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Stall (and other Speeds

Pfatt2

Member
I bought my 6A a few months ago and one thing I can't quite understand is how there can be such a wild variety of stall speeds for the same airframe.

My handbook states:
VNE - 182k
Vno – 156k @ 1650#
135k @ 1800#
Vfe – 96k @ 20°
87k @ Full
Vy – 113k
Vx – 83k
Va – 115k
Vs - 52k
Vso - 49k

Another person's POH states:

Vy - 103K (120 Kts best climb/cooling
speed)
Vx 65K
Vs - 60 Kts
Vso - 56 Kts

Everywhere I look, the numbers are wildly different. How can this possibly be and how can I find the actual numbers for those numbers I cannot ascertain for myself? An 8 knot difference in Vs and 7 knots in Vso?
 
I bought my 6A a few months ago and one thing I can't quite understand is how there can be such a wild variety of stall speeds for the same airframe.
My handbook states:
VNE - 182k
Vno – 156k @ 1650#
135k @ 1800#
Vfe – 96k @ 20°
87k @ Full
Vy – 113k
Vx – 83k
Va – 115k
Vs - 52k
Vso - 49k
Another person's POH states:
Vy - 103K (120 Kts best climb/cooling
speed)
Vx 65K
Vs - 60 Kts
Vso - 56 Kts
Everywhere I look, the numbers are wildly different. How can this possibly be and how can I find the actual numbers for those numbers I cannot ascertain for myself? An 8 knot difference in Vs and 7 knots in Vso?

Very likely a difference between airspeed indicators and/or static source.
 
To me that would either indicate a calibration problem with one of the the two air speed indicators, or maybe yours is in knots and the other is in MPH. Either way I would fly it to confirm the stall speeds.
 
Stall speed

They are not certified ac. Location and shape of pitot tube. And static port. Vary a lot. We’re as on certified ac both are set in stone as to location shapes etc…
Find the numbers for your particular ac test to insure there accuracy. And move on.

Hope this helps

RD
 
I don't like to think it, but I suspect some of he differences in speeds found in POHs are simply due to whose speed chart the builder cribbed theirs from :(

Buyers of used, probably a good idea to go verify all that. Ditto if the instruments were ever updated, for example from steam to glass.
 
I bought my 6A a few months ago and one thing I can't quite understand is how there can be such a wild variety of stall speeds for the same airframe.

My handbook states:
VNE - 182k
Vno – 156k @ 1650#
135k @ 1800#
Vfe – 96k @ 20°
87k @ Full
Vy – 113k
Vx – 83k
Va – 115k
Vs - 52k
Vso - 49k

Another person's POH states:

Vy - 103K (120 Kts best climb/cooling
speed)
Vx 65K
Vs - 60 Kts
Vso - 56 Kts

Everywhere I look, the numbers are wildly different. How can this possibly be and how can I find the actual numbers for those numbers I cannot ascertain for myself? An 8 knot difference in Vs and 7 knots in Vso?


I see you have Vno posted at different weights.. I don’t believe that’s correct. Vno shouldn’t change with weight, however Va does change (lighter weights, lower Va. opposite of what you have the Vno doing) as for determining your actual stall speed, you should just go and test them! If you’re uncomfortable doing that, you really should get a CFI or possibly another qualified pilot, and do them. There’s no reason to be apprehensive about doing stalls in RVs.

As for the differences in Vx and Vy.. with a fixed pitch prop, those speeds are best reached at a higher speed than with a constant speed prop, as the higher speed allows the engine to make more RPM. There are other factors that make it important to test and find the speeds in your aircraft, not just copy someone else’s results.
 
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Thank you

All excellent responses. Thank you for all of this.

I have seen at least 6 or 7 online POH's and every one of them is very different from the next one.

I have a CS prop and fully understand Vx and Vy won't be the same as in a FP prop, but when everyone's stall speeds are so different, how accurate will the other numbers be then? I don't want to be flying a bogus engine-out (best glide) speed when the correct speed would get me that extra thousand feet over the ground.

And as far as Vno, many of the POH's I have seen have them based on weight (???).

I just ordered the EAA Flight Test Manual :)

Thank you!
 
Matt,

As everyone has pointed out, no 2 RV's will have the same IAS for any condition unless they are built and configured identically, and have the same static source pressure error. The reason you can jump from one Piper Archer to another Piper Archer and fly the same speed is because those airplanes are built identically, have the same plumbing, same instrumentation and same pitot/static configuration. Not true for an EAB.

Good plan to fly the EAA flight test program! The only way to derive accurate information for your EAB is to flight test your EAB. We don't always think of it this way, but we're all experimental test pilots in the RV world, all the time :). Not all POH's are created equally, and a POH isn't even required for an EAB. If you buy an RV with a POH or detailed checklist, the only way you can verify the numbers are correct are to either look at the initial flight test data used to derive them, or run the test yourself.

I fly an RV-4 that has four airspeed indicators in the cockpit using various combinations of the primary pitot/static system and sensors, a boom-mounted Keil probe mounted on the wing providing a separate pitot/static source as well as two angle of attack systems that also compute airspeed. Interestingly, when I look at the data, none of the six that are recorded ever marry up until everything is corrected to calibrated airspeed; and even then I'm lucky if I can get 3 out of 6 to line up :rolleyes:. I'm not sure, but I think there is Voodoo (definitely smoke and mirrors!) involved in pitot/static rigging...

Another point to ponder is that airspeed is just a surrogate for angle of attack. Unlike IAS, AOA is constant from one airplane to another (of the same type, say RV-6/RV-6) for a given condition. The angle of attack for L/Dmax is the "fundamental" AOA for the airplane--it's designed in. Once you know L/Dmax (max range/max range glide), you can also determine an on speed AOA (Vref/neutral energy/max endurance glide) and Carson's (best efficiency) AOA. On speed is also Vx and L/Dmax is close enough to Vy during initial climb segment for gov't work. That's a lot of performance SA from a couple of fixed angles that don't care what attitude the airplane is in or what the G load is. As a matter of fact, you don't even need an airspeed indicator in an experimental airplane for day, VFR if you can accurately measure AOA: https://youtu.be/r8XcXDHLIIM

Drop a line or PM if you have any questions. I'll send you a data reduction tool that can compute your static source pressure error and provide CAS corrections from GPS horseshoe runs if you'd like.

Fly safe,

Vac
FlyONSPEED.org
 
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Vx and Vy speeds will be different for every engine/prop combination. They are not simply a function of aircraft aerodynamics. If you have a fixed pitch climb prop and you go to a cruise prop you need to go test to determine the impact on best climb rate and climb angle speeds. Since every RV seems to be a bit different (some are injected vs carburated, some have conventional mags vs electronic ignition, some have constant speed vs fixed pitch, some are cleaner etc) there is no reason to expect these speeds to be the same for all aircraft even if there were no airspeed indication errors. So they need to be determined by test for your aircraft.

Vne, Vfe and Va are provided by the designer and have to be respected and if airspeed errors are known they should be accounted for. Vs is assumed to be power off so it is dominated by aerodynamics so variations in engine/prop configuration should not have much impact but there will be small variations and pilot technique also results in variations. Vs should be determined by testing.
 
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