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02-14-2017, 08:01 PM
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Join Date: Jun 2007
Location: palm coast fl.
Posts: 945
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Flutter
one builder rebuilt his rudder and documented the process , he openly admits that he skipped the Pro Seal procedure . He crashed and died .
__________________
Rv8
N 666 TA
First Flight 2-3-2015 🚀
2017 donation paid
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02-15-2017, 05:24 AM
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Join Date: Dec 2015
Location: Auburn, AL
Posts: 200
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Quote:
Originally Posted by RV8JD
And as "g zero" mentioned above, the sealant could be important to the strength and stiffness of the -7's rudder.
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Gosh, I sure hope not! I used the VHB tape that Van's recommends on my rudder.
__________________
Steve T.
CFII/MEI/ATP 737,DC-10
RV-7 slow build
AUO
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02-15-2017, 05:40 AM
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Join Date: Jun 2005
Posts: 846
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FFT
What would really be cool is to do design work up on different models and then shake several of same model to see how they differ since no two exp are built the same.
Bob
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02-15-2017, 06:28 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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Quote:
Originally Posted by RV8JD
Again, I have no data to back up this supposition, or whether the rudder's torsional mode is even an important player in the flutter characteristics of this design.
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Have built one of each of these rudders. I was about to measure torsional stiffness until I read about the shear bucking failure of (the NJ tragedy) HS spar between the skin and fuse. Afterward the SB came out and added the "H" reinforcement to the spar.
I did measure the weight and CG of the two. The riveted TE, (aka 7-tall) is .016 skin, the folded (aka 8) .020 skin. The riveted TE is heavier by 400g and moves the CG from 3.18" to 4.05" from the pivot line. The 7-tall has some wiggle zone torsionally due to the skin looseness. I assume it is due to a combination of thickness and minor stretching from the dimpling and riveting the TE. All w/o glass caps, and with the same, by the print, counterweight.
If you think torsional stiffness is a key factor, then I will measure both, including quantification of the no-force wiggle zone. The planned method is fastening the CW end (top) with formed boards, then a weight and dial gage at the tip of the opposite end. Pivots will be included. Before doing this, what definitive value will it provide to Vne?
Suppose the 7-tall has a flop zone of .2-.3" and the slope of the force-deflection
is half of the 8, then what? Is it enough? What does it really tell us?
We should note that the 14 rudder has lots of internal bracing for stiffness. Beautiful design.
I will be doing first flight with the 7-tall, and some spin testing, followed by replacement with the 8 and more spin testing to validate the difference. Yes, I know Vans tested it.
Your experienced opinion is appreciated.
__________________
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.”
Last edited by BillL : 02-15-2017 at 06:38 AM.
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02-15-2017, 07:06 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Carl, great writing...
Quote:
Originally Posted by RV8JD
As I mentioned earlier, I'm convinced Van's airplanes are flutter free to the their design Vd, if properly built and the control surfaces are mass balanced correctly.
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So do I, the operative phrase being "to their design Vd", but that's like saying "Peace exists until the war starts." Most want a DMZ, a margin, a place to posture and give the other guy the finger...or maybe just not get shot for taking a wrong turn in the dark. A low flutter margin is the other guy with a hair trigger.
Quote:
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The -8's rudder drawing is below, and it has a folded TE, compared to the riveted TE of the -7......I'm not sure if the riveted TE or the folded TE would be stiffer in torsion.
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Also note there is a considerable difference in CG due to the 7's trailing edge wedge. The Canadian report fingered excessive filler and paint, which exacerbates the issue. Carl, could you share a few educational words on the effect of increasing control surface imbalance?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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02-15-2017, 09:10 AM
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Join Date: Jul 2009
Location: Niceville, Florida
Posts: 434
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Over-G vs Flutter Point to Ponder
From a purely operational standpoint, with the exception of some early RV-3's, RV's constructed IAW instructions and operated within the design envelope using acceptable handling techniques have proven to be free from flutter and structural failure. If limits are exceeded, then experimenting is occurring, whether intentional or not. As dangerous as flutter can be, so is 14+ G's available in the hands of a pilot that doesn't understand how to properly maneuver about three axis that may find himself in an unusual attitude, regardless of tail design and the amount of Bondo present. I can understand where conflating the two hazards is quite easy once speed limits are exceeded.
The "no man's land" Dan refers to in his metaphor can be effectively maintained in an RV-type by understanding the design envelope, using the Vne expressed as TAS technique, understanding when it's acceptable to exceed Vno (maximum structural cruising speed) and combining this with a handling skill set that allows the pilot to effectively manage energy. This requires the pilot to anticipate acceleration and deceleration and know how to control that with a combination of throttle, G, velocity and lift vector management (i.e., power, pitch and roll). Proper control application is also critical: how the airplane is rolled and how the stick is pulled. Another required skill his how to safely recover from an over-speed condition in any attitude.
