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Thread: GURNEY FLAPS, WICKERBILLS .....testing data and photos

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    Dave Calkins's Avatar
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    GURNEY FLAPS, WICKERBILLS .....testing data and photos

    Wickerbills have been discussed many times on the site, but no data or photos, so here we go. I made some that hang down about 3/4" long and installed them, took some photos and did some flying. I've been using them for about 3 flight hours and thought they were a great mod. I tested with them on the trailing edges about to where the aileron trailing edge starts to curve, then removed the ones on the ailerons, then removed the ones where the flaps would be on a flapped Cub.

    The airplane is a Wag-Aero Cuby, about a thousand pounds empty on 31's with AOSS, Baby Bushwheel, and Landes 32 gallon belly tank. Round tip wings, Cuby airfoil (not SuperCub airfoil. The Cuby's is similar, but with a larger leading edge radius), no flaps. I have sealed the elevator hinge gaps, which works well for increased low-speed elevator response. Large VG's located similar to MicroVG's locations on the wings, none on the tail (Gap Seal does that job nicely). 140 Horsepower, 80 inch prop. Engine/Prop combo work very well on this a/c. The a/c is well-behaved and a joy to fly. You SuperCub owners would not be dissapointed with the performance of this "Cub".





    Test data as follow:

    55 degrees F. Barometric Pressure 29.90

    The first number is with full TE wickerbills, Second number is minus aileron wickerbills, third number is with NO wickerbills. I believe there are anomolies.

    Takeoff speed indicated (ASI) 20 22 25
    Takeoff speed GPS 30 29 32
    Landing speed ASI 40 40 40
    Power off stall ASI 39 34 40
    Power off stall GPS 35 38 NA
    Power On stall (Nose very high, full power) ASI 30 20 35
    Power On stall GPS 34 38 NA

    Temperature increased to 57 for the last test. Air was somewhat bouyant (thermal activity?) on the test with the aileron wickerbills removed. Wind was light and variable for all three tests. I believe the ASI numbers to be more relavent relative to one another than the GPS numbers due to the varying winds. All testing was completed within a 1 hour timeframe.

    Landings were all at 40MPH indicated, with both wickerbill test approaches somewhat steeper than the non-wickerbill flight, and good flare authority with the wickerbills. Without wickerbills, 40 MPH was a stretch to see over the nose, with the nose high and need for a bit of power to arrest the descent and flare.

    Wickerbills took about 5 MPH off the top speed of this Cuby that will do 93MPH at 2450RPM without them.

    The tail shakes more on steep full power climbs WITH the wickerbills.

    DAVE
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    David thanks for the data I've allways wondered about how they worked now I don't Mike I

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    mvivion's Avatar
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    Interesting!! Keep the info coming, Dave.

    MTV

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    Rob's Avatar
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    Dave,
    One of the biggest banner outfits, (probably the biggest) runs gurney flaps on most of their cubs, at one point I think they ran over 50 cubs. Those cub don't tend to have the "nicest feel" but do tend to haul the biggest signs in the buisiness, and they are able to do it very slowly. Considering that each hop is +/- 4 hrs, and they do this all summer long and even year round, I would venture to say that they have more collective time with gurneys on cubs than pretty much any one person is ever going to amass... They also run exceptionally long wings, and ailerons modified to be like -12 ailerons. Since they are not in it for fun, or practice, the simple fact that they continue to run them would suggest something is going on. But it's really nice to see someone put numbers to it to see what and where that something is! Thanks!

    Take care, Rob

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    Dave Calkins's Avatar
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    Guys, thanks for the responses.

    What does not show up in the data are some handling qualities that I value. Namely, good over-the-nose visibility on short final, and ability to perform a steeper approach.

    I believe the data I posted had some anomolies. I would like to get a passenger to handle the data acquisition while I fly and also to do the testing in the very early morning rather than mid-morning on a 70 degree Anchorage, Alaska sunny day.

