Aileron add-ons - What is this?

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Tiger Tim

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Further to my post above, here are some pictures of the Active including shots of the rudder TE mod.
That Arrow Active looks like a delightful little machine. Now that trailing edge sharpness has been identified as the culprit in its weird rudder dead band, is there a plan for a more permanent solution?
 

wsimpso1

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are high stick forces safer...... less pio?
Define “safer”..

If stick force is near zero on center, the pilot can not tell where center is from feeling the stick.

If stick force is smaller than friction in the system, the control will not center, even when the pilot let’s go completely.

If the control is non-linear with either force vs response or travel vs response, making small corrections is difficult and usually requires frequent new corrections. In particular, controls that do nothing for the first bit of travel and/or force, then increase above that, are particularly difficult to fly smoothly.

if control force vs response is flat - zero or low force gradient vs response rate - or if the control force vs trimmed airspeed is flat, the pilot will have little feel to judge when inputs are adequate to get a desired output.

Then there is control harmony. Most of us like the forces and responses to be similar In all axes, and there are some who write of a preferred ratio set. If one axis has a dead band and requires a lot of force while another is light and sensitive, that is really difficult to fly well.

All of these can make flying uncomfortable and frustrating. So we do things to make controls work at even tiny inputs, to make the forces reasonable compared to the responses by the airframe, to make the forces increase smoothly and as linearly as possible vs responses, and to put the ratios between them in the desirable range.

In homebuilts, our small surfaces and modest airspeeds can make for too light elevators and rudder. Any aileron can be made too heavy, but they usually take dedicated effort to make too light. And deadbands exist on think control surfaces. So, we tend to do things like slightly thicken the surfaces to give centering feel and raise the forces around zero travel up to human thresholds. Then if control forces over the whole range are too high, we can aero balance surfaces with many devices. Sometimes one control gradient (force on the stick per g) in pitch is too light, and that is easily tuned with a bob weight. Sometimes another control gradient (stick force per unit speed change) is too small, which can be tuned with a down spring. Sometimes the down spring and any slop in the system is then compensated for with a fixed tab or Gurney Flap.

Vaughn Askue‘s book talks about all of these In some detail.

Back to the OP. Safe is partly a function of the forces and responses being harmonious. Landing in a gusting and rolling crosswind is plenty challenging event with good control response and harmony, but imagine it in an airplane where one control is too light, another with a dead band and way heavy. Similarly, flying a precision approach in gusty conditions with dead bands and poor harmony can be a recipe for loss of control…So yeah, having reasonable forces, reasonable gradients, and linear responses is safer. This is not to say heavier is better though.

As to PIO, some folks have found that raising the control gradient (by adjusting gearing that left less stick travel) makes for better control. Notably, Mike Arnold found this to work in his AR-5. Other folks have found that PIO was more easily avoided by reducing stick gradient (Adjusting gearing to give more stick travel) helped. These all can be aggravated by low gradients, high gradients, dead bands, control snatch, inadequate stability, too much stability, etc.

Billski
 
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flitzerpilot

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Hi Tiger,

Yes, the Active Mk.ll is a fabulous aeroplane and one of the fastest pre-war racing biplanes (well sesquiplane actually). The wings fold, which is why the upper wing has dihedral while the lower wing is straight, the narrow-chord lower wings folding against, and slightly under, the fuselage. The anhedral lower centre-section being the anchor point for the undercarriage legs matching the width of the upper c'section on its slightly splayed cabane struts. Fuselage cross-section is of a shapely elliptical contour, and the construction is all metal.

The aeroplane gained a bad reputation in the 1960's and 70's due largely to the forward CG with the installed Gipsy Major and the fact that the elevator crank was installed inverted, so creating much more down elevator than up, which did little for the landing characteristics and the aeroplane was broken on more than one occasion.

Various fixes were tried and only when Desmond Penrose, had the aeroplane completely rebuilt in the mid 1980's were the problems resolved. My late brother, Neil, many time British Aerobatic Champion and European Champion, loved the Active despite of its then faults and his low-level displays were electrifying. The first Active, the Cirrus-powered Mk. 1, had been owned briefly by Alex Henshaw in the 1930's, who was persuaded to wear a parachute (that his father had bought him) on an early flight, which was just as well as the aeroplane caught fire and Alex had to bail out. As he descended the burning Active spiralled around him getting ever closer, but happily passed him by before crashing in an inferno.

