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RV-8/-8A and RV-7/-7A Maneuvering Speeds (Va)

A suggestion...

Carl,

I'll not attempt to argue with your figures as I am no aeronautical engineer. However, if you think that Van's maneuvering speeds are incorrect I would hope that you will contact Dick VanG. We may all benefit from having both of you address this issue together. I am sending you Van's email via PM.
 
Wow, that quite a difference, I think I'll lower my Va to something less than the current 124 Kts (down to about 105) until this gets cleared up!
 
Based on the formulas above, it appears the RV-14 Va's are calculated
using the clean stall speed, Vs, not Vso as is the case with the -8/8a.
Regardless of aerobatic or utility load limit, which stall speed should
be used, Vs or Vso?
 
FAR 23.335 - Design Airspeeds

Clean stalling speed V sub s.

If the published stall speeds are flaps down and the calculations for maneuvering speed(s) by the OP are using those speeds, then I believe the Va speeds being calculated above are suspect.
 
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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.

In the case of the -8/8a it appears to me Van's used the gross weight approximate Vso of 58 mph and the Acro load limit to calculate the
Va of 142 mph. Not the clean stall speed (Vs approximate 64 mph)
and the utility load limit.

Regardless, your post has me recalculating the Va of my -8A. Thank you
for bringing this to our attention.
 
A related question for the aeronautical engineers: what about the effects of engine power on stall speed? As I understand it Vs and Vso are both reported at idle power. Power-on stall speeds are lower however. For cruise flight at normal power settings shouldn't the power-on stall speed be used to calculate Va?
 
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.

Maneuvering speed is the part of the flight envelope where the G limit intersects with the aerodynamic limit. Aerodynamic limit is just a technical expression for stall. Because this intersection is in the corner of the flight envelope, maneuvering speed is also called “corner velocity.”

Va/corner is computed by multiplying the stall speed by the square root of the G limit. As Carl pointed out, it’s based on Calibrated airspeed; and not all of us have accurate airspeed correction charts for our RV’s. With a traditional pitot/static system, indicated airspeed error increases as you approach critical AOA so when you multiply the IAS at stall for your airplane by the square root of the G limit, you are estimating Va as accurately as you can with the information you have available.

The important thing to note is that Va isn’t a fixed value: it varies with weight and G limits.

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.

Fly Safe,

Vac
 
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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.

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.
 
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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.

Thanks Vac!

If I'm following this correctly, the vertical component of thrust from the prop decreases the amount of required lift from the wing by same amount (level, 1G flight), so you can fly slower at the same angle of attack? Whereas in cruise flight the vertical component of thrust is negligible.
 
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.

One thing we both agree on: your first sentence!
 
To me this is all a little back to front. Va is a number generated from formulae specified in the certification standards, FAR23, CS23, etc. During the design stage it is one of the parameters that informs the design team of the loads the aircraft should be designed to withstand. For a certified aircraft (in general) the designer must demonstrate to the certifying authority (eg FAA) the requirements of the certification standard are met. For an experimental most designers use FAR23 to guide their design.

Once in service this is all academic, particularly for an experimental. Va is the speed at which the aircraft should just be able to generate enough lift to reach the max g limit. When the ASI of an RV-7 says 142mph will the wing stall at 6g? Seems unlikely for a whole host of real world factors. Who could predict what the stall load factor would be, might be higher, might be lower.

When flying in severe turbulence should I restrict the speed to Va - seems like a wise move, but what is severe turbulence? How do I tell when I'm about to encounter it?

When flying at above Va should I limit control inputs to less than full deflection available? Who would think full control (particularly pitch or yaw) is a good idea at Va? In a fully aerobatic aircraft perhaps, in an RV probably an unwise choice. When above Va should I avoid a combination of control inputs, yes - again seems like a wise choice. Is Va a magic number? No, of course not.

The real takeaway is perhaps that Va is well below the typical cruising speed of any RV, and so care must be taken with abrupt control inputs.

Pete
 
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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.

