The flaps up stall speeds should be used. I used the stall speeds from this Van's webpage, assuming they were flaps up stall speeds for the two different weights listed. Since Van's did not specify the configuration, they could indeed be the flaps down stall speeds.
The reason we like to know corner velocity (Va) is so we can look at the airspeed indicator and determine how much we can “pull on the pull.”
If I look down and see 150 MPH IAS, and my maneuvering speed is 128 MPH IAS, for example, I know that I can pull to 6G’s in my RV-4 if I’m less than aerobatic maximum gross weight and applying G on a single axis (in other words I’m not rolling and pulling at the same time). If I’m rolling AND pulling, then I can only apply 4G’s because even though Van’s doesn’t specify “asymmetric” G limits, I assume structural limits are reduced by 33%, and I don’t want to bend anything!
On the other hand, if I look down and see LESS than maneuvering speed, I can pull as hard as I want (not that this is a good technique, BTW!) and I know the airplane will stall before I hit the structural limit. This accelerated stall can occur at any IAS or attitude if I’m aggressive pulling the stick.
The good news is that if the nose isn’t buried and the airplane isn’t upside down, if you apply 2G’s per second (which is about as hard as you want to pull), the airplane is going to slow down rapidly as the G is applied. This is because of all the induced drag you are generating. This is going to effectively limit the amount of G you can pull. Picture a level turn at about 70-80 degrees of bank that you start at, say, 170 MPH IAS. It’s not likely you will even get to 6G’s because of the rate at which you are “bleeding” airspeed, and even at wide open throttle, you will be below Va/corner quickly.
Where things can get really bad is at high speed. Just at the top of the green arc in my RV-4, I can generate 10.7G’s if I pull really hard and fast—that’s sufficient to cause catastrophic structural failure. This is normal cruising speed in my airplane, so just imagine how easy it would be in an unusual attitude or botched aerobatic maneuver to have LOTS of airspeed.
Hi Alan,
That?s a good question.
Turns out, the critical (stall) angle of attack doesn?t really change with power (thrust)...at 1G, like the power on stalls you practiced as a student, you?ll see a lower indicated airspeed and higher pitch angle at stall with power; but the stall AOA is still the same as it was power-off. That slower observed speed and higher pitch is due to the vertical component of thrust. Because our RV?s have plenty of thrust, you can see some pretty significant pitch angles and difference in stall speeds.
It's interesting how complicated the RV community often makes issues that don't otherwise exist in the general flying community. No aerobatic pilot I've ever known, including myself, looks at their airspeed indicator and does a mental calculation of Va based on their flying weight, then using that information to ensure the structural safety of the airframe when pulling. They just know the G limits of the airplane and know what 4G vs. 6G vs. 8G, etc. feels like. They know how to pull the right G for the speed they are flying through feel and experience. Va is meaningless for aerobatic pilots. If slowing down to Va in strong turbulence makes you feel better, by all means. IMO, it just has little significance to actual "maneuvering" unless you are a very mechanical fly by numbers engineer type. But those types don't make for very good aerobatic pilots.
Some of us acro pilots even push nearly as hard as we pull. For airplanes with asymmetric +/- G load ratings, you think anyone thinks about Va when inverted? Ever seen an aerobatic aircraft designer publish a different set of numbers for Va in the negative G realm? Relax, feel, and fly the airplane. And there is no aerobatic maneuver that calls for fully deflecting the elevator anywhere near Va.
If you said this while sitting among a group of experienced aerobatic pilots enjoying a few beers, well you would probably get a few head nods and "that's right on brother" in response. But that's not the case. Many who are reading this thread have never been inverted and had no idea that a snap roll doesn't require full deflection of the elevator and/or rudder. And they have no idea what 4 or 5 Gs FEELS like. I will strongly take issue with your assertion that "Va is meaningless for aerobatic pilots." The finer point of knowing the exact Va for your given weight may be academic but respecting Va is just as important in my book as respecting Vne or maximum G.
The maneuvering speed is function of clean (no flap) stall speed. For utility category aircraft like the RV-9/9A, it is 2.1 (the square root of 4.4) x stall. For aerobatic category aircraft, it is 2.45 (the square root of 6) x stall.
(ignoring the discussion of the practicality of Va when it comes to maneuvering, and focusing just on the math)
Conclusion: Vans seems to have used full flaps stall speeds to calculate Va for RV-4,6,7, and 8, even though they specifically note that Va is a function of clean stall speed. Have we figured out why the numbers are reported as such?
By my calculation (I AM NOT AN AERONAUTICAL ENGINEER), if Vans actually used their clean stall speeds to calculate Va, it would have resulted in:
RV-4: 58mph x √6 ≈ 142mph
RV-6/6A: 59mph x √6 ≈ 145mph
RV-7/7A/8/8A: 64mph x √6 ≈ 157mph
Note that this is incorrect since the 64 MPH stall speed is at 1800 pounds, where the Design Load Factor is 4.4g and not 6g.
Similarly, the Va speeds calculated for the RV-4 and RV-6/6A are also incorrect.
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If you take the change in stall speed with changing weight into account, and the Aerobatic and Utility Category Limits into account, Flaps-Up Va looks like this:
This is all good stuff, I wondered about the RV-8 and the utility vs aerobatic category for Va. Does anyone know what or if Vb (rough air pen speed) exists for an RV-8? My mountain flying course CFI says to calculate a Vb for the aircraft which he says would be 1.7 Vs.