The present owner of the much prettier Mk. ll, will be carrying out further tests with the rudder modification and I'm sure he will create a more permanent and less protuberant trailing edge modification in due course. I'll keep the HBA Group informed.

PS. It was always my dream to fly the Active and Des Penrose was kind enough to offer me the chance after he had sampled the Flitzer Z-1 prototype (in 1998) and expressed great enthusiasm for it. However, so many experienced pilots historically had damaged the Active that I felt I had to decline. I was really only current on the Flitzer which is a total pussycat and I could never have forgiven myself if I has as much as scratched it.

Probably my brother's ghost would have had something to say as well! :)
 
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Riggerrob

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Why do Decathlon and Glastar have shovels on ailerons?
Is it to reduce stick forces?

Do any airplanes have similar shovels on rudders or elevators?
 

BJC

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Why do Decathlon and Glastar have shovels on ailerons?
Is it to reduce stick forces?

Do any airplanes have similar shovels on rudders or elevators?
Spades reduce stick force.

I have seen one aircraft with spades on the rudder, but I don’t recall the story behind it.

Elevators would more likely have aero balance from tips forward of the hinge line.

Aileron spades are used on several aerobatic aircraft, including many varieties of the Pitts. Changes to the hinge line chord-wise placement, plus a different shaped aileron leading edge and cove, have replaced the spades.


BJC
 

Pops

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My Falconar F-12 had ailerons and elevator about the same as the RV-4 but the rudder was large and very sensitive with zero brake-out force. Flew it for about 5 years and 750 hours. Flew it with my shoes off. If you touched the rudder enough to feel it you were yawing. With shoes on, you would be getting rudder and not know it from any feel of the pedals. Checked out my neighbor in it. He was having trouble in landing because he always had some rudder in it without knowing. Told him to take his shoes off. Said, he wasn't comfortable with his shoes off, told him if the airplane requires taking your clothes off, you take your clothes off, if you want to fly it. Took his shoes off and no more problems.
Great airplane for spinning, easy in and easy out.
 

wsimpso1

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Why do Decathlon and Glastar have shovels on ailerons?
Is it to reduce stick forces?

Do any airplanes have similar shovels on rudders or elevators?
They are called spades.

They are there primarily to reduce stick forces.

Spades have the neat feature of being easily unbolted and replaced with different shaped spades so that the break out force and the stick gradients can both be adjusted. Some aerobatic pilots will play with different shaped spades until they get it all working exactly the way they want. Others stay with the factory shape and just learn how to ”fly the hell out of it” (Bill Stein, aerobatic performer).

Billski
 

wsimpso1

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I have seen one aircraft with spades on the rudder, but I don’t recall the story behind it.
Given the huge strength difference between legs and arms, I would normally not see much need for spades on the rudder. Perhaps the owner flew It with hand control for rudder. Maybe the builder put in really stiff centering springs as a fix for inadequate yaw stability or flutter, then needed spades to get control gradients down. Ah well…
Elevators would more likely have aero balance from tips forward of the hinge line.

Second that. Spades are draggy. Center hinged or edge hinged rudders and elevators have a bunch of possibilities for tuning breakout force, gradient, etc. Come to mind are aero balance horns, bob weights, down springs, centering springs, trim tabs adjustable for amount of servo or anti-servo, etc. Bias springs and bob weights might not apply to rudders unless maybe when edge hinged. With all of these options that each add almost zero drag, why use spades?

Now ailerons, I understand needing to lighten them. We are about at our weakest pushing the stick sideways or pushing down/lifting up on the yoke. Even with hinges shifted aft, forward areas pushed into the airstream, etc, I can see how we might still go for spades to finish the tuning. Tuning by moving the hinge position is a big tear up that might make a couple little spades look desirable.

Billski
 

BJC

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The current aileron technology in aerobatic aircraft uses a cove that is not a circular arc, nor is the leading edge of the aileron. The geometry allows high pressure air to act on the leading edge of the aileron when deflected. Some air bleeds past the LE at small deflections, but the gap gets smaller as the deflection increases, thus applying more pressure to the LE. at higher deflections. Spades add too much drag.

If I can find a photo, I'll post it.


BJC
 

WINGITIS

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Still a Gurney Flap, but with it top and bottom. I am figuring the ailerons either had a dead band or the stick had no on-center feel. These things can happen because the boundary layer is pretty thick back there, and wiggling the ailerons inside the boundary layer either does not change wing lift or is below our threshold to feel it. Make the trailing edges a little thicker and/or put on a Gurney Flap top and bottom, and the ailerons become effective at lower stick travel and tend to want to stay on center. It feels better to the pilot.