The Canadian mishap was a pilot conducting some sort of "chase" formation that looked a lot like LOWAT BFM (low altitude basic fighter maneuvers, or 1 v 1 maneuvering) from the video evidence. The pilot did not appear to be a trained fighter pilot based on information in the mishap report. The ability to maneuver effectively in this type of fluid, three-dimensional environment requires some knowledge of energy maneuverability, aircraft handling characteristics, turn rate and radius management, ability to simultaneously assess and react to the other aircraft's maneuvering, and not allowing either the other aircraft or the ground to present a maneuvering hazard. It typically takes some specialized training to provide a pilot with the requisite skill set for this type of activity. A simple startle reflex at high speed is sufficient to generate more than enough G to bend something or start to remove airframe parts if the pilot suddenly realized that the ground was an immediate hazard. Another thing to consider is that overall G available (i.e., how much you can pull without bending or breaking anything) decreases if the airplane is rolling simultaneously--i.e., the ultimate load limit may be as low as 5.9 G's for aerobatic RV-types; however, Van's does not specify asymmetric G limits for any RV-type.
In another mishap, the pilot was at 6500 feet and 130 knots (150 MPH) and began a vertical maneuver of approximately 3500 feet which resulted in an airspeed of about 220 knots (253 MPH). While the exact maneuver was not described in the mishap report, if a pure vertical pull through (split-S) was attempted from these starting parameters and power and G were applied to control airspeed, a turn diameter of less than 1000' would result. Even with idle power, it may have been impossible to properly modulate airspeed without exceeding G limits as the maneuver was started at a speed above corner velocity (maneuvering speed). If G limits were honored, or insufficient G applied, then the turn circle would open up and the airplane will accelerate, perhaps accounting for the 3500 feet referenced in the report, which also correlates with the 220 knots mentioned. At 220 knots, the mishap aircraft was capable of generating in excess of 15 G's. Since "g available" is really how much lift the wing can generate, recall from pilot training that the amount of lift increases as the square of airspeed. The ultimate load limit for a properly loaded aerobatic RV-type is 9 G's. The mishap report indicated structural failure was a result of static overload, and no evidence of multiple dynamic loading fractures was found. No pre-existing fatigue damage was found.
As a point to ponder, consider that nothing really good happens during high-speed flight when design limits are exceeded in any airplane. It is good to have a healthy engineering discussion regarding flutter, structural integrity and design, but ultimately all of these mishaps share a common thread: handling error. The historical exceptions may be some early RV-3 mishaps involving the rear spar attachments.
It is incumbent upon the pilot to understand the limits and utilize appropriate handling techniques to either stay within them or recover if they are unintentionally exceeded. Hopefully, the information in the builder's manual was sufficient to answer the original poster's questions, and this post isn't off the beaten track too far--now I'll turn the discussion back to the engineers, pitch out of the fight and continue to read and learn!
Fly safe and have fun!
Vac
__________________
Mike Vaccaro
RV-4 2112
Niceville, Florida
Last edited by Vac : 02-15-2017 at 08:44 PM.
Reason: Removed Incorrect Reference to RV-8 Structural Failure
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02-15-2017, 12:46 PM
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Join Date: May 2012
Location: Ga
Posts: 662
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Thanks
I really appreciate the time that Carl and others have shared in this discussion.
__________________
Craig
RV-3 Sold
RV-4 Sold
RV-6a Sold
RV-9 IO-360 CS, Built and Flying
Aerostar 600A, Family Hotrod
Last edited by grubbat : 02-15-2017 at 07:40 PM.
Reason: Condensed to just say thanks
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02-15-2017, 02:26 PM
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Join Date: Aug 2016
Location: Vegas, NV
Posts: 16
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Nice writeup, Vac.
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02-15-2017, 06:36 PM
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Join Date: Dec 2011
Location: Alpharetta, Ga
Posts: 212
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RV14 Rudder
Quote:
Originally Posted by RV8JD
Bill,
Thanks for the great info.
Questions:
1) Just to confirm, are the mass balance weights the same for the -7 and -8 rudders by the drawing?
2) Are the -7 and -8 rudders the same planform size?
I did not know that the -7's rudder used 0.016" skins (versus the -8's rudder with 0.020" skins). That is really interesting. The -7's rudder will be less stiff in torsion compared to the -8's rudder because of this (if they are the same size). Also, the -7 rudder's torsional frequency will be lower because of this plus the heavier weight and aft cg of the -7's rudder (again, if they are the same size).
If you wanted to measure the torsional stiffness of both, that would be interesting information. But at this point I do not know whether the rudder's torsional mode is even an important player in the flutter characteristics of this design. Therefore, I don't know how we would asses whether the flutter margin is less with the torsionally softer -7 rudder. I don't want you to do a lot of work that we may not be able to use.
Good info on the difference in cg between the -8 and -7 rudders too. ("The riveted TE is heavier by 400g and moves the CG from 3.18" to 4.05" from the pivot line.") DanH had a question about that also, and I'll respond to his query in another post after this one.
Also, you made an interesting comment about the -14's rudder design with respect to stiffness. I'll order a set of CD plans for the -14, but in the meantime could you email me a drawing of the rudder, or point me to a drawing somewhere?
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Here's a link to the RV14 rudder plans. It would be interesting to hear the differences to other RV models.
https://www.vansaircraft.com/pdf/rev...4/RV-14_07.pdf
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02-15-2017, 07:57 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Some important differences with regard to the RV-3B rudder!
The RV-3B rudder:
Has a bent trailing edge, not the strip.
Doesn't have the shear clips.
Doesn't have the rudder horn brace.
Doesn't have the mass balance.
There are probably some smaller differences which I've missed.
The RV-3B has a Vne of 210 mph and .016 control surface skins.
Dave
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