Maybe I should clarify - I agree all pilots should know and respect Va and not fully deflect anything above that speed. I'm just saying the realities of learning and flying aerobatics leave Va with little practical value. You mention snap rolls. Nearly all aerobatic aircraft have a Va speed that is significantly higher than its recommended max snap speed, including RVs. Nobody should assume Va is OK for doing snaps even if they think snaps are done by fully deflecting the elevator, which they are not. Good instruction and self education takes care of that issue. You can't fix pilots who don't take that approach.

Regarding not knowing what 4 or 5G feels like, this is all part of the learning process. But aerobatic instructors do not teach as if, "OK now we are below Va and can try a maneuver". Not much can be accomplished at Va with two up in most aerobatic trainers. You quickly learn what G feels like and how hard to pull. G meters certainly help with this. If someone wants to get a feel for a new airplane by first starting below Va before pulling, then I can't fault anyone for that. It's just not the way aerobatics is generally taught, and you learn so quickly in one flight how to handle the controls properly, that's it's not really an issue. And anyone with even the most rudimentary level of aerobatic experience has no real reason to reference Va for maneuvering, since as I mentioned before, there is no maneuver that calls for anywhere near full elevator deflection near Va. Snaps are the closest thing and I've already mentioned that snap speed and Va don't correlate.

Just trying to ensure the RV crowd doesn't overcomplicate the acro flying...I mean they are already busy looking at their AOA every time they pull. ;)
 
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Variable Limitations

Hi Luddite!

Since we’ve still got folks losing control of airplanes and removing structural parts due to handling error, I think there may be some benefit in discussing these topics and glad Carl brought it up. For folks so inclined, learning to fly aerobatics is the single best way to explore the flight envelope; so it sounds like other than semantics, we’re in violent agreement :)

Every fighter pilot I know, including myself, knows what their speed is before applying G, and makes a mental calculation of how much G to apply after the lift vector is either set for a straight pull, or what my G limit is if I’m rolling and applying G. I even know where my speed is relative to corner velocity (a much more practical application of Va). Van is adamant about “smooth application of G” as am I, I just quantify it by onset rate. All that may sound complicated at first blush, but as you wisely state, after you develop a feel for it; it becomes second nature. It’s actually just a different way to think about handling than you might be used to, and might help one or two folks conceptulize the concepts. Physics is physics and I’ll always defer to the right answer or a better way to describe things!

RV’s are different from other many civilian types insofar as G limits, Vne and Va actually vary. In many cases, these variations can be designed out of an airplane (or operational restrictions can be placed on it); but since our community likes speed, our airplanes have a wide speed band, low drag, are each a bit different, and don’t come with a detailed flight manual, it’s incumbent upon the pilot to know what the limits are when “pulling on the pole” whether those limits are structural or aerodynamic.

Because I’m not the best pilot I know, it helps me to keep the airspeed indicator and G meter in my cross check and AOA cues help me precisely nail L/Dmax and on speed. I’m confident there are folks that can precisely manage Ps with just their finely calibrated elbow and butt, and I hope after some more practice, I might be one of ‘em.

Sincerely,

Vac

PS: I’d be happy to discuss over a beer how snap maneuvering correlates structurally with asymmetric maneuvering speed. I’ll be up at OSH and happy to buy!
 
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Any discussion is good of course, but I think this talk of mental calculations, corner velocity, L/D max, etc. overcomplicates and muddies the waters on how pilots are supposed to use Va in a practical sense when it comes to actual maneuvering. I'm still unable to glean concise and clear advice from this thread unless (consistent with my simple mindedness) the only useful application of Va during aerobatic maneuvering is to recognize that yes you are in fact either above Va and can possibly bend/break something if you have zero feel for what you're doing and pull like a gorilla...or no you are in fact at or below Va and will not pull the wings off if you handle the controls like you have zero feel and no clue what you're doing. ;)

Again, I just don't know any aerobatic pilots who have ever even considered Va while doing acro. It's all about G load and feeling for what G load is most efficient for what we're after, but airframe protection related to the very specific Va number doesn't enter the equation. We simply don't exceed the G load limits regardless of airspeed. Maybe fighter pilots are different. Regarding LD max, this is not necessarily always the goal, depending on what you're trying to accomplish in your aerobatic maneuvering. But again, I'm not connecting this separate topic to Va. If you are letting a newbie or some kid yank your airplane around, it might be wise to limit your speed to Va in case they handle the controls like they're playing a video game, despite your best efforts at briefing them.

