Tire deflection?

[Redfeather]

Hello, Engs.
I am trying to find how to calculate airplane tire deflection if the plane runs over a small manhole lid if the 5 inch diameter lid is recessed about 1/4" below the parking apron surface. The ultimate goal is to calculate the load on the lid.

Thank you for your time &b help.

 

[GregLocock]

The easiest, cheapest and most accurate way is to measure it.

However you won't believe me, so I'll treat this as an interesting but probably futile exercise.

What size tire are you talking about?

for example, a 1" diameter tire would behave very differently to a 24" diameter tire.

What load is on the tire? what speed is it running at? what psi is it?





Cheers

Greg Locock


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[CapriRacer]

Like Greg, I'm struggling to get a handle on the problem.

Airplane tires come in a wide variety of sizes. Some would completely sit on the manhole cover and therefore the load would be 100%. Some would completely span the cover and only a portion of the load would be on the cover.

But for heaven's sake, do NOT assume the load would be somehow calculated from the inflation pressure. Footprint loads are NOT simply the footprint area times the inflation pressure. That's a good estimate for passenger car tires but not for anything else. For example, truck tires are on the order of twice that value.

Perhaps context would help. What kind of airplanes are we talking about? Where is this cover? You said "parking apron" and I assume that means an area outside where airplanes would be parked - and not in a hanger or on a taxiway or runway.

 

[NormPeterson]

How any wheels are involved and is there any manner of load equalization involved? Since your scenario assumes a 1/4" depression, one rigidly suspended wheel in a pair would impart slightly less than half the total load on the pair.

But . . . is there any chance of the manhole cover being 1/4" raised above the pavement, where it would carry more than one tire's equal share of the load? . . . What about an impact factor to account for any suddenness of the loading occurring?


Norm

 

[FeX32]

The attached may provide some insight. The study was done on motorcycles, but I would imagine that apart from massive weight difference, suspension geometry and wheel placement differences, much of the dynamics would be similar.
They also seem to mention that direct measurement is difficult.

Cheers,


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[FeX32]

By dynamics I am referring to only the tire-surface interface.


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[GregLocock]

Well that's a fine lot of maths. However in the real world we've found that measuring tire deflection directly is so much more usful than attempts to infer it that we don't bother trying. You can use lasers to measure the distance fromt he hub to the ground, or you can use a high speed camera and spend a happy day clicking on the screen. The huge advantage of the latter is that you can see what the tire is doing, so you can see if your mental model is sensible. A decent digital camera with a high speed setting costs less than grand.

On the other hand strain gauges cost approximately $1 (plus several hundred dollars in installation) and will tell the OP the exact answer he is looking for, the force in the plate.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[FeX32]

On the other hand strain gauges cost approximately $1 (plus several hundred dollars in installation) and will tell the OP the exact answer he is looking for, the force in the plate.

I agree. It would be interesting to conduct both an analytical/computational approximation and an experiment and correlate the 2.
After a number of experiments have been correlated with models, the models can be used in the future to save time and money.


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[Redfeather]

Tire Experts.

Thank you for your help.

The parking apron is where the planes are parked for refueling, maintenance, etc until they are needed. The planes in question are C-5s. From the internet the tire is 49" tall x 17" wide. Also, the government has a specification defining wheel load by simulation of roving 200,000 lb load applied perpendicular to a 10"x 20" contact area. The speed would be taxiing or very slow.

Also, if the lid in countersunk 1/4" below the concrete should the concrete be radiused to reduce tire damage & wear?

 

[GregLocock]

Well thanks for explaining, in my opinion almost any obvious model would do, and yes, a 30 degree chamfer would be a nice thing to do.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[CapriRacer]

I agree with Greg. But I think it is unnecessary to radius the concrete - nice, but unnecessary. Rolling tires just aren't subjected to the same level of forces that driven tires are.

But I think this is more of a "designing airport aprons" problem - something that is a larger subject than just tires.

And I would think that there would be a "Best Practices" manual for designing Air Force runways, taxiways, and aprons. And on the off chance that we're not talking about the US Air Force, I would suggest you try to find someone who has access to such information. I can figure out a way to get the information needed, but it requires someone to instrument an installation. That would be interesting to verify the theory and could be used for future design.

 

[cibachrome]

Contact the US Army Aberdeen Proving Ground in Maryland. They have the ability to test a C5 tire using one of the spare platforms of their MTS Flat-Trac Roadway Simulator that has been turned into a tire tester.

Greg Schultz made up a framework to mount such tires and can roll a puck or a depression under the tire when rolling at ground speeds. Loads and deflections for the spindle and roadway are routine measurements.

(I've been there to do the very same thing on some slightly different tires).