Right Aileron Roll "Bump"

I have noticed a slight "bump" of "pressure" on the stick when executing an aileron roll to the right. It usually occurs half way or so around the roll. It feels like the stick is "bumping" into something soft. Of course, the stick is fully deflected and not moving. I slight release of side pressure and it does not bump.
It will not do this upon full deflection unless held, like in a roll. It does not do this on left hand rolls.

Any ideas?

JonJay said:

I have noticed a slight "bump" of "pressure" on the stick when executing an aileron roll to the right. It usually occurs half way or so around the roll. It feels like the stick is "bumping" into something soft. Of course, the stick is fully deflected and not moving. I slight release of side pressure and it does not bump.
It will not do this upon full deflection unless held, like in a roll. It does not do this on left hand rolls.

Any ideas?

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Typically RV's have an aileron buffet when they are deflected more than a certain amount. I haven't heard of one doing it in only one direction though.

My cure is to use less deflection. It doesn't seem to affect the roll rate very much.

Yup...

I get that effect rolling in either direction. I think Van's refers to it as "aileron snatch". If I back off the deflection a bit it goes away, and does not seem to affect roll rate appreciably.

JW

This usually happens with a full deflection roll. Use about 1/2 aileron or a little more and it doesn't happen.

You guys should see (feel) what my Midget Mustang does with a full-deflection roll. I get full-blown, undeniable aileron buffeting. The only thing I had ever rolled before was my old RV-3 and a Citabria (both with almost no buffeting). Got my attention the first time it happened!

Rod Woodard
Loveland, Colorado (KFNL)
Midget Mustang--fly [again] by Spring
RV-8 #81964--fly by Spring 20xx
www.mustangflyer.com

Aileron Snatch - but not to worry

Every RV I have flown exhibits this in full deflection rolling maneuvers. If you back off just slightly from full deflection it is no longer detectable. Here's an interesting snip that I found in a Sport Aviation article about the -7 with a confirmation on Van's position on this:

"Like its predecessors, during full-stick displacement rolls the -7 has a bit of an aileron snatch. You feel a small pulse or two in the stick about halfway through an aileron roll when using more than about half-lateral-stick displacement. Van believes this is caused by flow separation at the aileron leading edge.
There is no perceptible change in roll rate, which averages approximately 115 deg/sec, and you don?t feel the aileron snatch through the airframe. Don?t like the snatch? You can avoid it by using smaller lateral stick displacements, because less than half-stick displacement gives you most of the RV-7?s roll rate. If you do use full-stick to roll the airplane-and you know you will-the force required is approximately 20 pounds."

Friese ailerons

Just what others have said........use less aileron. My Air Tractor has huge ailerons and we spend all day making turnarounds. On occasion, when I get in a hurry and use a lot of aileron, I get a big snatch that makes the aileron fully deflect! Boy, that'll get your attention when you're down near the trees.... I think it's a typical Friese aileron characteristic.

Regards,

I wonder if there isn't some set of little vortex generators that could be attached to the leading edge of the aileron that could help the airflow stay attached to the aileron. Kind of like those low speed kits for Cessna and Piper.

inherent characteristics

Rivethead said:

I wonder if there isn't some set of little vortex generators that could be attached to the leading edge of the aileron that could help the airflow stay attached to the aileron. Kind of like those low speed kits for Cessna and Piper.

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Ditto on its common and not critical. Yea people (including Van) have written about it. Don't have time to look it up. Bottom line its inherent characteristics and the solution is less stick deflection in a roll. May be someone has the past "historical records" on this. Common and no big deal.

VG's could be glued on like a porcupine; not sure that would help but the trade offs of VG's for this are probably not worth it (extra high speed drag).

I know (as most do) the shape of the trailing edge radius is critical (too large radius bad, so check against the plans). Some RV'ers have put RV-9 style square extruded trailing edge ailerons on other models of RV's (RV-4, RV-6). There's some benefit with a square aileron trailing-edge; it's reported to be a little faster (less drag), but stick forces do go up a little. Not sure or recall the affect of this mod on the aileron buffet issue. Got to go, but its no big deal. The cure (if any) is probably worse than the symptom is my guess.

