REFLECTOR:Gear Speed

[reflector@tvbf.org]

At 02:27 PM 12/26/03 -0600, John Dibble wrote:


>Scott Baker wrote:
>>The air stream doesn't offer a lot of force against the split nose gear 
>>doors.
>How do you know this?  Have you ever measured how much force there is?

We measured the force on the forward gear doors on Berkut, which are very 
similar.  At 120 mph it was 12-15 lbs.

>>   The weight of the nose gear leg, casting, wheel, and tire offer a 
>> goodly amount of weight.
>So prove to me that the air stream force on the doors is less than goodly.

What we found was that the force on the gear was much more than the force 
on the doors.  Since the doors opened sideways, as soon as they cracked, 
the force went to nothing and they opened up nicely.  At higher speeds the 
gear would come partway out of the well, but not get anywhere close to 
overcentering.

We went to a system where springs were trying to open the doors, and the 
nose gear strut was keeping them closed.  As soon as it started to open, 
the doors opened immediately.  The door springs also helped push the gear 
down an inch or so.  But it's common in gravity fall extend systems to have 
to slow the plane down a lot, and for the over center spring to just be 
strong enough to kick the links into their over center position.

There's another failure mode that I feel I must mention.  As was previously 
said, gas struts get weak when it's cold, and can bleed down their pressure 
over time.  But every once in a great while they can seize.  A little bit 
of grit gets in the orifice and stops the piston from moving in either 
directions.  Imagine if the gas strut was suddenly replaced by a steel bar.

Now, I'm saying this as a general observation.  I haven't worked with the 
Velocity retract system, and it may well not apply.  In most applications 
I've seen (like Lancair) if a strut seizes in a normal gear extend, it will 
probably rip the mounting ball out of it's threads.  The real danger is if 
it happens on retract.  Then the hydraulics drive the rigid strut through 
whatever structure is behind it, or twist it into a pretzel.

We first made a deep cup that the body of the strut fit into, so if it 
siezed on extend it could pull out of the cup, and telescope to it's full 
length.  Later we made telescoping tubes the same dimension as the gas 
strut, but with just a compression spring inside.