    Originally I had installed the Gurneys only on the inboard parts of the wings. I was worried how they might affect the ailerons when installed there, so I tufted one wing, installed the gurneys on the ailerons, and proceeded very gently. It was obvious early on that they did not adversely affect the aileron performance or any other performance. There was apparent span-wise flow at high AOA, with some flow through the inboard end of the aileron cutout (aileron/flap junction area) and also flow through the aileron hingeline. Nothing unexpected flow-wise, but to find zero bad handling qualities was nice.

    An interesting discovery was that the gurneys produce enough lower surface pressure at the trailing edge of the aileron that in cruise both ailerons are about 3/4" reflexed, and both of the aileron direct cables have like 2 inches of slack. Yes, the balance cable is drum-tight!

    My seat of the pants impression of the gurneys is that they are working. I don't know how they work. Anyone? Circulation theory?

    The data doesn't show a great performance change, but maybe my seat is better calibrated than I thought, and is able to feel change in smaller increments than I would have imagined.

    Later. DAVE
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    jr.hammack's Avatar
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    dave,
    any concerns on loading of the ailerons hangers or spars?would interesting to see what cruise numbers with the ailerons change.
    have thoughts about trying on flaps also.

    jr.

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    Dave Calkins's Avatar
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    JR, the cruise numbers were the same 88 mph with gurneys on the "flap" area and ailerons, 88 with 'em on just the 'flap' area. Bird cruises at 93 without any gurneys.

    I think the aileron reflex due to stretching cables, etc. can account for the 88 with or without 'em on the ailerons.

    Yes, loading the aileron hinges, etc. is something to think about, but they're as tough as the flap hinges, so I don't see a problem.

    Later. DAVE

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    gurney flaps

    i have acub some what like yours exp no flaps stall seems to be around43 to47 clean power on 40. I would appreciate more info and would like to talk to you on the phone. My number is 406-592-3521, any time in evening, or if I could have you number I would give you a call.

    Thanks Pilk

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    Dave Calkins's Avatar
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    Pilk, check your PM. My phone number is there.

    I'm at sea level and the tests were done at about 800 MSL. The a/c was flown at about 1260 pounds.

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    Ruffair's Avatar
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    Having heard about these here on this site for some time....
    I can't remember what's been said about putting these on a
    certified airplane like a j3...?

    Can someone enlighten me as to stc's if any, or 337's....

    Are these homemade or...home depot?

    Thanks...
    Kem

  11. #11
    Dave Calkins's Avatar
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    Kem, these are experimental on an Experimental-amatuer-built a/c.

    Homemade of .032" 2024-T3 aluminum sheet and bent in a sheet metal brake, held on with #8 sheet metal screws into the aluminum trailing edge stock.

    I don't know of any certification for them.

    PS, the test-data/numbers don't tell the whole story.

    DAVE

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    Apparently an Alaska trick is to re-adjust flaps to 60 degrees. I have flown one of these birds - it came with Micro VGs - and it was a real surprise. In the flare it always felt like you could get out and walk beside it. Owner routinely stopped in the first hundred feet of runway. Sorry, no numbers. But it is really easy (if illegal) to try it.

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    Too bad you didn't have a THRUSTLINE mod!!!!

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    Dave Calkins's Avatar
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    Propwash, do you know something that i don't. DAVE

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    Quote Originally Posted by Dave Calkins
    Propwash, do you know something that i don't. DAVE
    No sir, I'm one of the of the ones that admits I know nothing. I was just being a jack@$$ about a previous topic on the thrustline mod, and Valdez. Nothing aimed at you. Pretty interesting what your doing, thanks for posting results, and not acting like a know-it-all, like some do at times.

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    Ruffair's Avatar
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    Rob wrote: One of the biggest banner outfits, (probably the biggest) runs gurney flaps on most of their cubs, at one point I think they ran over 50 cubs. ...........


    So I was wondering.... are they running experimental cub's..?
    Sounds like it with all the other mod's they had..

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    WhiskeyMike's Avatar
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    gurney flaps

    Quote Originally Posted by Ruffair
    Rob wrote: One of the biggest banner outfits, (probably the biggest) runs gurney flaps on most of their cubs, at one point I think they ran over 50 cubs. ...........


