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Why Fill Gear Leg Fairings with Foam?
- Thread starter donmtt
- Start date
Ah! So that was an attempt to deal with wheel shake on landing? Interesting. It will be interesting to see if the shimmy in my RV-7 (which is sometimes pretty bad and other times non-existent) is any different now that I've cleaned out the foam....
Welcome to -7 world; Please download a set of the plans and instruction manual asap. They'll really help you in the long run.
Just one of many ways to skin that feline.
Gear leg "shimmy" is solved by attaching another mass to the gear leg -- much like a tuning fork that you want to dampen its response. Van's describes this in DWG C3, Zones F-G x 5-6.
Internal fixation for the gear leg fairings -- a little "Great stuff" or 2-part pour foam at the top and the bottom 1.5 - 2" and things stay put...sort of.
Just one of many ways to skin that feline.
Gear leg "shimmy" is solved by attaching another mass to the gear leg -- much like a tuning fork that you want to dampen its response. Van's describes this in DWG C3, Zones F-G x 5-6.
That always solves it for me, but with tires + tubes coming in at $600+, it would be nice to find a solution that didn't leave a fair amount of tread on the throw-away's...
As time went on there were other approaches. Bonding wood to the gearleg, and then mounting the fiberglass fairing over it (but not bonding the fairing to the leg). Certainly is easier to get the leg fairing aligned with the airstream that way. A misaligned gearleg fairing can appear as a "heavy wing".
Be careful with foam if you choose that path. Several reporting's (I've seen the pictures), where the foam trapping moisture leads to severe gear leg corrosion. Only if the legs are properly prepared and primed/painted prior to foaming. I bonded wood stiffeners with proseal and fiberglass tape wrapped mine on my RV-4. One piece fiberglass fairings, no shimmy ever. Tires balance as others mentioned is important.
The foam was in the gear leg fairing, not the wheel pant.
And I wouldn't do it. It was an old and now discarded idea.
Drowosek
Member
I see what you mean. It all comes down to the near-total absence of hysteresis in metals, especially in hardened steel. For this reason, we used only fiberglass landing gear springs on our aircraft. The micro-movements of the plastic layers significantly dampen the energy of the vibrations, converting it into heat.
I assume you have fiberglass fairings and he has foam the entire length of the gear from wheel pant to side of fuselage?
I am guessing the intent of the builder. Many people try and DAMPEN the gear leg, a taper steel spring gear. Normally they work great per plans, with no extra "mod". However sometimes they can vibrate, shimmy "like" a bad wheel on grocery cart. I say like because there is no castoring wheel, it is rigid. But think vibration. Not going to explain the complex nature of vibration and this "SYSTEM" of gear, tire, wheel. Why because it is complex and I have a theory how to mitigate without adding gear "dampening".
Not sure how effective the foam is. I think little to nil. My GUESS is he thought this was beneficial for the reason of vibration.
However it is also a way to secure and support the gear leg fairing is/was another reason. Normally the gear leg fairing is just supported at the ends, near the fuselage and at the wheel pant fairing. I could be wrong, may be it is in the manual, but filling it with foam is a good way to corrode your gear leg.
Back to main gear shimmy. Some RV's do it more than others or only under certin contionds, FAST TAXI or hard braking. A common MOD is to bond, strap, a hard wood stiffener to the steel gear leg, not to make it stronger but dampen out the vibration, or shimmy.
My suggestion is RE DO IT... That foam will trap mosture and could cause corrosion of the steel.
First RV (RV-4) I had a wood working friend make a really nice double taper oak strip that was radiused to fit on taper gear leg. It was attached with strips of carbon fiber wrap (with epoxy) top, mid and bottom.
Second RV build (RV-7) I went nothing and no problem with shimmy. YOU DO have to balance your wheel tires way better than RED DOT at Valve Stem, good enough. I go further than standard static balance as well with call it semi dynamic balance. Tire pressure, surface, speed all can cause a vibration... Also the WHEEL FAIRING has to be balanced.
BoydBirchler
Well Known Member
When Grumman American came out with the aerodynamic improvements to the Traveler, they discovered their new fairings were pumping air down the inside of the fairing causing drag. The fix was to make a rubber fairing that fit tight around the fiberglass main gear, eliminating the interior flow of air , thus decreasing total drag. Foam filling our main gear fairings would possibly do the same.
Interesting, never heard the Grumman story but makes sense. Grumman was BTW (the Yankee) was Jim Bede design. The Grumman Yankee, Traveler, Tiger all are like RV's in many ways.
With that said, being in the RV building, owning and flying game since late 80's never heard this about this as a concern for RV. Not saying it does not exist. My post above is my understanding of the foam filled gear leg. The reason not 100%, but it was a "fad" for awhile. Someone did it others did it. Was it for air flow control? Not sure.
History, Van use to have metal gear leg fairings. They were shorter cord, not as aerodynamic as the fiberglass ones. The one piece metal ones kinked in the middle sometimes, so it was made into two piece with an overlap mid length. The TEAM ROCKET guys (the stretched RV-4 with (I)O-540 they made even longer chord gear leg fairings. I have them. Van addressed these thinking these could have negative effect in aerodynamics and spin recovery?
With that said I will seal one gear leg fairing ends using good cuffs, transition fairings, at the ends. You don't want AIR getting in there in the 1st place for sure. If that air in gear leg fairing PUMPING of flowing back out into the airstream again, that Interference Drag, mixing of two different airflows. You do not want that either. Thanks for the Aeronautical fun fact.
