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  #11  
Old 06-06-2018, 08:34 AM
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Alan Carroll Alan Carroll is offline
 
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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?
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  #12  
Old 06-07-2018, 03:26 PM
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RV8JD RV8JD is offline
 
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Default Errata

I emailed Van’s Aircraft regarding the correct flaps up stall speeds for the RV-8 and RV-7 series. They pointed me to their “speeds.pdf” document.

That page shows a higher stall speed than those I used in my first post which were based on those shown on Van's "RV–8/8A Performance" webpage. Using that data would yield a higher Va (at the Aerobatic GW) than what Van’s currently publishes for the RV-8 & RV-7 series (assuming that the stall speed given is at the Utility Gross Weight and scaling it down appropriately to the Aerobatic GW).

Van's does appear to have an inconsistency in their calculation of Va for the RV-8 & RV-7 series. But since the Van’s published Va (142 MPH) appears to be conservative at the Aerobatic GW, and other assumptions would have to be made to correctly calculate Va’s, I’ve elected not to show the revised speeds and plot here.

But note that Va at the Utility Gross Weight would still be less than the Van’s published Va, and that Va decreases as weight decreases. Ideally, I would like to see designers and manufacturers publish a plot of Va versus weight.

I will remove some information from my posts in this thread, since they were based on incorrect stall speeds.
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Last edited by RV8JD : 06-07-2018 at 03:47 PM.
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  #13  
Old 06-07-2018, 03:37 PM
Vac Vac is offline
 
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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.

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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Last edited by Vac : 06-07-2018 at 03:54 PM.
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  #14  
Old 06-07-2018, 04:09 PM
luddite42 luddite42 is offline
 
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Quote:
Originally Posted by Vac View Post
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.

Last edited by luddite42 : 06-07-2018 at 04:17 PM.
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  #15  
Old 06-07-2018, 08:06 PM
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Alan Carroll Alan Carroll is offline
 
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Quote:
Originally Posted by Vac View Post
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.
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  #16  
Old 06-07-2018, 11:31 PM
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Alan,

Yup!

Cheers,

Vac
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  #17  
Old 06-08-2018, 06:47 AM
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ronschreck ronschreck is offline
 
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Quote:
Originally Posted by luddite42 View Post
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!
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  #18  
Old 06-08-2018, 07:46 AM
Southern Pete Southern Pete is offline
 
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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

Last edited by Southern Pete : 06-08-2018 at 07:48 AM. Reason: typo
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  #19  
Old 06-08-2018, 07:55 AM
luddite42 luddite42 is offline
 
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Quote:
Originally Posted by ronschreck View Post
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.

Last edited by luddite42 : 06-08-2018 at 11:51 AM.
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  #20  
Old 06-10-2018, 08:09 AM
Vac Vac is offline
 
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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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Last edited by Vac : 06-10-2018 at 07:41 PM.
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