    So I was wondering.... are they running experimental cub's..?
    Sounds like it with all the other mod's they had..
    Banner planes run in RESTRICTED Category. You cannot use Experimental for commercial purposes. This brings up the business about paying for certified VG's etc. May seem like there's a cheaper way, but when you need the mod approved, someone has to pay lots to get it done. I'd LOVE to have the approval info on Gurney flaps. Contrary to popular opinion, you cannot make unapproved mods on RESTRICTED airplanes. In fact the ops limitations state that any mod invalidates the airworthiness unless and until approved in writing by an FAA inspector.

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    Rob's Avatar
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    Hi Ruffair

    Stearman600 knows the cubs / outfit I am reffering to. I believe he even has time in a cub or two of theirs
    And of course he knows the restricted / aerial advertising deal as good as anyone can...No experimentals...
    I can't speak for how they got approval for the mods on their cubs, and yes in fact many of the mods are to the extreme side... I can only guess that since this outfit has been around for a very long time, that most of these planes have been in their possession from a time of a more user friendly FAA... After all there once was a one time STC issued to mount a pair of Browning BARs as well as a pair of wing mounted shotguns to a supercub.... Probably not happening in todays FAA
    I will add though that I have been on the ramp waiting to launch for a busy (banner wise) venue, and watched that companies cubs get grounded by the local feds, I don't honestly know why, but it just goes to show, if you're going to stand out in the crowd you probably ought to have all your ducks in a row...

    Take care, Rob

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    Ruffair's Avatar
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    Thanks Rob and Stearman600,

    I figured Restricted might be it...

    I know of a fellow here in the SandHills of Nebraska that had the
    shotguns you talk about... His son has aquired them, wonder what
    they will be on next.... ha ha..

    Things just aren't like they use to be...

    Thanks again..
    Kem

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    ......I don't know how they work. Anyone? Circulation theory

    That's exactly how they work. They increase the wing circulation.
    Three things happen.
    1) They slow the air under the wing and move the stagnation line forward while speeding the air over the wing, thus increasing the pressure under the wing while decreasing the pressure over the wing.
    2) They cause the air to depart the trailing edge deflected more downward than the normal departure parallel to the camberline thereby increasing the downward momentum imparted to the air.
    3) They increase the pitching moment of the wing, requiring more tail download.
    JimC

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    mvivion's Avatar
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    Jim,

    Thanks for the straightforward explanation. Now, I think I can get my head around the concept.

    MTV

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    One more word about 2) above -- well, maybe a few sentences....

    The flow usually departs the trailing edge of the wing parallel to the camberline (not the chordline). The way the wickerbill gives more downward component is as follows. The wickerbill forms an approximately vertical trailing edge surface that is usually about 1% to 1.5% of the chord. That's about the most you can use without incurring a substantial drag penalty due to the bluff vertical surface. The vertical bluff surface causes a spanwise vortex to develop behind it due to the difference in flow velocity above and below it. Since the aftward flow at the bottom of the wickerbill is substantially slower than the aftward flow at the top of the wickerbill, the rotation of the vortex is clockwise as seen from the cabin looking out along the trailing edge of the right wing (counter-clockwise looking out along the left wing). The spinning vortex entrains the air along the top side of the vortex, and that entrained air spins off in a more downward direction than it would without the wickerbill. Making the airmass move more downward imparts more lift to the trailing edge of the wing. Since that part of the lift increment is well aft on the wing, it also loads the rear spar and lift strut more heavily than they would normally be, while partially unloading the front spar and lift strut.
    JimC

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    Dave, have you had a chance to get any more test data?
    I'd also be interested in seeing differences with 5/8 and 7/8 wickerbills.
    Thanks,
    JimC

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    Dave Calkins's Avatar
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    Jim, thanks for the notes on the theory of this stuff. It is immensely helpful to have you speak specifically about it. Otherwise I'd go on for years trying to pick up a few words about it.

