Here is the article, actually written by Ken Kruger, Van’s
chief engineer.
http://www.vansaircraft.com/pdf/hp_limts.pdf
It was written in response to a couple of turbo installations
that were showing up, one on an RV8, and as soon as the RV10 came out talk
started about flying high with turbo engines. Clearly, Van had not allowed for
these power plants when he designed the airframe.
While it is true that flutter is related to True Air Speed,
since the airspeed indicator is the standard reference instrument, Vne is
referenced as an indicated airspeed that leaves sufficient margin at high altitude
be safe. When the threat of turbo engines suddenly raised the useful altitude,
Van’s decided the article was necessary to keep the RV guys out of
trouble. It is interesting that the gliders have such a large altitude range
that they need to placarded accordingly.
There are other reasons for Vne besides flutter. I know from a
test pilot that the Piper twin Comanche will suffer structural damage to the
fuselage, just forward of the empennage, at Vne plus 5 at low altitude. Also,
the early Cessna 180s would suffer a runaway trim situation only a little over
Vne. In this case the stabilizer trim, which is mechanically operated by a
jackscrew, would run away in the “nose up” direction. I actually
talked to the flight test engineer who was aboard when this happened the first
time…he and the pilot both blacked out from the g-forces, and luckily
regained consciousness at the top of the climb and recovered the aircraft and
landed safely. It was one of the few cases of structural damage to a
strut-braced Cessna.
The RV8 is specified to have a max speed only 8 mph shy of Vne.
My own example will easily exceed Vne in level flight. So, I have spent some
time worrying about this, especially racing in the AirVenture cup race year
before last when we started in Mitchell, SD and raced eastbound. I went to
15,500 ft to catch a tailwind, but the problem comes in the decent…best
racing would be done with a decent at Vne…but doing so at 15,500 would
put me well over Vne TAS. That year, clouds forced me lower before the finish
line, and my final decent started at 9500 ft, which still put me over Vne, but
not more than I had previously tested.
I think the responsible kit-plane manufacturer will build a lot
of margin into this spec, considering build differences from one example to
another. You still never know about your particular example until you have
tested it. Or, just take Vne as a limit, one based on True Air Speed.
John Huft
RV spy