N908RV said:

Here's an interesting snip that I found in a Sport Aviation article about the -7 with a confirmation on Van's position on this:

"Like its predecessors, during full-stick displacement rolls the -7 has a bit of an aileron snatch. You feel a small pulse or two in the stick about halfway through an aileron roll when using more than about half-lateral-stick displacement. Van believes this is caused by flow separation at the aileron leading edge."

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To avoid terminology confusion, I must note that "aileron snatch" properly refers to a condition where the aerodynamic forces on the aileron will cause it to go to full deflection without any pilot input. In typical cases of aileron snatch, the aileron control forces will be normal until some critical aileron deflection is reached, then the aileron control force suddenly changes sign, and the stick will be snatched from the pilot's hand as the ailerons suddenly go to full deflection all by themselves. This is a very nasty characteristic.

RVs do not have aileron snatch unless the ailerons have been modified in some way not intended by Van. They may have some flow separation at high aileron deflection, and this disorderly flow could be felt through the control stick.

Search RVator

There is a "fix" if you want to call it that. Search the back issues of the RVator and there is a small article about it, sorry don't remember which. The effect was eliminated when a very small stall strip was installed on the leading edge of the aileron. It was surmised that the flow was seperating at max deflection. I don't remember any negatives, since the strip was out of the airflow at anything less than spprox 50% deflection. If my memory serves, it was just deemed to much work for really no appreciable gain since max deflection made no real effect on roll rate.

Thanks for all the input.

I appreciate as always the knowledge of the people on these posts. Glad to hear that it is normal. I do not intend to do anything about it as it is not a big deal at all to me and frankly it is a very minor bump, never anything scary.

Aileron serperation

To continue this Aileron separation thread during rolls - Van has experimented with vortex generators, mini ?stall strips" and other things according to the RVaitor article. I am seeing it on our RV and it is a noticeable annoyance.
So my question is ? has anyone tried a fix for this issue on there RV???
Some small ammount of flap tape or something?
Anybody?

Thanks

Happens with my RV-8 as well

Jon,

I have the same kind of airflow separation with fully deflected ailerons during rolls with my RV-8. It is a common occurrence with RV's. I just back off a bit on aileron deflection and it is not an issue.

Dan Miller

Enjoy the feel

Normal characteristics and all the cures are worse than the symptom. RV's handle well because they have lots of authority, light controls and fair well harmonized pitch/roll and yaw. Perfect? Far from it but one of the best flying planes I have ever flown. When you get into big stuff its all hydraulics, feel computers, stick shakers, yaw dampers and and ratio changers controlled by ADC (air data computer).

I love it when the plane talks through the seat or stick or feet.

Just another point to consider...no one should be using full aileron deflection unless you are at or below the published manuvering speed for your RV model.
maneuvering speed is the max. speed for full control deflection in all axis, not just pitch.

The flow separation still occurs at slower airspeeds but not as much.

Main point is that if doing rolls at higher than maneuvering speed (135 MPH IAS for the aerobatic RV's, if I remember correctly) you should not be using full aileron input, so you wont get the flow separation bumping.

Just one more data point, yesterday I noticed this "bump" on my -9 for the first time.

Since I have a different wing and aileron than the short wing RV's I thought I might be immune but that wasn't the case.

It wasn't real noticeable but it was there.

JonJay said:

I appreciate as always the knowledge of the people on these posts. Glad to hear that it is normal.

Click to expand...

You got that right. I just noticed the same bump a few days ago. Normally, I do not use full deflection for aerobatics and this was the difference. I was concerned as to what it was, now I know.

Once again VAF has turned a "concern" into a "non-issue". I even went so far as to do an inspection of the entire aileron mechanism looking for something, anything, and found nothing.

Thanks guys.

rvbuilder2002 said:

Just another point to consider...no one should be using full aileron deflection unless you are at or below the published manuvering speed for your RV model.
maneuvering speed is the max. speed for full control deflection in all axis, not just pitch.

The flow separation still occurs at slower airspeeds but not as much.

Main point is that if doing rolls at higher than maneuvering speed (135 MPH IAS for the aerobatic RV's, if I remember correctly) you should not be using full aileron input, so you wont get the flow separation bumping.