    After testing, I had not reinstalled any of the wickerbills. The owner of the aircraft requested that I re-install them, he liked them alot. The wickerbills have been reinstalled on the inboard portion of the wing, but not the ailerons. I have had the great fortune to fly this airplane any time I like, modify it at my will to optimize slow flight, and use it for backcountry skiing and hunting, the owner is very sharing with all his airplanes, and I benefit by my association with him, for sure. I'll get some solid numbers when time permits.

    DAVE

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    Gurney Flaps

    I don't know if anybody brought this up, but the Cessna Grand Caravan (C-208B) has gurney flaps on its flaps whereas the short body Caravan does not. Apparently, the gurney flaps were required to get the stall speed down within the regulatory maximum allowed for single engine aircraft when Cessna stretched the body and increased the gross weight.

    More validation for a simple device.

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    Speedo's Avatar
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    Quote Originally Posted by JimC
    ......I don't know how they work. Anyone? Circulation theory

    That's exactly how they work. They increase the wing circulation.
    Three things happen.
    1) They slow the air under the wing and move the stagnation line forward while speeding the air over the wing, thus increasing the pressure under the wing while decreasing the pressure over the wing.
    2) They cause the air to depart the trailing edge deflected more downward than the normal departure parallel to the camberline thereby increasing the downward momentum imparted to the air.
    3) They increase the pitching moment of the wing, requiring more tail download.
    JimC
    JimC,

    What are the implications of region of air where these three things happen? For example, if the downward deflection described in #2 were especially strong, could it force the tail down? Or is the tail typically far enough aft that it is not affected by the flow of air over the wing?

    If wickerbills increase the wing pitching moment (see #3), will a plane without wickerbills be able to pitch down more than and identical plane with wickerbills?

    In #2 you mention the change in direction of the momentum of the air. Would it be correct to refer to this in terms of a force vector? Is there some way of estimating the direction and magnitude of the vector given airspeed, aircraft weight, and height and length of the wickerbill? Is this one of those situations where empirical results, say from tuft testing, are the only way to gather the data?

    Sorry if these questions seem incoherent: I think they might be easier to ask if we were standing next to a plane.

    Eric
    Speedo

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    Comments inserted

    ........What are the implications of region of air where these three things happen? For example, if the downward deflection described in #2 were especially strong, could it force the tail down?

    That needs a two-part response.

    In cruise, no. It would be more likely to try to force the tail up. The increased pitching moment will want to force the nose down, requiring more tail download to compensate (more aft stick or nose up trim).

    During short field landings, the wickerbills when in combination with other flow control devices (drooped leading edges, slats, slots, VG's, etc. can contribute to blanking the tail at high angles of attack, thereby limiting short field landing capability by limiting the tail downforce required to achieve high angles of attack (which in a way, could be thought of as forcing the tail up :-)

    ..... Or is the tail typically far enough aft that it is not affected by the flow of air over the wing?

    In cruise, typically yes. During short field landings, sometimes yes, sometimes no.

    ........If wickerbills increase the wing pitching moment (see #3), will a plane without wickerbills be able to pitch down more than and identical plane with wickerbills?

    ??? Both configurations can be easily pitched down enough to do an outside loop and fail the lift struts in compression. I may not be interpreting your question correctly. Would you rephrase it, please? At a given speed and load, a plane with wickerbills will fly more nose low than a plane without, and the wings will also be supporting the additional load from the increased tail download.

    ........In #2 you mention the change in direction of the momentum of the air. Would it be correct to refer to this in terms of a force vector?

    Yes.

    ...... Is there some way of estimating the direction and magnitude of the vector given airspeed, aircraft weight, and height and length of the wickerbill?

    Yes. One of the typical ways would be by means of a software program like XFoil (Mark Drela's free airfoil design and analysis code -- contrary to some of the statements on Google, UNIX isn't reqired to run it -- you download the version suitable for your platform), and then adjusting the output for finite aspect ratio.

    ........ Is this one of those situations where empirical results, say from tuft testing, are the only way to gather the data?

