood and fabric wing.

Are vortex generators really making a mess of your wing? Could they rather be energizing the boundary layer enabling the attched airflow to be organized and adhere to the wing at a higher angle of attack, instead of separating and causing reverse and disorganized airflow?

http://youtu.be/9Whd_KnsLKE

After watching this YouTube video, I wonder about a retractable spring loaded VG set up for the flap, that would extend when the flap is extended, re-energizing that airflow as it np ends around the flap.

[Battson]

Yes about the VGs - they trip flow from laminar to turbulent. The turbulent one is draggier, but creates more lift. So less efficient, gliders may or may not use laminar flow wings.
http://i.stack.imgur.com/hYRmk.gif

[Battson]

On my 4 place I am building the flap to extend inboard to the root like the Patrol. Specifics were approved by Bob. I hope for better slow flight and no significant weight change.

Sent from my SCH-I535 using Tapatalk

I think that's a good idea, and less onerous than most of the mods I suggested. I could easily retro-fit such a thing I imagine.

So - what is the main benefit in your view? Is it as simple as increasing the span of higher effective camber, and increasing the total lift that way?

Are you building a Bearhawk with the intention of lots of STOL flying?

[alaskabearhawk]

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.

[nichzimmerman]

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



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[Jflyer]

http://wingsandwheels.com/tapes-seals.html

Gliders improve their performance significantly by sealing up the wing. They seal gaps at the ailerons, flaps, elevator, and rudder. In addition, they seal up push rods both at the aileron (and flap) and at the wing root where push rods pass through the area. They want the wind totally sealed up. I think the theory is uncontrolled airflow causes turbulence which increases drag.

In the very low speed arena, the pressure differential is max. Would sealing up your wing provide some improvement? My expereince in a glider with no tape sealing the wing root - fuselage joint resulted in additional noise, turbulence and decreases glide performance. Before that experience I thought those guys were a little anal. Now maybe I am bit anal....My Patrol will have a sealed wing.

Real world numbers:
I've flown a number of Scweitzer 1-26s. Not the most sleak and high performance ships out there but a great one up design. Tube and fabric fuse, metal wings.

The "1-26 Assiciation" recommends taping the wing root to fuse joint top and bottom with good vynil duct tape. The Assiciation claims 5MPH decrease in Stall speed.

This sounded a bit exaggerated to me until I added the tape to my 1-26. The stall speed dropped exactly 5mph.

Appareny that area between wing and fuse causes an incredible amount drag due to air flow being tossed In and out, in various directions and especially down inside the root of the wing and then out again at the various openings of the aileron bays and dive break assemblies.

We just use the wide high speed tape found in the aviation isle at Ace Hardware. It comes in enough pretty colors to closely match the ship's colors.

[jaredyates]

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.

[Bcone1381]

Up to a certain point, moving the CG from forward to aft improves a whole lot of performance numbers. Faster cruise, less downforce on the tail thus lessoning the overall lift the wings must develop to maintain level flight, better elevator authority at slow speed. I will be building a ballast system into my Patrol in order to have good control of it. BTW, the McDonald Douglas MD-11 has a tail fuel tank. It manages the fuel in that tank to obtain a CG at 98% of the Aft Limit in cruise to improve efficiency.

[Battson]

Very interesting idea! It certainly makes sense, from an efficiency perspective. Not sure it would help the stall speed much given you need to add weight, but very possibly will help control, and sounds very unique!

[Bcone1381]

After doing some math, I agree with your assessment. a 30 pound weight would effect it only about 4 inches of the 11 inch CG envelope on the Patrol.

[John Bickham]

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

[Mark Goldberg]

The Pitts style gap seal seems to give more authority. I had a good report on that from a flying kit Bearhawker. Mark