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Ok - You know I trust you completely Scott. I thought Va was a certification primarily for elevator deflection and potential separation or failure due to wing loading or elevator loading prior to stall. I was not aware that all control inputs where limited or even tested for as a roll does not lead to stall,theoretically.
In your, off the record, opinion, can full deflection of aileron possibly cause a failure of anything if below Vno? It just does not seem that there would be enough force, strictly by feel, where as it is most obvious when you yank on the elevator. Anyway, I am sure your statement is a surprise to many of us who have done rolls at +-160mph indicated, full aileron deflection.

JonJay said:

Ok - You know I trust you completely Scott. I thought Va was a certification primarily for elevator deflection and potential separation or failure due to wing loading or elevator loading prior to stall. I was not aware that all control inputs where limited or even tested for as a roll does not lead to stall,theoretically.
In your, off the record, opinion, can full deflection of aileron possibly cause a failure of anything if below Vno? It just does not seem that there would be enough force, strictly by feel, where as it is most obvious when you yank on the elevator. Anyway, I am sure your statement is a surprise to many of us who have done rolls at +-160mph indicated, full aileron deflection.

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I know you're talking aileron, but I think I'd be careful using stick force as an indicator of airframe loading. One example is the tandem RV with a backseat passenger/aft CG. The longitudinal stability approaches neutral, and almost seems to diverge with pitch deflection (in the -8).

As for 160+ mph full aileron deflection, that's beyond my comfort limit. And I consider myself a moderately aggressive (acro) RV pilot.

2 cents

Aileron deflection adds load on the wings just as elevator deflection does - but it results in a net torque around the fuselage producing a roll rate rather than a climb. The total wing load of lift from angle of attack plus lift from aileron deflection, plus parasitic and induced drag must be considered.

Sure, makes sense, but...

airguy said:

Aileron deflection adds load on the wings just as elevator deflection does - but it results in a net torque around the fuselage producing a roll rate rather than a climb. The total wing load of lift from angle of attack plus lift from aileron deflection, plus parasitic and induced drag must be considered.

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...is it part of the limiting factors tested when manufacturers publish a Va?

Depends on the manufacturer, I would assume. I have heard from many aeronautical engineers on this, with opinions on both sides. Some say yes, some say no, they all believe they are correct. Only Van himself could tell you for sure for the RV.

JonJay said:

...is it part of the limiting factors tested when manufacturers publish a Va?

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For a type-certificated aircraft, the designer is required to ensure the structure is strong enough to handle full deflection of any of the controls at speeds up to Va. There is no guarantee of structural integrity if you apply full aileron, or full rudder at speeds greater than Va.

For amateur-built aircraft, there are no regulatory structural design requirements, so we cannot make any assumptions about what strength the design may have. We can only go by what the designer publishes. What does Van say about Va in the Builder's Manual (I'm on the road, many thousand miles away from my Builder's Manual).

Per the Manual - RV6

Kevin Horton said:

For a type-certificated aircraft, the designer is required to ensure the structure is strong enough to handle full deflection of any of the controls at speeds up to Va. There is no guarantee of structural integrity if you apply full aileron, or full rudder at speeds greater than Va.

For amateur-built aircraft, there are no regulatory structural design requirements, so we cannot make any assumptions about what strength the design may have. We can only go by what the designer publishes. What does Van say about Va in the Builder's Manual (I'm on the road, many thousand miles away from my Builder's Manual).

Click to expand...

"...137mph full control can be applied. Speed at which full elevator would cause a 6G load. Calculated by multiplying clean stall speed by 2.45 (the sq. root of 6)."
Judging by this statement, I would doubt any tests beyond G loading the wing have actually been done.

The manual provides Aerobatic Entry Speeds, but also re-explains that these are "entry" and "full and abrupt controls should not be done above Va". Entry speeds for Aileron Rolls and Barrell Rolls 120-190 mph.

So, I guess I can Roll at 190mph, but only deflect my Ailerons 95%. I am of course being sarcastic, but I dont think my wings will fall off, or Ailerons buckle at 160mph and full deflection, at least they havent, or I would not be writing this. Now, we have learned you dont, nor necessarily want full deflection for an Aileron roll, so , that point is mute.

