I think you missed my point, but agree
airguy said:
Careful - not technically accurate as you have stated it.
Flutter is entirely dependent upon the airstream's true speed. Vne is usually referred to in INDICATED airspeed, not true airspeed. The TRUE airspeed version of Vne will never change, but of course indicated Vne will decrease as you increase your density altitude. To induce flutter, the air flowing over the surface must exceed a specific TRUE velocity, regardless of the indicated velocity which is dependent upon ram air pressure, and thus density. You can induce flutter at ridiculously low indicated air speeds if your altitude is sufficient and you have enough power. This was one of the limiting factors for the U-2 aircraft, at operational altitude they would have only a few knots of envelope left between stall and Vne.
If you read I state indicated flutter speed decrease with altitude. Be careful here not to out smart yourself, there are many factors that affect flutter, not just altitude. However traditionally Vne is based on indicated speed for low speed GA planes. RV's being fast do push the envelope a little. Thats why we have mass balanced control surfaces.
GA planes, normally aspirated generally have ONE Vne and do not make altitude corrections (like high flying aircraft like Jets and gliders, yes gliders).
Vne covers several airframe limitations, like strength and flutter. You really don't know what is the limiting factor is unless you have the all data.
I can say RV's have shown to have very high flutter speed, where RV/Rockets are going 250 mph. Also RV's tend to protest with a buzz before going divergent and destroying the aircraft instantaneously. To be clear my opinon is flutter is not something to be take lightly. My motto is follow Van's limitations, weight, speed and so on.
RV's are faster than most GA planes. RV's can build more speed in descent than a C-172, so be careful. In level flight its unlikely Vne will be exceeded or flutter margins reduced at any altitude, with a non-turbo-ed engine.
Van knows a normally aspirated RV goes slower (true and indicated) as it climbs due to loss of HP, but if you climb into the teens and dive to Vne indicated, you are reducing your flutter margins. What is the flutter speed? I don't know? You would have to test your RV until it flutters!
NO THANKS. All we know is Vne is OK below say 10 or 12 thousand. If you do the full flutter test, let me know, I want to get an insurance policy out on you.
One RV-4 was "tested". Being dove at altitude the TAS was in the 240's or 250 mph range. He got the "Buzz". He reduced power and raised the nose. No damage. Lesson learned:
We all should reduce Vne (mentally) as you climb and be careful with high speed high, altitude descents. To start a descent just lower the nose for a +5 or +10 mph increase. That will give plenty of descent. As you descend start pulling power back and maintain speed or increase slightly (2 mph per 1000 ft).
Consider Vne as a moving target or as a TAS speed limit, at least in respect to flutter. This only comes into play at high altitude and in a descent. For most RV'ers who fly at 8,000 to 10,000 ft operations, Vne indicated is acceptable in smooth air, but don't forget about about Vno.
EXAMPLE:
RV-8 w/ 200 HP, Vne 230 mph indicated; max level flight speed at 10,000 feet (std day) is 210 mph true (182 mph indicated).
Assume the Vne = 230 mph TRUE, at 10,000 feet its only 200 mph indicated. With a 182 mph IAS cruise, 200 - 182 = 18 mph margin to Vne in cruise. To start a descent out of 10,000 you have 18 mph to play with. Another way to look at it is reduce Vne by 2 mph for every 1,000 ft
For normal operations Vne indicated covers most RV's very well as long as you fly around 10,000 ft or less. Super high flying RV'ers in the teen's, with big HP engines do need to take extra care. RV's are fast. Equipped with big engines you're closer to flutter, that's a given. This is why VAN does not like bigger engines than recommended in his planes.
To avoid the HIGH DIVE descent that scare's (Boo) your passenger and pops ears takes descent planning. Start down at 3 to 1, allow 3 miles for every 1,000 feet. At 10,000 ft above field elevation start down 30 miles out, PLUS add 5 miles to slow down for pattern and 1 mile for every 5 mph tail wind.
I think we agree, but my point is NORMAL operations and Vne give protection, not withstanding your U2 dreams.
BTW: when jets fly at max altitude as you point out, stall and high speed buffet (like flutter) are a few kts apart. This is called the "coffin corner". Boo happy Halloween.