I know the nose gear thing has been done to death already but here is a new angle on it, which just might save your nose gear. The key takeaway is:
Don?t brake on bumpy strips (if you can avoid it). Read on to understand how I came to that view.
In deciding whether to build a TW or tri-gear RV-7, the key issue for me was whether the nose gear would be able to cope with grass strips. I read everything I could find but it left me feeling that I?d be gambling with $50k and 4 years work if I operated a 7A out of a roughish strip. I became intrigued and set about analysing the gear design. What emerged is that the nose gear has relatively little vertical travel ? just 4? (1?? for the leg and 2?? for the tire).
There are two common situations that require a lot of suspension travel: Landing and bumpy ground. I?m going to assume that nobody expects to land on the nose wheel on bumpy ground and get away with it, which means we can treat nose wheel landings and bumps separately.
Van?s deals with the landing case by advising pilots against using the nose gear as a landing gear. That?s good advice, because the RV-7A has much less suspension travel than would be needed to meet the FAR 23 three point landing requirement for certificated airplanes. Put simply, it?s not designed for nose wheel landings.
Van?s suggests the 7 instead of the 7A for rough strips but how big do the bumps have to be before it counts as a rough strip? A way to think of it is that you have a maximum of 4? of suspension travel and part of that is taken by the factors that pre-load the gear (static weight, braking, residual compression from any earlier bumps and forward stick positions). Any remaining suspension travel must be greater than the bump height, if the leg is to survive. Tire pressure also plays a role, because it determines (in part) how much of the travel is taken up by the pre-load.
The graph below shows the compression in the gear, as a function of braking and tire pressure. This is for worst case loading conditions of 1800 lb all up weight, 375 lb static nose load and a 45? CG height. The travel available to absorb bumps is simply the 4? total minus the value from the graph.
The gear compression with no braking is around 0.5?, leaving scope for bumps of up to 3" or so high, before the leg fails. However, the capacity to deal with bumps decreases under braking, dropping to just 1? under maximum braking (defined as when the rear wheels lock-up). The blue (25 psi) line flattens at the top, because the tire bottoms on the rim.
Pulling the stick back will reduce the load on the nose provided there is enough airspeed. However, that still leaves braking at low speed, where the elevator authority is low. Conversely, forward stick positions will increase the nose gear compression but I would argue that that represents poor piloting technique. The other source of pre-load is unrelieved compression from a previous bump, or from dropping down on to lower ground, but that?s going to have to be the subject of a future post.
In the meantime, avoiding braking on bumpy strips could be a good plan, especially at low speed.
Just getting my flame suit now?
Raiz
Suspension travel is the key to RV-7A nose gear collapse
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