[PaulRomano]

I had not really intended to extend the RV-4 envelope in terms of Vne (or tickle the dragon) beyond what Vans had specified. My intention was to test/demonstrate my tapered wing as capable of replacing the original wing and meeting the original Vans parameters in terms of Vne (gross weight and hence range parameters increased). On paper, or by the numbers, my wing is slightly more flutter resistant than the original wing I replaced. Vans publishes a Vne of 210 MPH and the standard of testing in regards to Vne is (as I understand it, again I am without credentials) 110%. That is how the testing parameters of 231 MPH IAS was arrived at. This was theoretically doing nothing with the wing that Vans had not already demonstrated on the original wing, assuming they too tested to 110% the published Vne. But - at what altitude?

Every "store bought" non pressurized piston powered plane out of Wichita or Vero Beach (or any manufacturer selling certified planes in the US) has a red line at the clockwise end of the ASI. This is an IAS limit, that's why its painted on the ASI. Then Vans publishes a letter or bulletin (as stated above I think it was in regards to turbo charging RV's) about the risks of high True Airspeeds that can be achieved at altitude, especially if you can keep your ponies strong up there. So now there is some question (and corresponding discussion here on VAF at the time) of just what the airspeed limits are of the RV series. According to the "consensus opinion" and seemingly best answers I could find, the Vne on certificated airplanes is an IAS limit (hence being marked on the ASI) and valid up to the altitudes at which the aircraft is intended to be operated. Is this up to its technical service ceiling or only up to its intended/predicted use. ??? Dunno So Van's article about the hazards of high TAS causes me to wonder what is safe to do with my RV-4. I called them (a decade or so ago, near the time they published the "notice/bulletin") and asked them to what altitude the published Vne applies. They were "strongly resistant" to providing an answer to the question. So from that point on I flew my original wing with a self imposed TAS limit of 210 MPH.

Upon completion of the new wing I'm doing Phase 1 flight testing, in an assigned test area, with parachute and test pilot. The perfect opportunity to determine/demonstrate an altitude to which the 210 MPH Vne applies. So I/we somewhat arbitrarily chose 10,000' This is an altitude to which the RV surely was intended to operate, well below the requirement for oxygen, and only moderately above the terrain in a large portion of the northwest. Vans declined to tell me an altitude to which it applied, so I decided 10,000 was a reasonable number to "demonstrate" for myself and then utilize as a limit. Above 10,000 I would continue to use a conversion table so as to not exceed the TAS demonstrated at 10,000' I did not consider this to be "high adventure", "irresponsible" or "tickling the dragon".

Above it was stated that Vans has since clarified that the Vne is a TAS limit. This, if accurate, strikes me as a very unexpected answer to the question I posed to them of "to what altitude". I'd never have expected them to reply "Sea Level". This makes the standard practice (and likely FAA requirement for certified aircraft) of placing a red line on the ASI pretty much irrelevant or nonsensical since there is almost nowhere you can fly at sea level without getting your airplane really dirty. I suspect Vans has stated that the Vne is actually a TAS limit out of an "abundance of caution" and did not intend it as such "back in the beginning".

So am I "tickling the dragon"? (NOT a rhetorical question) How many RV's have descended through 10,000' or even 12,000' or higher during the first 4 decades (prior to the announcement that the Vne was a TAS limit) with the pilot dutifully controlling pitch and power to keep the needle just below that red mark? If Van's truly believes and intends that the published Vne is a TAS limit then, yes, doing at 10,000' what is approved at Sea Level is pushing boundaries, by a lot - but has probably been demonstrated by hapless pilots for 40+ years.

Vince, your comments on pushing/expanding the limits are appreciated and well received. I have to now wonder if I am doing so or not. Comments invited!! Perhaps there is a "Van's guy" visiting here that can elaborate on this from an "insider opinion". If you're out there but not willing to comment publicly I welcome your PM input.

Steve--you are more than welcome to spend some time with me in Colorado. I can go over in detail the various ways we look at things, and how you can do some basic calculations for your aircraft. I have one set of the waterjet cut LE fixtures, Vince has the other set in his facility in Indiana. Fly in to KLMO after Phase I, or I can pick you up at DIA. I can talk about most things, only a few will be off limits. None of what I do is special.

Follow Steve and Bob's guidance, they are the best.

Just beware, my niece calls me Uncle Gruncle.

 

[Taperguy]

Hey “Taper Steve”…..this is an AWESOME thread with the unveiling of your design - simply stunning work….so I’d hate to see it de-railed by the usual discussion of IAS vs. TAS for Vne. There are hundreds of threads and thousands of posts on the topic here on VAF, with people clubbing one another over the head with various opinions because they learned that Vne was ALWAYS an IAS becasue that was what they had in the Cessna or Piper in which they learn to fly. The truth is that as the performance envelope opens up (faster, higher) those rules just don’t apply … they are simplifications that work on low, slow airplanes. I have played with flying machines from 65 mp Cubs to Mach 25 spacecraft….the aerodynamic world is far more complicated than the majority of pilots will ever know!