Really doesn't seem to be needed

I haven't timed it, but subjectively, that last 1/4 or so of aileron seems to do little for roll rate. As soon as I found the aileron bump I used decreased control deflections and the rolls seemed just as crisp and felt better. Does anyone feel like they need full deflection?

Friese Ailerons

The Friese aileron is a VERY old design. You'll find it on BT-13's, T-6's and DC-3's. It was used by engineers in the 1930s, and was pretty quickly replaced by better designs.

The Friese aileron design seeks to negate adverse yaw by adding drag to the inside wing in a turn. This is done with an offset hinge and gapped nose. The offset hinge geometry causes the nose of the aileron going TE up to stick down into the airflow, adding drag to the inside of the turn. The radius of that protruding nose is sharp, and tends to cause flow separation at moderate angles of deflection. This is what we feel in a high deflection roll. These ailerons are also relatively draggy.

Airframe loads: The big deal here is twist on the wing. Ailerons when deflected cause large changes in pitching moment on the wing. These torsional loads must be carried into the fuselage where they are reacted by the couple between the main spar and the rear spar attachment points.

Also, the RV wings are fat and stubby which helps in torsional stiffness since we get a large spar box (main spar, ribs and rear spar making a box). This box, when skinned, carries the torsional loads spanwise on the wing. We're pretty stiff here. This also helps control structural flutter.

The best fix to this problem would be to get rid of the 75 year old aileron design and replace it with an elliptic nose type of control surface commonly found on modern aircraft.

Aileron bump

The Design of the aileron keeps us in a relatively coordinated turn. Think of the "bump" as the airplane talking back to you...."Hey, you don't need that much aileron at this airspeed". The flight control surface is "dumping air" that is in front of the down wing aileron. It is very normal! Notice at slow speeds,( nose high and slow) you do not feel it when you roll on your back. There is not as much air to "trap" in front of the control surface. It is a good design.

"easy fix"

Bill Wightman said:

The best fix to this problem would be to get rid of the 75 year old aileron design and replace it with an elliptic nose type of control surface commonly found on modern aircraft.

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"for any design, everything is a compromise"
Agreed that this may be good fix for this problem. But the off-set hinge also provides a forward pivot point where a static balance can be added to limit flutter dynamics and add that light control feel- we all enjoy so well.
But, as I posted earlier, That Van has already found the simple fix in his article of aileron separation in "24 years of RVator".
I also roll to the" bump" then back off slightly where the bump separation is depleted. As for most maneuvers in a RV, the loads are pilot limited.
So Van?s explanation is that the separation bump comes with the approach of roll rotation speed dynamics of the upward deflected aileron.

Please follow my thinking: So to take this further- lets just say that you would agree that the separation bump is a function of roll rate with high entry speed. As we approach the ?bump? the acceleration twisting loads are increasing to equilibrium while the aileron deflection point is stable- the wing is experiencing twisting rotation loads - then here comes the separation bumping: I am thinking the bump force adds vibration characteristics at a time of twisting loads to the wing: Not a good thing

The RV 6 series wing is designed to take 6 G?s positive (plus a 50% safety factor.) That is around 9 G?s at positive and 4.5 negative before something gives? So this is an ?approximate? function of strength to twisting loads that the wing can handle also.
So I?m thinking here that what the separation bump is simply telling me that the aileron airflow is separating at this rotation speed, That the bumping is a vibration dynamic to avoid and really not anything about the true loads of twisting force that I should avoid. My conclusion is that one dynamic is not related to the other. And the vibration could be a more of a problem-
So if this postulation is correct:
I?m back to the original question, why not cure the ?bumping? with vortex generators "IF" the wing can handle the full roll rate twisting forces?

So to take this further- lets just say that you would agree that the separation bump is a function of roll rate with high entry speed.

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I would not agree with that assumption. The "bump" is caused by flow separation on the lower surface of the aileron that goes TE UP. Its not trapping air in front of it either. It is flow separation caused by exposing a very tight radius to the free stream (the water pipe Van uses as a counterbalance in the nose of the aileron is the radius I refer to here). That separated flow is very unstable and unsteady and is what you feel in the stick.

