B

bobdobbs

Member
Hello,
I was wondering if anyone would be interested paying to run a flutter analysis for the RV-9. As it stands Van's has only ran one flutter analysis on any of his airplanes, the RV-8 and that was done using the largely discredited Engineering Report 45. This is intrinsically related to the Vne which currently stands for the RV9 series at 210 mph.

I am wondering if there is interest in running a subsonic flutter analysis on the wing and the tail surfaces to verify the Vd and Vne. The total cost looks to be $12,000. The proposed engineer to run the analysis is Martain Hollmann MS, "Dr Flutter", who has done flutter analysis on:

* Stallion
* Adventurer (post crash)
* Lancair IV
* Lancair 320
* Lancair 360 with big tail and small tail
* Lancair 360 with big wing and small wing
* Lancair ES
* Star*Kraft 700
* Sadler Vampire (single place all metal)
* Cygnet glider
* Killdeer (four place all metal)
* Cessna C337 with extended wing tips, STC?d (six places all metal)
* Rigel AA300
* Barr Six
* Thunder Mustang
* Wheeler Express, (post crash)
* BD-10 Jet, (post crash)
* Mach Buster
* RotorJet
* Geoduck
* Rans S-7 Courier, FAA Certified


His website is:
http://www.aircraftdesigns.com

Was wondering is this valuable enough for people to chip in $100 or $200 dollars for the report. Alternatively I will probably take the

http://www.aircraftdesigns.com/Classes/HANDS-ON-FLUTTER-ANALYSIS/aircraft.tpl.html

This december, but the results wouldn't be an aero engineer's certified report, available for the FAA review, but some guy giving some numbers. ;)

Bob
 
bobdobbs said:
Hello,
I was wondering if anyone would be interested paying to run a flutter analysis for the RV-9. As it stands Van's has only ran one flutter analysis on any of his airplanes, the RV-8 and that was done using the largely discredited Engineering Report 45.
Bob
Click to expand...

I don't know where you got this information but it is totally false.
 
Last edited:
Scott...

rvbuilder2002 said:
I don't know where you got this information but it is totally false.
Click to expand...


...that is good to know. I'm sure the flutter characteristics of all of the models have been well analyzed.

Have any of the reports been published?

Or are they considered proprietary?
 
az_gila said:
...that is good to know. I'm sure the flutter characteristics of all of the models have been well analyzed.

Have any of the reports been published?

Or are they considered proprietary?
Click to expand...

I am not aware of any of the reports being published.
 
I don't see the need for this analysis.

If there was a problem with any of the RV's, they would be falling out of the sky left and right.

Other than the unfortunate accident with the demonstrator RV-8 some years ago and some issues with the early RV-3's, I do not recall any stories of RV's coming apart in the air. Usually they have to hit something. (I could very well be wrong here, but like I said, I don't recall any such accidents other than the ones I mentioned.) To this day I don't know if the NTSB or Van's every conclusively determined why the wing separated on that RV-8. (Please don't start speculating, there has been enough of that already.)
 
I would actually be fascinated to know how much margin there is above the stated 200 knot (True) limit on the -8 ( and the other models as well). However, I am not sure that I am confident enough in analytical methods to accurately predict flutter to the point where we'd know much more than we already do. In other words, I think that the margin of error in an analysis would probably outweigh the margin we'd see. I am afraid that I am a bit "old school" when it comes to flutter limits (probably because I finished Aero Engineering school almost 30 years ago...) - I believe in testing more than I do anything else, and I know that specific flutter conditions can vary with fit, finish, and balance on an aircraft. A production line can produce very similar airplanes, and therefore a limit determined by testing can reasonably apply to all copies of the design. Experimental, Amatuer-built planes are much more unique, so what holds true for one might not hold true for another.

I'm not sure that we'd gain much real margin form the testing, because we'd have to add so much pad on it for variations from plane to plane.

Paul
 
one benefit would be to show altitude dependence

One benefit of having Hollman do the analysis is that he would show what the results are at several altitudes. Even if Hollman's actual flutter speed isn't completely trustworthy, for the reasons that Paul says, the trend in flutter speed with altitude would still be right. This would help establish an altitude dependent Vne, with roughly constant flutter margin.

The widely-held and often-stated belief that flutter speed is a constant true airspeed is WRONG. Choosing a constant TAS V_ne is conservative, and is often used in the GA industry in the absense of actual testing at different altitudes. But it is very restrictive at the upper reaches of RV altitudes.

Flutter speed ( TAS) does increase with altitude, just not as fast as the TAS for a constant EAS. A reasonable approximation to the actual boundary is the average of a constant TAS and a constant EAS.

I've mentioned this before, with very little evidence to support it. A data set that I evaluated 25 yrs ago which showed this trend was lost. However, I have now worked through a sample analytical 2 DOF flutter problem that does show the trend I expect when the correct aero damping terms are included. I can send the analysis to anyone that is interested and able to understand 2DOF ODE's and Routh's stability criteria.

But what is being suggested here is better - get 120 guys to chip in $100 each and have Hollman do the analysis. Just make sure he repeats the analysis at 9k, 12k, 15k, 18k ft so the altitude trend will be evident.
If he shows a 40 kt margin with the factory-chosen V_ne, fine. I would NOT recommend expanding the envelop to a higher sea level V_ne based on that, but I would consider deriving a constant-margin V_ne adjustment with altitude based on his results.

If I had my pick, I'd say to do the RV-7/RV-8 wing, since that will be the most common ( right now I think there are more -6's (?), but eventually the newer wing will be more prevalent). The RV-9 wing would probably be the most interesting to the factory, since it is higher aspect ratio and departs some from the heritage of the rest of the models. In either case, the trend with altitude will be very useful to a wide audience.
 
Interesting stuff Steve! What we really need is to be transported back to the days of the old NACA, when the aeronautics researchers actually got money to do analysis and research liek this and publish the results - oftne it was for (or directly applicable to) particular models of aircraft. Alas....there seems to be a lack of understanding these days that the first "A" in "NASA" stands for "Aeronautics" ! If somone at an Aeronautics Center had a little slush money to do such a study....;)

Paul
 
N941WR said:
...To this day I don't know if the NTSB or Van's every conclusively determined why the wing separated on that RV-8. (Please don't start speculating, there has been enough of that already.)
Click to expand...
"The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
the intentional or unintentional sudden application of aft elevator control by an undetermined aircraft occupant that exceeded the design stress limits of the aircraft. The aircraft gross weight, which exceeded the maximum allowable for aerobatics, and airspeed, which exceeded the maximum maneuvering speed for the weight, were factors in this accident."

"Wing flutter analysis found no flutter within the aircraft's flight envelope. The analysis included a simulated failure of the aileron control rod.

Horizontal tail flutter analysis found no flutter within the aircraft's flight envelope. The analysis included balanced and unbalanced conditions, as well as fixed stick and free stick modes.

Elevator trim tab flutter analysis found no flutter within the aircraft's flight envelope.

The report concluded that: "The RV-8 aircraft was shown to be free from flutter to speeds above its design envelope with the amount of balance weights provided by the manufacturer on all control surfaces." "

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001211X10121&key=1
 
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