What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

VNE

TAS is for Vne, IAS is for stall. That is why the U-2 flies just below Vne, just above stall, and turns s l o w l y . Take a look at Kevin Horton's draft POH, he has a beautiful graph for this.

Where is that little animated guy eating popcorn?
 
Interestingly however, the label for the chart is for airspeed indicator markings... I've always interpreted Vne as a TAS number also though.

Bob
 
The above referenced article is one of the most confusing and misleading things I have ever read. A lot of it concerns the danger of high altitude cruising with turbocharged engines, and the resulting high TAS. But even normal-engined RVs can climb quite high (the 10 to over 20,000'), and then descend at very high speed. So this issue is not restricted to turbocharged aircraft. The correct answer is (C), neither of the above. There are many different flutter modes, and their threshold of onset depends, in general, on the parameters rho (density) and V (true airspeed). Many of the most important modes on typical single engine airplanes have thresholds where these variables can be combined into the form rho V squared and nothing else. Since this is the definition of indicated airspeed, for these modes Vne can be expressed as an indicated airspeed (only). But for other modes, life is not so simple, and the threshold depends on rho and V in an algebraic expression which is different from rho V squared. For these modes you need to specify, for each rho, a V. But since you have specified rho and V you can calculate IAS, and specify an IAS for each rho (rho usually being changed into an altitude). But it incorrect to say you can specify Vne in terms of either TAS or IAS. It also depends on altitude (rho).
One of the most important modes that does not scale as rho V squared is where the wings flap like a bird (along with a torsional twisting 90 degrees out of phase with the flapping). Aircraft like gliders, with long, flexible wings, are most suceptible to this mode. Most single engine airplanes have relatively stiff wings and do not easily excite this mode.
Certified aircraft must be able to specify a Vne in IAS, good from the service ceiling to sea level; or, placard the plane with altitude/airpeed limits if they cannot comply with a single IAS number. (see FAR part 23). Most normally certified aircraft, even high ceiling turbocharged ones, can get by with the single IAS for Vne. So what is the story for RV's? Since RV construction is similar to Piper's, one might reasonably assume that a single IAS will suffice for Vne. But absent testing, no one can say for sure. This is the price we pay for flying EAB aircrft.
 
Last edited:
This topic has been discussed many times here on the forums by a lot of very well respected aerodynamicists who have all given it their own spin but in the end agree that Van's interpretation for his design is the one that needs to be listened to. Bob - the tail design of the RV's is very different from the low-wing metal Pipers (the Cherokee and descendants all have stabilators), so I don't know why you'd make assumptions about RV's by comparing them.

WHat is the story for the RV's? Absent my own analysis and testing, I'd trust the designer.
 
Ironflight said:
Bob - the tail design of the RV's is very different from the low-wing metal Pipers (the Cherokee and descendants all have stabilators), so I don't know why you'd make assumptions about RV's by comparing them.
.
Click to expand...

I think what I said was that one might reasonably assume...but absent testing no one can say for sure.
But what I meant was that I know of no Piper, Cessna, Beech, or Mooney single engine aircraft that has found it necessary to specify Vne as anything other than a single IAS number, so one might assume that that works for all conventionally constructed aircraft. But that is an assumption. My opinion is that in the absence of high speed, high altitude testing, Vans is being conservative. It would be helpful if they would say at what altitudes and airspeeds testing was done.
 
It would be helpful if they would say at what altitudes and airspeeds testing was done
Click to expand...
Yes - but that would have been on 1 prototype, with many minor (and some major) differences from "homebuilt" subsequent examples.

The RV-7 Canadian accident report does give some insight into the testing and calculations Vans did.

My take on it is that although the VNE is stated by Vans as 200KTAS, there will be some flexibility / margin, either intended or not, for 200KIAS at reasonable altitudes.

The point I take from the RV-9 article was that by modification, or just flying beyond Vans' "reasonable" expectations, you can erode completely those margins, and encounter flutter.
 
Andy Hill said:
Yes - but that would have been on 1 prototype, with many minor (and some major) differences from "homebuilt" subsequent examples.

The RV-7 Canadian accident report does give some insight into the testing and calculations Vans did.

My take on it is that although the VNE is stated by Vans as 200KTAS, there will be some flexibility / margin, either intended or not, for 200KIAS at reasonable altitudes.

The point I take from the RV-9 article was that by modification, or just flying beyond Vans' "reasonable" expectations, you can erode completely those margins, and encounter flutter.
Click to expand...
Just have to comment on this statement:
My take on it is that although the VNE is stated by Vans as 200KTAS
Click to expand...
I am not sure if you are referring to a specific RV Vne with those numbers or if you are just using an arbitrary number to make a point. But to clarify something that the uninitiated may not be thinking about, Van's documented speed numbers for ALL of the RV line are documented in MPH NOT in KTS! This is a very important concept to comprehend when reading the published V speeds and taking them into account when flying an RV at the margin of the safety "envelope".
 
The -7 and -8 have a 230 MPH VNE which converts to 199,86 knots.
And section 15 of my plans states that this is TAS.
 
Last edited:
BillL said:
TAS is for Vne, IAS is for stall. That is why the U-2 flies just below Vne, just above stall, and turns s l o w l y . Take a look at Kevin Horton's draft POH, he has a beautiful graph for this.

Where is that little animated guy eating popcorn?
Click to expand...


Ok here we go...

VNE at high altitude is a function of sonic flow and Mach number, not flutter margins like on RVs. Same thing happens in jets, as you climb the redline IAS moves down, but max max number MMO, remains the same.

But yes, it's all TAS.
 
You must log in or register to reply here.
Back
Top