RV-A skid plate design

vlittle · #1 12-17-2005, 10:32 AM

The other thread is so big that's it tough to find new postings. Here is a link to my design for an RV-A nose gear skid plate design. You can see a photo of my implementation below, and details are on my site.

Vern Little -9A

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Highflight · #2 12-18-2005, 06:52 AM

I see what you're doing, and I understand the philosophy.
I'd just like to suggest a consideration.

While resins in many respects are quite strong in many applications, impact resistance is not usually one of their strong suits. And we're talking BIG IMPACT here, not just a scuffing.
A properly bracketed, curved piece of steel might be better (if anything) because while steel may bend, it should still work as intended while a piece of catalysed resin will very possibly shatter. Even if the resin didn't shatter, I can imagine it being dislodged by the impact.
(To test my theory, mix up a big ball of material and let it cure. Then set it on the ground and whack it as hard as you can with the heaviest sledge hammer you can find. That will be only a fraction of the impact a nose gear might see if it drops into a hole.)

If that happens, you could then have either one big chunk, or a bunch of loose material (inside the fairing), jamming and deflecting the wheel itself and perhaps causing accelerated damage and subsequent flip where the standard arrangement might have been ok with just fairing damage.

Just thinkin' it through to a logical "conclusion".

This is one where I would definitely ask for Van's blessing before going with a chunk of resin in that area.

vlittle · #3 12-18-2005, 08:16 AM

Quote:

Originally Posted by Highflight

I see what you're doing, and I understand the philosophy.
I'd just like to suggest a consideration.

While resins in many respects are quite strong in many applications, impact resistance is not usually one of their strong suits. And we're talking BIG IMPACT here, not just a scuffing.
A properly bracketed, curved piece of steel might be better (if anything) because while steel may bend, it should still work as intended while a piece of catalysed resin will very possibly shatter. Even if the resin didn't shatter, I can imagine it being dislodged by the impact.
(To test my theory, mix up a big ball of material and let it cure. Then set it on the ground and whack it as hard as you can with the heaviest sledge hammer you can find. That will be only a fraction of the impact a nose gear might see if it drops into a hole.)

If that happens, you could then have either one big chunk, or a bunch of loose material (inside the fairing), jamming and deflecting the wheel itself and perhaps causing accelerated damage and subsequent flip where the standard arrangement might have been ok with just fairing damage.

Just thinkin' it through to a logical "conclusion".

This is one where I would definitely ask for Van's blessing before going with a chunk of resin in that area.

Yes, I did consider this. the problem with a steel or aluminum skid plate is getting it to fit good enough to transfer any impact load to the front of the nose fork (and not the back part that would tend to roll the fork) before the nose cone collapsed. Another consideration is that I used flox, not glass fibers for the skid plate.

Flox is more flexible than glass fibers and when combined with epoxy resin, can resist impact quite well.

I did contact Van's on an earlier design, and they denied that there was an inherent design problem, so that was a dead end.

Finally, any impact strong enough to shatter the skid plate would probably blow the tire and shatter the rest of the wheel fairing as well due to the impact. This skid plate is much stronger than the nose cone itself, and is designed to improve the margin of safety, not function under unlimited extreme circumstances.

No skid plate design can solve the inherent problems with the nose gear design. My 'fix' is simply to provide a theoretical increase in the design margin that was relatively easy to implement.

That being said, I'm confident that others can improve on the design. Testing any of the designs is difficult, but the debate on line is healthy.

Thanks for the feedback.

Vern Little

gmcjetpilot · #4 12-18-2005, 09:12 PM

Cant hurt and might help ride out a pot hole vs. dig in. It comes under no technical objection, no harm and might help. Hard to test and hope you never find out if it does not work if you know what I mean. Good idea George

JoeG · #5 01-01-2006, 11:59 PM

Quote:

Originally Posted by vlittle

No skid plate design can solve the inherent problems with the nose gear design. My 'fix' is simply to provide a theoretical increase in the design margin that was relatively easy to implement.Vern Little

There is a very effective design mod for eliminating this problem. Like most good solutions, it is the obvious answer... Build a 9 instead of a 9A!!!

Sorry, that was just too easy to pass up!

Highflight · #6 01-02-2006, 01:18 AM

As long as this was brought up again, one thing I thought of earlier was that perhaps someone who works with metal castings of some sort could make a mold and cast a solid piece out of aluminum.

