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08-05-2014, 08:37 PM
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Join Date: Mar 2013
Location: Iowa
Posts: 164
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08-06-2014, 06:53 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by sheepdog
Let's try a different aircraft closer to an rv airfoil, like an f4u corsair.
Fabric on wings behind main spar, no rear wing attach points, fabric on elevators and ailerons, getting fun.
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And let's also not forget that the Corsair ailerons are made of wood - the metal ones fluttered in flight test.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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08-06-2014, 07:00 AM
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Join Date: May 2007
Location: Quincy, Florida
Posts: 680
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flutter
OK....let's leave the P51 and talk only about RVs. First, I'm only familiar with the RV8. I know VNE for the RV8 is 230 MPH. I think VNE for the RV6 is 210 MPH, yet it is said that the RV6 has the strongest wing. The RV7 has the same tail and wing as the RV8 (I think....not sure about that). I've always heard that air speed should be kept at or below VNE to prevent flutter. Maybe I'm beating a dead horse, if so, I apologize. It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
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08-06-2014, 07:13 AM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Quote:
Originally Posted by DEWATSON
....It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
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Not all RV-s have the same surface designs. Differences abound and subtle ones can play a huge part in flutter. Even with scaling the design to different sizes, the differences that result in mass and in stiffness can be very important. And frequently, the stiffness of the fuselage or wing can play a significant part in the resulting flutter speed or characteristics.
As you indicated, it's very involved.
Dave
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08-06-2014, 08:01 AM
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Join Date: May 2006
Location: Houston
Posts: 2,012
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Quote:
Originally Posted by DEWATSON
OK....let's leave the P51 and talk only about RVs. First, I'm only familiar with the RV8. I know VNE for the RV8 is 230 MPH. I think VNE for the RV6 is 210 MPH, yet it is said that the RV6 has the strongest wing. The RV7 has the same tail and wing as the RV8 (I think....not sure about that). I've always heard that air speed should be kept at or below VNE to prevent flutter. Maybe I'm beating a dead horse, if so, I apologize. It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
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Maybe this will help.
Aerodynamic flutter is like ticking off a woman. To truly understand the condition leading to this requires knowledge of many interrelated variables and several intangible conditions. The variables are similar with certain groups of women, but no two are truly the same.
Best to simply stay well clear as once you trigger the condition, your odds of surviving are low.
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08-06-2014, 08:09 AM
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Join Date: Jan 2005
Location: Columbia, SC
Posts: 804
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Another perspective ...
Quote:
Originally Posted by DEWATSON
OK....let's leave the P51 and talk only about RVs. First, I'm only familiar with the RV8. I know VNE for the RV8 is 230 MPH. I think VNE for the RV6 is 210 MPH, yet it is said that the RV6 has the strongest wing. The RV7 has the same tail and wing as the RV8 (I think....not sure about that). I've always heard that air speed should be kept at or below VNE to prevent flutter. Maybe I'm beating a dead horse, if so, I apologize. It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
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First, a few words to look into on this matter:
Harmonics
Natural Frequency
Excitation
AX-O and others have given good info on this but here is a totally different look at it (from a LOT safer perspective :-) )
On my RV6, there is some natural frequency associated with multiples of 9 KTS rolling (ground speed) that EXCITES my right gear leg to shake ("flutter"? :-) ).
If I do nothing, I feel like the wheel pant will tear apart. If I speed up or slow down, it goes away until I reach a harmonic.
Now, even though there are many RV6's built the way mine is the suble differences in mass make a big difference as to whether the shake occurs as well as the magnitude. For example, I recently changed tires from recaps (heavier) to first run new (lighter and different treads) and the magnitude "feels" like it is twice as much though still at the same speeds.
Now, take all the stuff said about structures and mass in other posts and you can see where not only might flutter occur at different points for RV8 v RV7 v RV6 but even within the RV6 family it may be different. The analysis that many do helps to see where flutter is not likely to occur. All other regions are off limits. A roll of the dice.