Regarding max recommended snap speeds, I've never thought about that in the context of asymmetric G/load and maneuvering speed. Never seen any analysis on it either. Interesting question. Snaps put high loads on the tail and longerons even if min. G is used to initiate a snap. "Rolling G" is easier to visualize and analyze in terms of G load plus aileron use, but snap dynamics are different, since they are not aileron driven and snaps very quickly break and unload. Aerobatic aircraft suitable for snapping typically list a recommended max snap speed and pilots should respect that regardless of what the higher Va (asymmetric or otherwise) number is.
 
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(ignoring the discussion of the practicality of Va when it comes to maneuvering, and focusing just on the math)

The original poster to this very old thread looks to have deleted his post, but from the discussion it looks like he pointed out what I was also about to post/ask about:

In Van's Construction Manual, under G load testing, they list the following Va maneuvering speeds:

RV-4: 134mph
RV-6/6A: 134mph
RV-7/7A/8/8A: 142mph
RV-9/9A: 118mph

Vans goes on to state:

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.

I am assuming by "function" they mean "factor". If you reverse the calculation to "back out" stall speeds from those given Va, you get:

RV-4: 134mph / √6 ≈ 55mph
RV-6/6A: 134mph / √6 ≈ 55mph
RV-7/7A/8/8A: 142mph / √6 ≈ 58mph
RV-9/9A: 118mph / √4.4 ≈ 56mph

Compare to Van's suggested airspeed indicator markings for stall speeds:

RV-4: 54mph full flaps, 58mph clean
RV-6/6A: 55mph full flaps, 59mph clean
RV-7/7A/8/8A: 58mph full flaps, 64mph clean
RV-9/9A: 49mph full flaps, 56mph clean

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
 
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Knowing Va

Interesting rehash Drake!

Was about to post a video I show my students of an airshow pilot snatching a wing off his airplane in an inverted push. (ouch!) But then noticed it was RT that took the video so given world events I leave that to anyone who wants to find that video on their own: "Dramatic video of pilot surviving crash after plane's wing breaks off mid-air"

Some earlier posts about whether it matters. I do believe that not everyone doing aerobatic maneuvers knows their current airspeed limits based on their current weight, perhaps feel gets more emphasis than the hard numbers. Feel is terribly important I agree, it's a big part of how we perform. But when it mattered I for one always also knew the numbers, it's how you stay safe. At least I knew the numbers I was given for my machine. Know your limits right?
 
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(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:

One complication: the Va calculation should use Vs expressed in CAS, while the airspeed indicator markings are most likely IAS. At low speeds IAS is typically smaller than CAS.
 
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.

------------------------------------------------------------------------------

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:


i-BzbnwTP-L.jpg
 
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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.

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.

------------------------------------------------------------------------------

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:


i-BzbnwTP-L.jpg
 
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.

Van's does not publish Vb, Turbulence Penetration Airspeed (aka: Design Speed for Maximum Gust Intensity), and it is not required to be published for Part 23 aircraft (yes, I know E-AB's are not required to meet those standards). Vb is defined for Part 25 Transport Category aircraft.

But here is a good article discussing Vb for light aircraft:


and another:


"A simple rule of thumb would be to split the difference and fly a speed that is approximately half way between Vs1 and Va. However, a slightly faster speed will help improve controllability in very rough conditions. Since airspeed will be varying considerably in turbulence, it is not important that the pilot try to maintain an exact speed, but rather work to maintain near a level attitude and not exceed either the Va or Vs1 limits."


I saw another article that approximates Vb as 1.6Vs, so 1.7Vs is as good as any other approximation.
 
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