"Thumping" on aileron during roll

I've noticed this lately. In an aileron roll, rolling pretty fast but not full stick deflection, somewhere around about inverted I feel two "thumps" or tugs on the ailerons in rapid succession. It almost feels like what you get when you override an a/p servo. I have no control binding or anything.

What is this? Is some surface stalling or something?

It's an aileron stall, which also results in a yaw during the roll. It's caused by too high an entry speed into the maneuver. Your entry speed should be below maneuvering speed....

walkman said:

I've noticed this lately. In an aileron roll, rolling pretty fast but not full stick deflection, somewhere around about inverted I feel two "thumps" or tugs on the ailerons in rapid succession. It almost feels like what you get when you override an a/p servo. I have no control binding or anything.

What is this? Is some surface stalling or something?

Click to expand...

Talked about before.. http://www.vansairforce.com/community/showthread.php?t=21686&highlight=Aileron+bumping

Fred.Stucklen said:

Your entry speed should be below maneuvering speed....

Click to expand...

You do not need to limit rolls to Va and below. Van wrote an aerobatic article containing a "How to Roll" section, with the general "getting started" recommendation of 170mph IAS and "firm aileron input; half stick deflection or more". RVs have a long history of doing aileron rolls above Va with full deflection. Not making a recommendation to ignore the technical definition of what Va entails, but I can tell you that never have I known pilots of aerobatic aircraft to limit full aileron deflection to Va and below. You won't be competitive doing that...not that this matters for RV pilots. I know of lots of airframe problems in various aerobatic airplanes resulting from abusive control inputs, but none related to aileron input alone. I wouldn't roll an RV at Vne with full deflection, but a little common sense is in order. It's too easy an answer to say that Va applies to all controls when it's obvious that each control would have a different "Va" airspeed...even though this may not actually be tested by the designer. They don't need to, given that Va airspeed represents the "lowest common denominator", which is elevator usage in most airplanes. Apply full elevator well above Va and I do believe you will bend/break something. Ailerons not so much. I would imagine it would take an airspeed way over Vne to break something with full aileron deflection alone. But that does not mean you cannot reduce the fatigue life of certain wing components if you do full deflection rolls at high speed. But you'll do that doing aerobatics period, compared to an airplane that does nothing but cruise straight and level. RVs have been around a long time, and I would not worry about issues that have not shown themselves to actually be an issue with respect to airframe integrity. Opinion. Nobody would criticize anyone for conservatively limiting full aileron deflection to Va and below. But that doesn't mean that rolls should be done only at Va or less.

Excellent

I thought it might be something like that. Thanks for the link

Just back the aileron off the stops a bit and the bumping will go away.

Just out of curiosity, where can you find Va for the RV's? I just checked the Vans site for the -6, and I couldn't find it. Is it in the builder's manual?

I don't know how to find the Va for an RV-6 but the builder of my RV-6 put a mark on the airspeed indicator at 141 MPH and that seems fairly reasonable for a Va value. I figured he probably did some math to come up with that number.

I've rolled the following RV models: -4, -6 and -8 and they all make the same characteristic "thump-thump" with full aileron deflection in the roll. I just simply accept that as a typical RV behavior and don't pay it much mind... but I don't roll them much anymore because doing acro just makes my tummy queasy the older I get.

Airspeed Markings Document

Neal@F14 said:

I don't know how to find the Va for an RV-6 but the builder of my RV-6 put a mark on the airspeed indicator at 141 MPH and that seems fairly reasonable for a Va value. I figured he probably did some math to come up with that number.

Click to expand...

Here is a useful document from Van's. The info is also published in the construction manual.

Van's Airspeed Indicator Markings Document

141 is higher than the published Va for the 6.

Skylor

Maneuvering Speed

Va is not static, it varies with the gross weight of the airplane. Va is a limit that corresponds to allowable G. The formula to compute Va is: Vs x the square root of the design limit G. For example, if Vs=55 MPH IAS and the symmetric G limit is 6, then Va would be 135 MPH. Keep in mind that IAS associated with stall changes with gross weight (critical angle of attack remains constant). So at lighter weights, Vs DECREASES as does Va.

Another thing to keep in mind is difference between symmetric and asymmetric G. "Symmetric" maneuvering is pulling about a single axis. A loop or split S are examples of maneuvers about a single axis. Assymetric G occurs with simultaneous maneuvering about two or more axis. Any maneuver that requires roll AND pull is "asymmetric." A barrel roll is a familiar example. Asymmetric G is sometimes referred to as "rolling G" but this is a bit confusing since some rolls are symmetric. If, however you are "pulling" and then input aileron (or vice versa), you are maneuvering about two axis. Vans doesn't specify asymmetric G limits but we can look to MILSPEC or FAR 23 for guidance.

MiLSPEC reduces allowable G under asymmetric conditions by 20% and FAR 23 stipulates a 4G asymmetric limit in the aerobatic category. Manufacturers can, of course specify higher limits, but since no limits are specified, the most conservative ESTIMATE would be the 4G FAR 23 specification. So in the first example, Va for an asymmetric maneuver (e.g., snap roll) would be 110 MPH IAS (55 x the square root of 4).

Summary: Va is a fluid number that is affected by gross weight and G allowable. G limits depend on gross weight and whether or not manuevering is symmetric or asymmetric. If you know IAS for stall at varying weights, you can compute IAS for Va based on G limits. G limits are specified by Van's (although asymmetric limits must be estimated).

Fly safe,

Vac

Neal@F14 said:

I've rolled the following RV models: -4, -6 and -8 and they all make the same characteristic "thump-thump" with full aileron deflection in the roll. I just simply accept that as a typical RV behavior and don't pay it much mind...

Click to expand...

I wonder if the trailing edge radius or other factors affect whether this happens or not. I've never experienced this in the four RVs I've rolled with full aileron (my -3, two 6's, and an 8).

i have felt the bumps as well in mine, good question answered

I wonder if the presence/absence of buffeting is closely linked to the control deflection limits. It's possible that another .5 degree is enough to make the stall happen more often?

In any case, if I roll with entry speeds between 140-160 mph I can't feel the buffet at any aileron deflection. Over about 160 for entry and I can make it buffet if I push the aileron to the stop. You really don't need that much deflection to do gentlemanly rolls though, and as such I haven't felt the buffet in quite a while.

sandifer said:

I wonder if the trailing edge radius or other factors affect whether this happens or not. I've never experienced this in the four RVs I've rolled with full aileron (my -3, two 6's, and an 8).

Click to expand...

From that other linked thread, it appears its primarily due to the Friese aileron design, which makes sense...but its an interesting question Eric, especially the "other factors" part. I'd add aileron span to that "other" column. My 6 has clipped wings and slightly shorter ailerons, and the bump is absent or almost imperceptible, and that's throughout a fairly wide entry speed range. In contrast, in a 7 that I helped test fly through Phase 1, it was quite pronounced at full deflection, but went away just off the stops, as others have noted. Need more flights in an 8 to compare, and will try it in a 4 soon. Its a good test mission!

Now in the 1...

...I'm kidding, I did not roll the 1...honest!

Cheers,
Bob