    No. But it is one of those situations where test flights are very useful and well worth doing (thanks, Dave). Also, over the years I've noticed that folks who aren't particularly into aerodynamics and flight mechanics tend to doubt the value of computation and focus instead on pilot reports. Both methods work.
    JimC

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    Speedo's Avatar
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    Quote Originally Posted by JimC
    ........If wickerbills increase the wing pitching moment (see #3), will a plane without wickerbills be able to pitch down more than and identical plane with wickerbills?

    ??? Both configurations can be easily pitched down enough to do an outside loop and fail the lift struts in compression. I may not be interpreting your question correctly. Would you rephrase it, please? At a given speed and load, a plane with wickerbills will fly more nose low than a plane without, and the wings will also be supporting the additional load from the increased tail download.
    JimC
    I'll try again (but no guarantees that I'll make any more sense the 2nd time):

    If I understood your explanation, wickerbills create a pitch up, regardless of airspeed, right? If one puts wickerbills on their plane, could they limit the amount of nose down pitch that can be attained (mimicking the effect of limiting the forward travel of the stick)? Could a pilot find himself wanting to achieve more nose down pitch, but be unable to because of the wickerbills? I hope rephrasing the question made it easier to understand what I'm driving at, but if I'm taking this thread down a bunny trail I'd be happy to take the line questions off line.

    Eric
    Speedo

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    skywagon8a's Avatar
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    This is an interesting discussion of which few of us know anything about. How would deflecting the flaps a degree or two compare to the use of the wickerbills? They both generate a nose down pitching moment. The flaps could be fully retracted at cruise eliminating the extra tail download, while the wickerbills would still generate extra down load on the tail with the resultant reduction in cruise speed.
    N1PA

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    .......If I understood your explanation, wickerbills create a pitch up, regardless of airspeed, right?

    Wrong. They create a pitch down. You have to increase the tail download to compensate.
    JimC

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    .....How would deflecting the flaps a degree or two compare to the use of the wickerbills?

    The deflected flaps would not achieve as high a maximum lift coefficient as the wickerbills. You can land slower with flaps plus wickerbills.
    JimC

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    Quote Originally Posted by JimC View Post
    .......If I understood your explanation, wickerbills create a pitch up, regardless of airspeed, right?

    Wrong. They create a pitch down. You have to increase the tail download to compensate.
    JimC
    Just to add some data points for those who may be using the search function:

    I now have 50+ hours on Wickerbills on a flapless Wag Aero Cubby (20mm depth, 60" wide, not installed on the ailerons). I like them so much I never take them off.

    The above statement is not what I experience at all. In fast cruise, I come close to running out of nose-down trim. The airplane pitches up with them installed, not down.

    With the gurney's removed, the airplane requires minimal and mostly centered trim.

    Your mileage may vary.
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    skywagon8a's Avatar
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    Quote Originally Posted by motosix View Post
    . In fast cruise, I come close to running out of nose-down trim. The airplane pitches up with them installed, not down.

    With the gurney's removed, the airplane requires minimal and mostly centered trim.
    What is the difference in cruise speed with and without the Gurney flaps?
    Does your Wag Cubby have a trim tab or a trimming stabilizer like a real Cub? That could explain why you come close to running out of trim.
    N1PA

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    Quote Originally Posted by skywagon8a View Post
    What is the difference in cruise speed with and without the Gurney flaps?
    Does your Wag Cubby have a trim tab or a trimming stabilizer like a real Cub? That could explain why you come close to running out of trim.
    Jack screw for the tail.

    3 to 4 MPH cruise hit (down to 69-70MPH from 73-74MPH), possibly 3 MPH off my stall.

    The gurney's effect the entire flight envelope both in good and bad ways. The biggest difference is the ability to fly at the edge of the stall much more responsiveness with much lighter controls. I absolutely love them at any speed under 65MPH and can tolerate them the rest of the time. They are annoying as hell when you want to actually get somewhere, but they are great fun when landing/thermaling/ridge-riding, or when you have no where to be...