A piece like that wouldn't add any more weight than a chunk of resin (maybe lighter even), and my mechanical intuition says that it might work more as intended if ever needed.

Pilot Dane · #7 01-18-2006, 07:49 PM

Think back to the mid 90's... The nut on my nose wheel was hitting the fairing so I cut the Van's fairing and glassed in a bubble over the nut area. Seeing the danger of digging the nut into the ground I laid up five or six layers of heavy marine cloth to a finished thickness of about 1/4". Well, while flying off my time and learning how to land my shiney new RV I porpoised pretty bad and did not catch it in time and I bonked the nosewheel. I did not think anything of it at the time but taxied back to the hangar to cool my jets. A close look revealed that I had ground a flat spot on my reinforced area. It appears as though the gear leg flexed enough to allow the nose wheel to rotate back & up until the nut hit the pavement. Without the fiberglass ski I am certain it would have dug in and been much worse.

My ski was nothing more than a thick area in the bottom of the nose fairing. There is nothing backing it up. The fiberglass flexed until it was backed up by the nut, and it was enough to save my bacon.

Live and learn and keep the nose up...

fodrv7 · #8 01-18-2006, 11:52 PM

It is admirable that a number of ?A? fliers are putting a lot of effort into reducing the risk of rollover in the aircraft.
Sitting here with my bum on the ground (Tail-dragger) I can?t help thinking they are overlooking a bit of year 11 Trigonometry and that their efforts may be in vain.

If the Nose STRUT bends back two things will happen.
1. The Wheel Base is reduced and therefore placing an increased load on the nose Strut.
2. The Strut will effectively lengthen; relative too the main gear, loading the Strut even more.

If the Nose STRUT bends back it must be because something besides the wheel is in contact with the ground. Whatever it is that is dragging on the ground, smoothing it or strengthening it is not going to reduce the friction and the more it bends the more it is loaded and the more it will continue to bend. We are not talking about snow skiing here.

I doubt that any mods other than making the nose fork rigid with a trailing link will solve the problem.
Meanwhile, I think it would be best to avoid rough fields.

If you are interested here are my calculations:
1. IF the Nose Strut bends back and remains Straight (Not Curved) it will protrude nearly 1? below the main gear. Of course it will bend in a curve, but it gives you an idea of how your aircraft comes to be poised over the nose wheel.
2. From a spreadsheet I knocked up.
Nose wheel Load Load /Main Wheel
Normal Position 300lb 400lb
Bend Back 5? 330lb 385lb
10? 360lb 367lb
15? 413lb 344lb
20? 471lb 314lb
Note that a 10? rear deflection puts as much weight on the Nose wheel as is on the mains.
Pete.
PS.
I am happy to send anyone the Spreadsheet and I welcome all to check my Trig.
P

Highflight · #9 01-19-2006, 05:12 AM

Quote:

Originally Posted by fodrv7

If the Nose STRUT bends back it must be because something besides the wheel is in contact with the ground. Whatever it is that is dragging on the ground, smoothing it or strengthening it is not going to reduce the friction and the more it bends the more it is loaded and the more it will continue to bend. We are not talking about snow skiing here.

I'll take issue with that statement because it really is about skiing of some sort. Try snow skiing with just the boots and see how far you get.

While Pilot Dane's observation about how his "solution" avoided a more serious event is anecdotal at best, there is an obvious difference between what would happen if a sharp edged object digs into the ground (the nut) and what would happen if what hits the ground were a smooth transitional piece that would allow some "skiing" action.
In fact, to take this to an extreme example, imagine if you put a ski on the nose gear instead of a wheel and you can see that the darn thing would NEVER dig into anything. Of course, taxiing would be problematic...

Anything that would present the ground with some kind of skid plate rather than a ski pole (again, the nut) would have to offer a better chance of avoiding more severe bending.

fodrv7 · #10 01-19-2006, 04:09 PM

Vern, You are quite right. Reducing the friction (Resistance) of any fixed part of the nose wheel is going to improve things.
If however, the resistance is still sufficient- a largish hole for example- to bend the strut back, then as I have pointed out, the load on the Nose Gear and therefore the resistance will only increase.

After all that is the whole point of trailing arm suspension. Deflection decreases the load.

Pete.