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  #1  
Old 10-06-2006, 03:31 AM
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Jonathan Cude Jonathan Cude is offline
 
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Location: KS
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Default Flap Rod - Danger or Not?

Sorry for Breaking the message up... it must be too long or something... here goes, its a 3 parter...

So I built the Flap Pushrods for my RV-7A. I have to say I thought they looked really small. I have heard buzz of these suckers failing and thought maybe I would look into it instead of paying attention in class... here's what I got.
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RV-7A :: Penn Yan XS-IO-360 :: Tip-Up :: SkyView

Last edited by Jonathan Cude : 10-06-2006 at 03:37 AM.
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  #2  
Old 10-06-2006, 03:33 AM
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Default Flap Rod Equations, Sizing, and Material Info

Material Properties
Aluminum 6061-T6
Yield Strenght = 36KSI
Youngs = 10,000 KSI

Section Properties
Rod Length (pin to pin) = 6.625"
Rod OD = .3125"
Rod ID = .1965"
Rod Wall Thickness = .058"
Rod Cross Section Area = .25*PI*(.3125^2-.1965^2) = .04637 in.^2
Rod Cross Section Moment of Inertia = .25*PI*(Ro^4-Ri^4) = 3.949E^-4 in.^4

From Euler's Buckling Equation:

Critical Load = (E*I*PI^2)/(L^2)

E=Young's Modulus
I=x-sec moment of inertia
L=length of column

If we insert the values into the equation, we get:

(10,000,000*.0003949*3.1416^2)/(6.625^2) = 888#

Critical yield stress at this load is calculated from the equation:

(Critical Load)/(x-sec Area) = 888/.04637 = 19.15 KSI
***NOTE: Critical yield stress < Yield stress, therefore buckling failure will occur first!
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  #3  
Old 10-06-2006, 03:35 AM
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Jonathan Cude Jonathan Cude is offline
 
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Default Flap Rod Considerations

Things to think about:

How about a standard safety factor of 1.5... now the critical load is reduced from 888 lbs to 592 lbs.

For the rod to fail within the 1.5 safety margin (ie exceed 592 lbs), the flap must transfer more than 592 lbs to the pushrod. A hinge moment equation exists to calculate this, but is beyond my thoughts at the moment (no pun intended.)

To save the world and my fingers the rest of the derivation, I made some assumptions and approximations using dynamic pressure and the area of the flap... now I know this is rough, but it can put things into perspective hopefully (any advanced aero guys chime in here please!!!) to give a feel for what we are looking at...

The RV-7/7A Flap might need speeds in the neighborhood of 200 mph to generate the force necessary to exceed the 1.5 margin (592 lbs)... I shudder to think about this, but offer the idea that maybe something else is going to fail before the little flap rod buckles.
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RV-7A :: Penn Yan XS-IO-360 :: Tip-Up :: SkyView
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  #4  
Old 10-06-2006, 03:36 AM
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Jonathan Cude Jonathan Cude is offline
 
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Default

Realistically a correctly built part should be just fine. However a homebuilder could easily make a less than accurate part with a critically weak area at the end of the threaded portion (ie off axis tapping) that could cause a failure. Tight tolerances are needed and need to be identified on the plans.

In conclusion: a precisely built part is just fine. Van is probably too optomistic with the average builder skill for this part.

Recommendations:

1. Tap rods using a lathe.
2. Use hex or thicker walled rod
3. buy some 4130 pre-made you see in the classifieds.

I made mine in the lathe and they are on the plane (not flown yet!)

Sorry for the length... all discussions, questions, criticisms are vastly encouraged!
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SedanFloral.com
RV-7A :: Penn Yan XS-IO-360 :: Tip-Up :: SkyView

Last edited by Jonathan Cude : 10-06-2006 at 11:58 PM.
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  #5  
Old 10-06-2006, 05:39 AM
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Default They're strong

Jonathan,
There's an easier way. I picked up the phone, called Van's and was told that they'd been tested to 170 MPH and held. (Those itty-bitty aluminum ones that scared me), so I don't lower flaps until I get down around 120 MPH and then no more than 15 degrees. Then, after 100 MPH, the rest.
Regards,
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  #6  
Old 10-06-2006, 05:53 AM
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Default 4130

i agree with all you have said but since i dont have a lathe and 45.00 for 1.25 worth of tubing i'll get the thicker tubes and make a set of my own. i doubt the aluminium would fail but they are miserably small. van probally visualized a tappered tap that would self center as the tube was being tapped. thus putting the threaded portion past the possible thinned wall. i have a set of hand tapped one that can be tested, less the rod end bearings.
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  #7  
Old 10-06-2006, 06:11 AM
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Default

I believe that Van calls out max flap extension speed of 110 mph for up to 20 degrees, and 100 mph for 40 degrees. I think the supplied aluminum tubing is OK, (there are a lot of RV flying with it for many hours). If it were really a problem, Van would have changed it. Having said that, when I built my -6 in the early '90s, I thought the pushrod looked pretty flimsy, so I used 4130 tubing (maybe 5 dollars).
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  #8  
Old 10-06-2006, 06:35 AM
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Default Ignorance is bliss

Sometimes I think it's better to not be bright enough to scare yourself!

I too thought the tubes looked a little flimsy and I noticed that the factory 9A that I got a ride in had hex tubes. But then I checked the NTSB records and could only find one case of an actual failure, and it was caused by not making the hole in the fuse big enough.

Since then, I've seen several at fly-ins with improperly small fuselage holes and even seen bent tubes. It sends a shiver up my spine. Especially when I realize that I've only seen a tiny fraction of all the RV's in the world and if I've run across one with bent flap tubes, there must be dozens.

And yet, only one actual incident.

Thanks for doing the math. It makes me feel better about those guys flying around with bent tubes, now that I know the ultimate limit is 200 mph, Vans tested to 170, and apparently a lot of guys ignore the 100mph Vfe and regularly drop flaps at 120 with no ill effect.

Posts like yours prove that there are lots of smart guys out there doing plenty enough worrying for all of us.

I envy folks who don't worry about stuff and just enjoy the ride.
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  #9  
Old 10-06-2006, 07:20 AM
RV_7A RV_7A is offline
 
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Default

Just so everyone knows. Vans sells the hexagonal rods already drilled and tapped for use with the flaps. Its just another part that they are to "fat, dumb and happy" to upgrade in the kit for everyone. Just pick up the phone and order. Problem solved.

-Jeff
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  #10  
Old 10-06-2006, 08:48 AM
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Jonathan Cude Jonathan Cude is offline
 
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Default just to be sure

Quote:
Thanks for doing the math. It makes me feel better about those guys flying around with bent tubes, now that I know the ultimate limit is 200 mph, Vans tested to 170, and apparently a lot of guys ignore the 100mph Vfe and regularly drop flaps at 120 with no ill effect.
My math leaves the ultimate speed sort of unknown, I would be amazed if the true ultimate flap extension speed is within +/- 10% of 200 mph.

All in all, glad to hear Van went to 170 with the "little" rod. Still, the Vne for the flap should be followed... can't think of a reason to fly around at 170+ with my flaps down unless there was an emergency of some sort.

In regards to bent rods... I bent one myself making the cut-out hole. A bent rod in a flying plane really makes me shudder. Its critical load strenght has probably been significantly been reduced.
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RV-7A :: Penn Yan XS-IO-360 :: Tip-Up :: SkyView
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