    Interesting to note that my build is slightly nose heavy when compared to a real J3/11. Lyc O235, +3gear, 29 ABWs, but no header tank and instead dual 13gallon wing tanks. The nose-up pitch is counter intuitive, and possibly there is another dynamic specific to my airplane that others would not experience.
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  35. #35
    Dave Calkins's Avatar
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    Ho motosix.

    Are your elevators counterbalanced or j-3?

    also, what is your empty CG?

    thinking about the grey Cubby with the wickerbills, and balanced tail, and also my current C85 J-3 with unbalanced tail

    thanks

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    Quote Originally Posted by Dave Calkins View Post
    Ho motosix.

    Are your elevators counterbalanced or j-3?

    also, what is your empty CG?

    thinking about the grey Cubby with the wickerbills, and balanced tail, and also my current C85 J-3 with unbalanced tail

    thanks
    Counter-balanced 18 elevators with gap seals (gap seals make a huge difference on this airframe). Standard Wag rudder which is a copy of the J3.

    As far as the FAA knows, I was at the forward limit of a J3 CG although my paperwork was completed using an PA18 worksheet (don't ask, wasn't me...). Empty weight shows 890-something pounds, but I have since removed 50#s worth of starter, ring gear, battery, alternator, etc etc off the end of the nose which made a world of a difference in how she flies. I need to get some scales and do a complete weight and balance now that she sits on 29s with bigger gear, but seat of the pants tells me she is still nose heavy.

    I gave some more thought to your initial comments Dave, and most of my observations are right in line with yours. The thing I utilize the most are the slightly steeper approaches, and while the numbers might not appear drastically different, the feel of the airplane is very much different. My current limit (without a headwind) is the airflow over the tail and its inability to hold the nose up to the angle of attack I really want at touchdown. A quick blast of throttle at the flare is my current fix, but over-sized elevators might help. A bit more weight directly over the tail post might help as well. Maybe even a tiny gurney flap on the upper trailing edge of the elevator?

    A side effect not mentioned in this thread is that gurneys make ground effect feel like a parachute (they amplify the effect). If you even the slightest bit fast hitting ground effect, you will float for e v e r . . .. and ever. And ever.

    A 0290 on 31s would pretty much be the winning ticket for this airframe IMHO. A O320 is within a pound of two of my O235 without all of the accessories, but the fuel burn really would limit the range with my 12 gallon wing tanks.

    FWIW, my normal DAs are 7K on the ground in the summer months and cooling these things is a challenge. That means my performance numbers (outside weights) may not line up well with others.

    -dave
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    Pulled out some notes to get you better data:

    #941 when last weighed, 47# on the tail.

    J3 empty CG is 10.6 to 22.7" using leading edge of wing.

    Mine was 10.08" from the leading edge, empty. (I don't expect that is what it is now, but it is close)
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  38. #38
    skywagon8a's Avatar
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    Quote Originally Posted by motosix View Post
    J3 empty CG is 10.6 to 22.7" using leading edge of wing.
    Empty or Loaded maximum gross CG range??

    The TC A-692 gives 8.1" to 20.7" for an empty CG range.
    N1PA
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  39. #39

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    10.6 number was incorrect apparently. I like yours much better, but my airplane doesn't fly any nicer knowing it...
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  40. #40
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    Very topical for me. I like to test wing mods in isolation to the opposite wing. Please don’t berate me for being dangerous and irresponsible. I fitted vortex generators to the top of one of my flaps. No noticeable change in flight characteristics so I removed them. I then fitted flap gap seals to one wing. A very noticeable lift increase on that wing when the flaps were deployed. So I installed them on both wings. I then fitted a gurney to one flap. I used some double sided tape and some gaffa tape as back up. That wing demonstrated less lift when the flaps were deployed. The joke was on me. Because the flaps are balanced, and the gurney worked, that flap didn’t deploy and the opposite side did. I had a little laugh at myself but will definitely be installing them on both.

    Sky wagon taught me about flying with more aft c o g. Vast improvement. I actually fly around with my tail wheel full of water. I can’t get anymore weight of the nose. But won’t this increased lift aft of the c o g work against my efforts to fly aft c o g?

    cgoldy
    Back Country O-375 wide body extended wing cub
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