I will admit I chuckled a bit at the thought of 10,000’ being a good ”higher” altitude - we live at 4500’ and rarely fly at LESS than 10,000’ in our RV’s for anything but a local trip down the river. In answer to “what do folks do?” Most people now have EFIS’s in their panels, and most all of the EFIS’s allow you to display TAS in real-time - and many even allow the redline to be displayed dynamically on the IAS tape (showing the current IAS redline that matches the current TAS), so it is actually very easy to know where you are relative to that TAS redline (which is established because flutter is dependent on TAS, not IAS)….

But alas….I don’t want to de-rail your thread - just wanted to let you know that there is mountain of discussion on the topic already, and I’d far rather hear more about how you built this thing!

Paul

Paul,
Thank you for your "direction" of the conversation. No need to stumble into the head clubbing that apparently has already transpired, perhaps repeatedly. On the apparently tired topic of TAS vs IAS I'd just like to say that your phrase:

they are simplifications that work on low, slow airplanes

Is the best explanation I've heard yet in regards to the mystery of why all the Pipers/Cessnas have it marked as an IAS but in other cases......

I'm curious as to what Mach 25 spacecraft you "played with" and in what way. Sounds fascinating!

and I’d far rather hear more about how you built this thing!

Happy to oblige. I made over 400 pages (MS Word) of notes while building this to be sure if I did it again (and I'd be doing it at lease once more for the OTHER WING) I'd remember all the tricks and avoid all the mistakes from the first one. If I copied and pasted it here it would bore everyone to tears and destroy the thread. I'd be happy though to address any specific questions from you or others. But I could start with this: (In the opening thread I sort of told "why". Here's the beginning of the "how")

While having a steak and beer one evening at the local Applebee's I began to seriously consider building this thing. On the way home I stopped at Lowe's and got a roll of masking tape and small tape measure and went to the hangar. I taped off what I thought "looked about right" in terms of where the leading and trailing edges should run, in other words, how much taper, and measured what would be the resulting chord at the tip. At home I sketched that planform out on graph paper (Hillbilly CAD attached) and then made a few calculations as to what the volume of the leading edge would be, as that would be the limiting factor in terms of fuel capacity. The fuel capacity was right at what I felt I needed/wanted for the intended "mission" so I deemed the project "plausible".
Then I "leaned on" an engineer (thank you Eric) to see if it was practical to build a spar that would fit in the RV-4 "spar box" and handle the intended loads, to include landing (if necessary) with the wing fully fueled. The math showed this was practical so Eric performed a more complete structural analysis of the project.
Meanwhile I removed the wings from my RV and made careful patterns of the wing attach points and bolt patterns so the new wing would fit the old plane. Then the wings were put back on the plane of course so it could continue to be used during the building of the new wings.
Then the drilling, and drilling, and drilling of spar caps began.....

 

[RV8JD]

The Vne piece has been beaten up pretty well. Just one dogpile comment: if we use TAS as is shown in the table above, and as appears widely accepted as the safe path, we’ll be conservative down low, and safer up high.

The conservative approach for flutter is to assume a constant TAS limit when the critical flutter mode(s) are either not known or not well understood.

Some flutter modes have flutter speeds that follow a TAS line with increasing altitude. An example of these are the so-called "explosive" flutter modes, where there is a large decrease in aeroelastic damping for a small increase in airspeed. "Aeroelastic" damping is Structural Damping plus Aerodynamic Damping. Structural Damping is usually a constant, the value of which depends on the construction design and materials used in the structure.

Other flutter modes follow the so-called "half and half" speed line with increasing altitude, roughly midway between EAS (CAS/IAS for us non-Mach challenged RV's) and TAS. An example of these would be the so-called "hump" flutter modes, where there is a small decrease in aeroelastic damping for a large increase in airspeed. It is called a "hump" mode because it looks like a hump when plotted on an Airspeed vs Damping plot.

And some flutter modes follow more of an EAS line with increasing altitude.

For most of the RV’s, using a constant TAS Vne limit from Sea Level to the low Flight Levels, I believe leaves excess flutter margin on the table at the higher altitudes. I’ve noticed in my RV-8 that this limits the descent profiles I can fly in descending from those altitudes, and it's easy to exceed Vne if not careful.

I’d like to see Van’s understand the flutter boundaries with altitude better and sharpen their pencil a bit (not changing any structure or the design) and tailor Vne to expand the operational airspeed-altitude envelope a bit to take advantage of the excess flutter margin I believe exists, as opposed to the constant TAS Vne from Sea Level on up based on their original IAS Vne speeds that they have adopted. Maybe a constant IAS up to a certain altitude and the a constant TAS above that (not unlike the RV-12s and some other aircraft).

A side note: When flight flutter testing using stick raps to excite the aeroelastic modes with no instrumentation (i.e., accelerometers) to asses modal damping (other than the pilot’s butt), low-damped or “hump” modes can sometimes be backed away from if the speed increment to the next higher test speed is small. But explosive modes usually cannot, since for a small change in airspeed there is a large decrease in aeroelastic damping and flutter occurs very quickly with no warning.