On that airplane, it appears to be a little fiberglass piece glued to the surface.

If you knew you needed one when you were building, you would modify the foil to taper to a thicker trailing edge instead of to a thin edge.

Dan Gurney was a very successful race driver, car constructor, and entrant. Won LeMans and Grand Prix of Belgium one week apart in 1967. He was also exceptionally tall as international race drivers go. Huge number of accomplishments in there, but now we know him for:
  • Gurney Plate - A usually small vertical plate or lip on the trailing edge of an airfoil or car body. First applied to a foil where they wanted more downforce but they could not add chord or span and they already had the foil built. Bend an angle on a strip of aluminum and rivet it to the trailing edge;
  • Gurney Bump - A bulged spot on the roof of a car to fit a seated tall driver within the car. These were usually a fiberglass teardrop with a flange, cut out the roof and riveted in.

Billski
I guess Gurney may have been the first to put a right angle "GURNEY LIP" onto a racecar wing but others before him invented the flap!
 

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  • PERHAPS WHERE GURNEY GOT THE IDEA FROM.png
    PERHAPS WHERE GURNEY GOT THE IDEA FROM.png
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D Hillberg

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In this case, he thought the controls felt heavy versus the other Velocity aircraft he has flown (I think this is his 5th). I suggested he trim them back a little at a time to get the control feel he wants.

Update: He informed me he wacked them off and is now happy how it flies.
Got tired of snagging his fingers every preflight.....
 

Jonny o

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Riggerrob said:"
Conclusion, for structural and handling reasons, you are often better off with thick trailing edges."

Au contraire, mon capitan! Best scenario (theoretically) is knife edge trailing edge as rounded trailing edges can generate the the circular flow described in the Kutta Condition discussion.

“Best”at what Reynolds number?
What is beneficial at slow speed can work the opposite at higher speeds.

Saying a modification:
Has more drag.
Increases control force.
Increases control authority.
Increases lift and etc. are helpful.
 

wsimpso1

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I guess Gurney may have been the first to put a right angle "GURNEY LIP" onto a racecar wing but others before him invented the flap!
Wings, spoilers, etc did not show up in volume in race cars until the late 1960's. Yeah, like 30 years after various lips on airfoils were patented. And in racecars, Dan Gurney was credited.

Now if you want to call it Zaparka lip, more power to you. Personally, I think that name should be reserved for movable trailing edge lips, not the fixed ones we are discussing here.

Was there a patent or other crediting for a fixed lip?

Billski
 

WINGITIS

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I am not sure whether Dan Gurney got a patent or not, given previous literature he would have been unlikely to have it awarded.

What I meant was that its not a FLAP he invented its a LIP, there is heaps of documentation on flaps at right angles creating a vortex from the thirties...

The ZAP FLAP is the Zaparka version.

Here is another link from a 1933 NACA document covering a German paper from 1932:


So where Dan got the information is not known, but he did use it in LIP format, hence a "GURNEY LIP" which is what it is known as in some places....

According to WIKI, Liebeck had named it the "GURNEY FLAP" later on.

As well as NASA were calling it a Gurney flap in 1988.....


It does generate more lift or down-force if inverted, but at that EXTRA lift it has a bigger CD and a lot more drag, not an issue on a powerful race car.

But various tests also show that at the equal lift point it does have a bit less drag at some AOA....

Its affect on the CM curve are another issue...which does not affect race car applications.

So one can see why it would be used as a trim tab on a plane or helicopter blade, do not expect to see one on the full semi-span of a wing on an aircraft.

At that point a better airfoil would be the answer...


Having a tiny flap for trim that is wide but can be adjusted as per ZAPARKA could be useful and others have looked at that including a study on PLASMA actuation of the same affect(ATTACHED)

Also attached is a NASA paper from 1988, that covers the topic.
 

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  • 19890004024 GURNEY LIP TEST IN A WATER TANK GOOD.pdf
    6.1 MB · Views: 1
  • flow-control-over-an-airfoil-using-virtual-gurney-flaps.pdf
    6 MB · Views: 1

WINGITIS

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A couple of recent patents..
 

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  • US10829197 2018 GURNEY PATENT.pdf
    840.7 KB · Views: 1
  • US20170355445A1 2017 GURNEY PATENT.pdf
    651 KB · Views: 1
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