Roll rate is what we get due to the differential lift on the wings. It is opposed by something called "roll damping". That is a fancy way to refer to the counter-forces built up on the wings as roll rate increases. The up-going wing has its angle of attack reduced due to the rolling maneuver. The down-going wing has its AOA increased in like manner. These changes in AOA drive lift forces for each wing in the opposite direction of the roll. That is what forces a roll rate to stabilize at a certain point. Roll damping. And a rectangle wing planform has lots of it, requiring bigger ailerons for a given roll rate.

The twist I mentioned before is spanwise twist. Its twist like if you took the wingtip in your hands and tried to force its leading edge up and trailing edge down. This kind of twist produces diagonal wrinkles in the skin on the wing. This has no relation to accelerated rolling motion. Its analyzed as a steady state load. Roll and/or yaw acceleration is a player for secondary structural loads such as tip tanks, under wing stores, etc.

he RV 6 series wing is designed to take 6 G?s positive (plus a 50% safety factor.) That is around 9 G?s at positive and 4.5 negative before something gives? So this is an ?approximate? function of strength to twisting loads that the wing can handle also.

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Bending strength has little relation to twisting strength, Brad. In a traditional structure like ours, bending loads are carried by the main wing spar. Spar caps and shear web do that job.

Wing twist, or torsion, is carried by what is called the "wing box" or "spar box". That is the "box" defined by the four "walls": Main spar, rear spar, ribs (2), and wing skins (top and bottom). There are thus a series of "boxes" in each wing. Without the skin, these boxes could not carry torsional loads. After you skin the wing, it becomes stiff in torsion.

Deflecting the ailerons loads the wing in torsion. The torsional loads are carried by the wing box into the fuselage where they are reacted by the main and rear spar. Note your rear spar attach is literally a hinge: a single bolt! That's because it is by design unable to carry any bending (that is all forced into the main spar). The rear spar carries only shear loads for reacting drag and torsion forces. Thats it.

The reason I explained all of this is to give you more insight into how wings are basically designed and how they work structurally. Bending and torsional strength are designed to be basically exclusive of one and other.

On a happier note, you should also know that our aileron design -although quite old - is safe. The bumping you took note of is not a safety problem. However, I have no good "bandaid" fix for it.

Bill Wightman said:

The Friese aileron is a VERY old design. You'll find it on BT-13's, T-6's and DC-3's. It was used by engineers in the 1930s, and was pretty quickly replaced by better designs.

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It may be an old design, but it seems to be used on quite a few modern aircraft. Cessna uses them, so does the Marchetti SF260, Glastar, the Russian radial Pitt's M-12.........................and I'm 99% sure that the AirForce's T6-A Texan II has them also. Reading various literature, it seems that many designers still prefer the frise aileron.

I stand to be corrected, but this is derived from what I've read or seen.

L.Adamson --- RV6A

Mr Adamson, Beech (T-6 II) makes some of the nicest flying ailerons in the business. Although control surface design is sort of a black art, the Beech surfaces are the offset "elliptic nose" (my language) type. None of them have hinge axes hanging below the bottom of the wing.

Take a look at a T-6II and see if there is an aileron hinge hanging down below the underside of the wing. If there is, it *might* be a Friese. If not, then its probably the same design Beech has used since day-1. If I get around one soon, I'll take a look.

Cessna too?? I haven't seen that yet. None of the Cessna singles use them, nor do the 300 or 400 series twins. Maybe the AgCat??

Never the less, you are correct in stating that a few "modern" aircraft use the Friese aileron. Not many. I still wonder why anyone who knows aileron design would use outdated technology when there's been 70+ years of better ideas.

Bill Wightman said:

Never the less, you are correct in stating that a few "modern" aircraft use the Friese aileron. Not many. I still wonder why anyone who knows aileron design would use outdated technology when there's been 70+ years of better ideas.

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I suppose it's the same reason, that most modern airplanes still have the main wings up front, and the little ones in the back...

But..... regarding Cessna's. I read that when the 206 went into production again, that the flap length was increased, which removed a bit of aileron surface. Cessna then incorporated the frise aileron to get a little more "umph" without heavier control feel. However, this is something I read. I have not looked.

L.Adamson

Piper uses friese ailerons

My 74 Piper Warrior uses friese ailerons where most other Pipers I have looked at do not.