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RV-8/A Stick bumping during aerobatics

B

brendan.bardellini

I'm New Here
Hi Everyone,

This is my first time posting on VAF as I am a new RV-8 owner. I recently purchased an RV-8 with the main purpose of getting into light aerobatics. I have noticed a slight lateral movement (bumping) in the control stick when performing aileron rolls. This happens in both directions. It's mild but I was wondering if anyone could shed light on what might be causing it?
Any input would be appreciated.
 

aileron flutter

Am having a mild aileron flutter during full deflection aileron rolls on an RV8? Has anyone enchanted similar airflow disruption and was there a solution? Cheers, Bob
vansairforce.net vansairforce.net
 
Simply put. Aileron “flutter feeling” in the stick or buffet. Nothing to worry about. If you don’t like it just ease off a little bit. Enjoy! They’re fun aircraft!
 
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That’s normal. Only happens to me with full quick deflection. Wouldn’t call it flutter though as it doesn’t oscillate for me its just one bump. To be honest I am not sure what exactly causes it. If someone with more aerodynamic knowledge then me could explain it that would be great!

Oliver
 
Not only is it normal in RVs, but it's a good thing -- it means you're getting the stick all the way over to the stop when doing aileron rolls.

One of the "common errors" I see when teaching rolls is that the student will not use full control deflection. Which is understandable, because how often do we use full deflection under normal circumstances, right? But insufficient aileron input can make the roll rate sufficiently slow to allow the nose to fall far enough to cause visual loss of the horizon. That's where disorientation can take place and the student can get tempted to split-S out the bottom. That can be especially hazardous in RVs due to rapid speed build up.

--Ron
 
What is going on here is a normal characteristic of Friese ailerons.

The aileron is hinged on the lower surface, at a point about 25% of the way back from the nose of the aileron. This produces a very nice aerodynamic balance that helps reduce aileron actuating forces, AND because the nose of the aileron extends below the wing surface when the aileron is deflected TE-up, the flow around the nose of the aileron produces lower pressure there, which again tends to reduce aileron actuating forces. The nose extending below the wing surface also tends to produce a little more drag, helping to alleviate adverse yaw.

All good. These are all part of why RVs have such nice light aileron feel.

Except when you deflect the aileron to full deflection, and the nose extends so far below the wing surface that the flow can not stay attached all the way around the nose and turn upward on the upward-deflected aileron. So the flow separates. The aileron stalls. This separated flow produces the buffet you feel in the stick.

With some practice, you can find that perfect aileron deflection that gets nearly full effectiveness and doesn't stall - just back off a little from full deflection. Personally, I don't really like 2-second aileron rolls anyway. My aging head has a hard time keeping up. What is wrong with a nice 3-second aileron roll?

If you have an RV that does not do this, it means that the control stops (or something else) are limiting the deflection of the aileron so that it is not deflecting far enough to stall.
 
What Steve said...plus you could also be lacking aileron stops, and the ailerons deflect more than normal. I inspected an 8 two weeks ago with no stops.
 
scsmith said:
The aileron is hinged on the lower surface, at a point about 25% of the way back from the nose of the aileron. This produces a very nice aerodynamic balance that helps reduce aileron actuating forces, AND because the nose of the aileron extends below the wing surface when the aileron is deflected TE-up, the flow around the nose of the aileron produces lower pressure there, which again tends to reduce aileron actuating forces. The nose extending below the wing surface also tends to produce a little more drag, helping to alleviate adverse yaw.
Click to expand...
Great explanation! My visual mind can see that!
scsmith said:
All good. These are all part of why RVs have such nice light aileron feel.
Click to expand...
The BEST feel: responsive without being twitchy/sensitive! One of the many things pilots love about RVs.... 😊
scsmith said:
With some practice, you can find that perfect aileron deflection that gets nearly full effectiveness and doesn't stall - just back off a little from full deflection. Personally, I don't really like 2-second aileron rolls anyway. My aging head has a hard time keeping up. What is wrong with a nice 3-second aileron roll?
Click to expand...
Agreed! I find I can do more 3 second rolls that 2 second rolls as my head tells me: OK; that's enough for today!
scsmith said:
If you have an RV that does not do this, it means that the control stops (or something else) are limiting the deflection of the aileron so that it is not deflecting far enough to stall.
Click to expand...
I don't feel this with SuzieQ and am now wondering why. She has unrestricted aileron deflection until the stops. Could this be type-specific? More prone in the -8, for example? Yes: I would take a peek at the wing to see if the control stops are there....:oops:
 
Brendan,

You wouldn't happen to have an autopilot with automatic envelope protection. Would you?
 
