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Thread: Lift distribution change for speed

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    Registered User BJC's Avatar
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    Lift distribution change for speed

    My Glasair Aviation Sportsman 2+2 has a wing span of 35 feet and an AR of 9.1. With Lycoming IO-360, the quoted top speed is 145 knots.

    A popular mod is to rig the ailerons slightly TE up relative to the chord line to shift the lift distribution and increase cruise and top speed.

    Rigged per specifications, i.e., with the ailerons’ TE aligned with the chord line, my airplane has a GPS-verified top speed of 147 knots. Raising the ailerons’ TE by approximately 1/4” increased the top sped to 149 knots. Others have reported gains of up to 5 knots from raising the TE by 1/2”. As soon as I can get back to my annual condition inspection, I plan to raise the TE another 1/4” to see what additional speed gain is available. I’m hoping to be able to claim a top speed of at least 150 knots rather than 149.


    BJC

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    Re: Lift distribution change for speed

    Uh oh! Wait till Billski hears this LOL

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    Registered User wsimpso1's Avatar
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    Re: Lift distribution change for speed

    What do you expect Billski is going to say about it?

    Normally I will advise that folks stick to the plans unless there is a known issue and a known good fix. No serious airframe changes. Sounds like that is Byron's plan. On top of that, how many of us have seen control surfaces this much out of streamline just to make the plane fly straight?

    Byron is certainly qualified to make the decisions, run this sort of experiment, and do the test flying safely.

    Technically, this change unloads the outer part of the wing slightly, loads up the inner part of the wing a little, reduces bending moments, reduces pitching moments, which are all favorable. It might have some modest effect on flutter, but I would expect it to be pretty small - the sailplane guys adjust reflex all the time.

    The downsides are that folks who do this are deviating from the plans, but not by much. This may induce some separation at both ends of the ailerons, but it appears to be a net plus for low CL drag. Other pilot/owners have already done the adjustment and found it to do what they wanted, presumably with no nasty side effects. And if the rest of the envelope takes a hit in handling or performance, it is easy to put back to stock configuration. On top of all that, Byron is doing it in steps, so he got an indication of what the change does to the airplane without taking the full step the first time - reduces surprises in the test phase...

    If you can make such a simple rigging adjustment to pick up 2% on top end (without any gotchas), it does indicate the designers were looking at other issues than top speed or maybe just "missed" when they loaded up the tips, maybe to make stall speed as low as they could.

    Byron, please tell us how this adjustment affects approach and low speed handing, stall behaviour, stall speed, and roll rates.

    Once done, are you going to consider twisted ailerons and flaps (per Cessna) and adjusted alignment of the wing tips?

    Billski
    Billski's opinions expressed here are available free and may be worth the money you paid for them. Understand that they are based upon a successful combination of education and a lifetime of experience using that education, but I can not know everything about your circumstances. Your choices are yours alone, and you must be the final judge on what you do. No whining...

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    Registered User proppastie's Avatar
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    Re: Lift distribution change for speed

    I wounder what might be a considered standard measurement error in a test like this. Also consideration of nose up nose down AOA in level cruse which will cause speed changes.

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    Re: Lift distribution change for speed

    Quote Originally Posted by wsimpso1 View Post
    ...

    Byron, please tell us how this adjustment affects approach and low speed handing, stall behaviour, stall speed, and roll rates.
    The relatively small change that I have made so far has not noticeably changed any characteristics other than the top speed.

    Once done, are you going to consider twisted ailerons and flaps (per Cessna) and adjusted alignment of the wing tips?
    That would be the elegant thing to do, but, no, I will not be doing that. My guess is that the designers stuck with a no-washout wing and straight flaps and ailerons for ease of construction. One of their design goals was to have an airplane that retained aileron control through slow flight and into the flaps-retracted stall. They succeeded at that.


    BJC

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    Re: Lift distribution change for speed

    Turning the ailerons up is an old RC trick for adding wash out to help with tip stalls on bad handling planes. Without you butt in the seat, your only cues on how it flies is visual. Something like a WW2 fighter in judged competition will show “fixes” on the airframe and points are deducted. Once to the flying portion of the contest, a little rigging change and you have something much tamer in line with the rules. Watched a very nice 1/5 scale Fw190D spin in from too tight a turn; high speed tip stall without tricking the ailerons. 25ft is not enough altitude to fix it. Splat. On the Sportsman, It’s very nice that something so simple can get results.

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    Registered User BBerson's Avatar
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    Re: Lift distribution change for speed

    What does it do for normal cruise speed? Or do you cruise at top speed? I knew an airplane dealer that said he always delivered aircraft to owners at full power cruise.

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    Re: Lift distribution change for speed

    I cruise at anywhere from 120 knots to 145 knots, depending on weather / winds, planned distance, time of day, etc.


    BJC

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    Re: Lift distribution change for speed

    I have raised the ailerons on a couple of my homebuilts with good results. I start with 1/4" at the inboard trailing edge.
    Pops

    If its not there, it cost nothing, weighs nothing, and is 100% reliable.

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    Re: Lift distribution change for speed

    Some have added a 'cruise flap' control for trimming the ailerons. Presumably this aircraft benefit from a range of settings. Does your rigging affect roll at all?

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    Re: Lift distribution change for speed

    Quote Originally Posted by BJC View Post
    The relatively small change that I have made so far has not noticeably changed any characteristics other than the top speed.
    Quote Originally Posted by pictsidhe View Post
    Some have added a 'cruise flap' control for trimming the ailerons. Presumably this aircraft benefit from a range of settings. Does your rigging affect roll at all?
    I’m familiar with flaps that reflex for cruise, but I have not seen / noticed ailerons that reflex for cruise. The early RV-3’s drooped ailerons with flaps, but that feature was abandoned.

    See above re. roll.


    BJC

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    Re: Lift distribution change for speed

    Original Carbon Dragon had a 5 deg. positive setting for the Flaperon...

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    Re: Lift distribution change for speed

    Quote Originally Posted by BJC View Post
    I’m familiar with flaps that reflex for cruise, but I have not seen / noticed ailerons that reflex for cruise. The early RV-3’s drooped ailerons with flaps, but that feature was abandoned.

    BJC
    The Schleicher ASK13 sailplane sports a control differential that lowers the ailerons (a bit) when you pull the stick (and vice versa). In theory this allows slower flight but decreases twist when you want to have it most (i.e. when flying at high AOA).

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    Re: Lift distribution change for speed

    Quote Originally Posted by BJC View Post
    I’m familiar with flaps that reflex for cruise, but I have not seen / noticed ailerons that reflex for cruise. The early RV-3’s drooped ailerons with flaps, but that feature was abandoned.

    See above re. roll.


    BJC
    Neither have I, on full size light airplanes. On competition RC sailplanes the four servo wing is common. That is reflex both ailerons and flap for speed and a lot of different mixing to tweek the wing for whatever speed and load combination.

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