James
__________________
James E. Clark
Columbia, SC
RV6 Flying, RV6A Cowling
APRS
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08-06-2014, 02:13 PM
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Join Date: Feb 2008
Location: Pensacola, FL
Posts: 374
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Quote:
Originally Posted by jclark
First, a few words to look into on this matter:
Harmonics
Natural Frequency
Excitation
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Like I said about the ex!
__________________
RV-? in planning stages.
RV-14 #140050 SOLD
Permanently willing to contribute fuel for RV rides
Helicopter ATP/CFI(I)
ASEL/AMEL Commercial/Inst/CFI(I)
Aerobatic Instruction available
Atlanta based.
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08-06-2014, 07:53 PM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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All the jokes and confusion won't make sense, but a new perspective to the complexity of the phenomenon will be revealed with this link.
http://www.ltas-cm3.ulg.ac.be/AERO00...elasticite.pdf
The jokes may become funny. 
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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08-06-2014, 10:11 PM
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Join Date: Aug 2006
Location: Taylor Texas
Posts: 811
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Well, MAYBE! Be Careful.
Recall that several fellas have spoken to frequency - increase frequency and increase Vne. Increase frequency by adding stiffness. Increase stiffness by adding structure. Increase structure, and increase weight. The process follows that path.
So, let's have a look at the P47 - a ship with almost no Vne. It is heavy. Google an exploded view of this ship - especially the aft section - and count the parts. Looking at that view, you MIGHT get a clue as to how to get your RV up to 300KT. In a dive.
Later, a follow-on version of the P47 was developed, and it's empennage fell off in dive testing: the aft fuselage failed as one engineer had predicted. The pilot survived by following that engineers' instructions (the heavy G forces would oscillate - "jump out when the Gs are negative") - I talked to him about this flight. It was not a fun ride, or so he told.
So, it is not gonna be easy to simply 'increase your Vne'. You WILL be making some parts. Keep in mind that the RV empennage is based on the Midget Mustang design, which is light, and doesn't really go that fast (0-200 power). Parts count remains the same with the RV (3,4,6,7,8), so Vne remains pretty much the same for the empennage.
I would stop there - but you can see where it all leads. You cannot simply fix one area - the aircraft structure aft of the spar is a system. Be ready to work back from the main spar to, and including, the emp structure, and then you can do your testing for a higher Vne.
The RV wing structure is ready for just about anything you think it can take. I had heard a very fast number associated with the aileron flutter margin, but I cannot speak to the wing itself, tho it is associated with the aileron system and it's flutter number. Seems the RV4/RV6 wing is VERY solid.
I have seen, and supplied replacement parts for, several F1s that have been flown to ridiculous speeds. I will volunteer that beefing up those parts will certainly allow those parts, and maybe the ship, to survive those speeds a second time, but the spar-aft system is not 'fixed'; other internal parts are required to make those speeds 'safe'. I have also been personally involved with one particular flutter event; the part that failed has been beefed up, but which part might fail next?
This flutter discussion is not to be taken lightly: gravity always works.
FYI the P-39 had an aft fuselage problem that let the empennage fall off. More than a 'few' aviators died proving this fault. Big, heavy, external skin doublers allowed that ship to remain in service. This is an example of how to handle flutter, or more to the point - aero elasticity. In this case, the empennage was up to the task, but the structure fwd was not - same as the more powerful version of the P-47. Point B cannot be stronger than Point A, lest Point A fail 1st.
"Gravity always works, and the ground is always hard."
Good luck, and Carry on!
Mark
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08-07-2014, 07:56 AM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Quote:
Originally Posted by F1Boss
....Parts count remains the same with the RV (3,4,6,7,8)....
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The RV-3 is considerably different. The rudder only has two hinges, not three, for example. Both stabilizers lack the middle ribs. Neither the elevator nor the rudder are balanced (but at least on the RV-3B, the ailerons are balanced).
I suspect there are thickness differences and probably other changes as well.
Ain't no way I'll take mine past its 210 mph Vne.
Dave
RV-3B - Still building wings.
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