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  #1  
Old 05-10-2018, 03:53 PM
RV8JD's Avatar
RV8JD RV8JD is offline
 
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Default

...........
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Carl N.
Arlington, WA (KAWO)
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RV-8, 1938 Tach Hours (Pic 1),(Pic 2) - Sold

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Last edited by RV8JD : 02-01-2019 at 10:27 PM.
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  #2  
Old 06-05-2018, 06:02 AM
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ronschreck ronschreck is offline
 
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Default 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.
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  #3  
Old 06-05-2018, 06:27 PM
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Walt Walt is offline
 
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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!
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  #4  
Old 06-05-2018, 07:43 PM
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Chino Tom Chino Tom is offline
 
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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?
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  #5  
Old 06-05-2018, 08:40 PM
KatanaPilot KatanaPilot is offline
 
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Location: Locust Grove, GA
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Default 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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Last edited by KatanaPilot : 06-05-2018 at 08:58 PM. Reason: Added text
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  #6  
Old 06-05-2018, 11:53 PM
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Chino Tom Chino Tom is offline
 
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Quote:
Originally Posted by RV8JD View Post
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.
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  #7  
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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  #8  
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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  #9  
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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  #10  
Old 06-07-2018, 08:06 PM
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Alan Carroll Alan Carroll is offline
 
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Location: Madison, Wisconsin
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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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