Stronger Wheel Pins

[Andy J]

Did some high speed taxi runs and short hops today in preparation for the first flight of my 91inch Chipmunk. These went well although on inspection of the model I found that the wheel hubs are now rubbing on the inside of the oleos so assume the wheel pin has bent as I hopped down the runway.

 

On inspection each pin is only 4mm in diam, so what is the recommended fix. Do I drill out the oleo to accept say a 5mm pin or try to find a stronger steel in 4mm?

[Andy Gates]

Andy,

 

Is it the pin or the lower strut that is bent?

Either way, it is obviously not up to the job for your machine.

 

Whichever it is, it needs to be manufactured from stronger material either by increase in size or better quality.

Try speaking to a local engineering workshop - they may be able to help / advise you.

[Paul De Tourtoulon]

You will also need to make a new bottom piece for the legs out of steel the aluminium will also bend.

 

 your bent bit needs to be made out of at least 5mm piano wire.

[Geoff S]

On smaller models I've often used 4mm or 5mm high tensile cap head allen bolts. 

[Don Fry]

Your current point of failure is the top hat with two holes in the top, looks like an aluminum unit. Needs to be steel as a starter. And longer so it sits deeper onto the leg? Currently you axle bolt looks straight. It should be stiff, 12.9 steel.
 

Heavy machine. My take, if I tried high speed taxi, and hops, the return to ground would not be a kiss. It would be a wallop. So I’m unsure how much stress you put on it. You could be far better than me. Bottom line, it’s heavy enough to break something is it hits the ground too hard. A weak point, easily fixed is not necessarily bad. 

[Ron Gray]

I’m not sure if they are the correct axles for that leg. Normally the portion that fits in the leg would not be threaded as the 2 grub screws in the bottom of the leg hold the axle in place. I would be inclined to use 4mm or 5mm HT bolts as per Geoff’s post above if you haven’t got any other suitable material.

[Andy J]

Like the idea of HT bolts so will tap out the oleo to accept a M5 cap head bolt. 

[Andy J]

Having looked again at how little alloy there is on the oleo, rather than using a M5 tap to fix the axle to the oleo bottom end boss  I simply drilled it out to 4.81mm to match the shank diameter of the cut down M5 cap head bolt. So will see if that fixes the issue. If it does fail again think I will have to investigate if I can manufacture a bottom end of the oleo in steel.

[Ron Gray]

You could also cut down the oleo springs by 2 or 3 turns, this will make the ‘suspension’ softer and more likely to absorb bumps. It’s surprising what a difference that can make.

[Andy J]

Good point Ron as they are quite stiff. Mind you the model is heavy and the max travel is only about 30mm. Must get round to buying some second hand scales.  Did look at some the other day in a charity shop but couldn't get them to work but sure plenty are available on ebay.

[Ron Gray]

Most ‘cheaper’ oleos are oversprung so compress very little under the plane’s weight. 

[Andy J]

Did do quite a few bounces as I touched down on the last fast run so sure one or two managed to compress the oleo's. Now put some expo on the elev channel to avoid that in the future.

[Bruce Collinson]

Electronic digital scales are ridiculously cheap on Mr. Bezos’ site. He probably needs your £10.   Mine are I think 5 kg but there are bound to be versions which will weigh more.

 

one the one hand they are undoubtedly Chinese.

 

On the other, I tested mine with a pair of calibrated weights from top end N. American digital scales which cost a lot more, and still use a strain gauge, and they are spot on at 25-75 grammes, and realistic at 2-3 lb.  They are extremely useful.

 

I see that the PRC has a much worse drought than we have.

 

BTC

[Martin McIntosh]

I use digital luggage scales which easily weigh a 25kg suitcase. About £3 on ebay.