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Old 02-18-2020, 09:52 AM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 8,643

Originally Posted by Aviaman View Post
I tend to get into the weeds in technical discussions. You motivated me to read the Kruger article again.
Let me discuss it as I understand the article.

Here’s what appears on page 1 of the article “Flying High and Fast”.

“No, the real problem is not mechanical. The real danger is exceeding the Never Exceed Speed, noted as Vne.” So far, no mention of Vno, but will come back to that.

Yes, at extreme altitudes TAS will become very high with sufficiently high IAS. The example he gave was with a turbonormalized engine, which is very unlike what we are discussing. A normally aspirated engine dramatically loses power with altitude. Because of this, my 360 does not exceed Vne, or even come close, in level flight at any altitude that I have flown. In any case, my EFIS computes and displays TAS. So IMO, exceeding Vne is not in the cards, (except in a dive.)
Now, maybe my engine underperforms (carbureted, fixed pitch prop) and others can go faster. But my own experience doesn’t support the possibility of exceeding Vne in level flight.

About Vno. That is the upper limit of the green arc (or the lower limit of the yellow arc). It’s generally understood to mean only go above that limit when in smooth air. Is that a guarantee of no high loads from unexpected gusts like what you described ? No. Flying in the yellow arc is a matter of judgement and the assumption of risk. However that limit was defined for the RV-9 Just like it was for any other standard category plane. (I have experienced sudden gusts like you described, so caution is advisable). There is no reason to regard that limit as defined differently from other aircraft. For the RV-9 its 180 mph IAS. One should give due caution to that limit as one would with any other plane.

I suspect this hp limit on RV-9s was motivated by Vans fearing that RV pilots would think RV-6 and 7 performance was to be expected or considered safe in the RV-9. No, that wouldn’t be a good idea. It is not stressed for acrobatics, and has lower load limits.
The article was written as applicable to all RV models so it does apply to the RV-9 as well, but don't confuse that article with being the reason for the 160 HP recommendation for the RV-9.

The RV-9 is a slightly different breed of RV and has a lower max. HP limit than the other models for a different reason. I think I have now thoroughly explained the reason.
As you mentioned, Vno should be obeyed while flying any aircraft, to which I agree, but having been present and participated in the static load testing of the RV-9 wing and other major air-frame components, and having direct knowledge of many of its design details, I can stand behind my explanation of why there is a smaller engine recommendation specifically for the RV-9.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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