DanH said:
What Steve said...plus you could also be lacking aileron stops, and the ailerons deflect more than normal. I inspected an 8 two weeks ago with no stops.
Click to expand...
I recall that the aileron stops are illustrated in a rather "subtle" way on the plans. If you are not accustomed to pouring over plans, you could easily miss them.
Roadjunkie1 said:
I don't feel this with SuzieQ and am now wondering why. She has unrestricted aileron deflection until the stops. Could this be type-specific? More prone in the -8, for example? Yes: I would take a peek at the wing to see if the control stops are there....:oops:
Click to expand...
It could be that your aileron stops are installed in a way that limits the deflection enough to prevent separation.
Since the -7 and the -8 have essentially identical wings, I would think the same behavior would be felt on both. I know Bob Mills' Rocket wings on his RV-6 didn't seem to do it. I didn't measure his aileron deflections on his old wings though.
 
I agree with most pilots here.. almost every RV I’ve flown seems to “bump” the ailerons when you roll.. I just don’t use full aileron and accept a slower, more gentle roll anyway.
 
scsmith said:
It could be that your aileron stops are installed in a way that limits the deflection enough to prevent separation.
Click to expand...
Or it could be that the 'soft' aileron stops get in the way......also known as my legs.........! 😂 :ROFLMAO: 😂 :ROFLMAO: They are still in the 2-3 second 360 roll area.....😊 (RV grin....) Still feels pretty 'snappy' to me!

I have done feet-on-the-floor (well, let's say no rudder input, feet on the pedals) rolls just to see how well she does. This airplane is such a good design that they were actually pretty decent! Does she do better with a little rudder input? Well, yeah.......but still..........😊😊😊

LOVE this airplane.....but I digress......
 
scsmith said:
What is going on here is a normal characteristic of Friese ailerons.

The aileron is hinged on the lower surface, at a point about 25% of the way back from the nose of the aileron. This produces a very nice aerodynamic balance that helps reduce aileron actuating forces, AND because the nose of the aileron extends below the wing surface when the aileron is deflected TE-up, the flow around the nose of the aileron produces lower pressure there, which again tends to reduce aileron actuating forces. The nose extending below the wing surface also tends to produce a little more drag, helping to alleviate adverse yaw.

All good. These are all part of why RVs have such nice light aileron feel.

Except when you deflect the aileron to full deflection, and the nose extends so far below the wing surface that the flow can not stay attached all the way around the nose and turn upward on the upward-deflected aileron. So the flow separates. The aileron stalls. This separated flow produces the buffet you feel in the stick.

With some practice, you can find that perfect aileron deflection that gets nearly full effectiveness and doesn't stall - just back off a little from full deflection. Personally, I don't really like 2-second aileron rolls anyway. My aging head has a hard time keeping up. What is wrong with a nice 3-second aileron roll?

If you have an RV that does not do this, it means that the control stops (or something else) are limiting the deflection of the aileron so that it is not deflecting far enough to stall.
Click to expand...
Ya.. what he said……
 
rvbuilder2002 said:
Assuming I am remembering correctly, that was the number from Van himself, so you can debate with him if you disagree.
Click to expand...
I'm well aware of roll rate numbers from lots of different manufacturers not exactly reflecting reality, Very common. Actual video will show otherwise.
 
luddite42 said:
More like 3 sec.
Click to expand...
Yeah, I have noticed in the past that you are quite aware... it is not at all clear to me why something like this is so important 🙄
Lets just say it isn't as fast as aircraft purpose built for competitive levels of aerobatics but compared to most other certificated aircraft designed and approved for aerobatics (Citabria, 150 Aerobat, Beech Sundowner😧), the aerobatic RV's have a pretty sporty roll rate.
 
rvbuilder2002 said:
Yeah, I have noticed in the past that you are quite aware... it is not at all clear to me why something like this is so important 🙄
Lets just say it isn't as fast as aircraft purpose built for competitive levels of aerobatics but compared to most other certificated aircraft designed and approved for aerobatics (Citabria, 150 Aerobat, Beech Sundowner😧), the aerobatic RV's have a pretty sporty roll rate.
Click to expand...
Not that important, roll rate accuracy is just a "thing" in the acro community. I've done a million rolls in all the aerobatic RVs except the -14 and know how fast they roll compared to all types of other aerobatic planes. You can't beat them for an all-around mount.
 
luddite42 said:
. I've done a million rolls in all the aerobatic RVs except the -14 and know how fast they roll compared to all types of other aerobatic planes.
Click to expand...
literally, or figuratively? 😉 if you did it in 20 years that would be 136.986 rolls/day. If it was over 30 years it would be 91.324 rolls/day.

good discussion and thanks SC Smith for the explanation. It sure surprised me the first time I rolled my RV4 and came back and did some research immediately to make sure I didn’t have a problem.
 
Well you guys really got me on the technical inaccuracy of a common figure of speech. Surely there's one RV-4 pilot with a GoPro left in the world. ;)
 
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