Long-EZ main gear melt downs
January 1992
We continue to hear from at least one builder/flyer each quarter who has had this problem. We have reported on this problem several times in past Canard Pushers yet it continues to happen. The bad part is that each flyer who we hear from acts as though they had never heard of this problem and why weren't we warning people about it?! It is quite frustrating for us at RAF because this is a problem that, frankly, does not need to happen - should never happen, in fact.
If you have a Long-EZ (or know of someone operating a Long-EZ) who is using the heavy duty brakes, this is what must be done to fix this potential problem. You must install heat shields between the axle mounting flange and the glass/epoxy main gear strut. This shield is purely a radiant heat shield and, as such, must be large enough to prevent the hot brake disc from "seeing" the gear strut. A fan-shaped, 1/8" thick aluminum heat shield that extends up above the brake disc by at least 1/2" works fine. You don't need any more than that. We have seen more gear legs damaged in this way than we care to think about and @ of them had exactly the same damage: namely, the epoxy had been boiled or vaporized out of the glass strut directly opposite the brake disc. The damage was confined to an area the same shape and size as the disc. The damage can, and does, occur even through fiberfrax insulation. I repeat, this damage is caused by radiating heat from the red hot brake disc and is locally confined to a small section of gear leg directly opposite to and the same shape and size as the disc.
To our knowledge, this damage has only ever occurred when tight fitting wheel pants were installed. Apparently with no wheel pants, the disc gets enough cooling air flowing over it to keep it from getting hot enough to do this kind of damage. So - if you have tight fitting wheel pants, expect your brake discs to get very hot and protect the gear with an aluminum shield. In addition to the radiating heat damage, it is possible to generate enough heat inside an unvented wheel pant that this trapped oven-like heat can soften the epoxy and cause the gear strut to bend, usually at the highest point in the wheel pant. To protect against this kind of damage, you must wrap the strut from axle to the top of the inside of the wheel pant with Fiberfrax insulation, held in place with silicone (RTV). We have found wrapping over the Fiberfrax with aluminum tape makes a neat job and helps hold the Fiberfrax firmly in place. This will help the "oven heating" problem (as opposed to red hot radiation), but you must provide a place for this hot air to "chimney" out of the wheel pant. A vent of some kind is needed. This vent should be placed at the highest point in the wheel pant when parked, whether you park 3-point or nose down. This position may change a little depending on the wheel pant design. The important thing here is that the vent must be high to allow the trapped hot air to flow out and pull cool air in around the tire. These two fixes together will help prevent a softening of the epoxy-type failure.
The NACA scoop-type inlets and outlets we have all seen on wheel pants may have some value but you really need the cooling after you come to a stop. Cooling the brakes during braking probably has some value but these NACA-type cooling scoops are generally too low to allow good chimney venting when parked.
The single most important thing is not to conduct extensive braking/taxi testing with wheel pants installed. Do all initial taxi tests with no wheel pants. Once the airplane has been test-flown and signed off, generally you will not find a need to do extensive taxiing/braking. If you do have to check-out a new pilot, for example prior to his or her first flight in their own EZ, remove your wheel pants before you allow someone to practice for their first flight in your airplane.
If you have to taxi a long way with a strong crosswind, for example, the full length of a 10,000 foot taxiway on a day with a 90 degree, 30 knot crosswind, you will have to ride one brake all the way. Under these circumstances, you might consider removing the affected wheel pant as soon as you park. This small inconvenience is tiny compared to getting stuck in some remote area, miles from home, due to a failed gear leg.
And if you are unfortunate enough to fail a main gear leg due to heat, contact Mike Melvill at Scaled Composites to borrow his steel splint that was made specifically to ferry a Long-EZ home with this problem. So far, it has been used on two Long-EZs and one Cozy and it will fit left or right Long-EZ main gear legs!