The pitfalls of building a Replica Spitfire

Took the broken casing to a local engineer today he said a fault in the casting caused the faliure hense why the relief valve didnt operate just need to get a new case and all will be well.:)

Looks pretty damn good with that prop wizzing round though! I’m not sure we would get away with that here in the states!!
I do have an article somewhere on the perils of ground running a Merlin at high speed then chopping the throttle Or prop drive in this case! the big end bolts will fail on a Merlin on deceleration not under load. Royces wern’t really sure why, but just said they did…………..Paul……………..

That`s right, well, mostly. Royces almost always knew what caused failures in thier engines. It is a common problem caused by vacuum loading the conrods, high rpm with the throttle closed or even just high rpm off load puts a stretch on the rod bolts. If a racing engine is going to fail in this way it is often on the overrun going into a corner at high rpm.
That is why the Merlin and Griffon manuals stipulate that terminal velocity dives where the engine goes over its normal maximum rpm must be carried out at either full throttle in the case of the Merlin, or a certain percentage minimum with the Griffon if I remember correctly. There is nothing in the books about not shutting the engine down quickly, in fact one of the tests is to do just that to ensure the engine does not stall.
The Alvis Leonides apparently has a potential problem with the valve gear if you snap it shut, but I have never seen anything in the manual about it.

Pete

I wasn’t aware the rev limit for the engine was 2750, I always thought it 3500,it may be that my rev counter is not reliable as the engine seems to run happily at indicated speeds of 3000+ without any trouble from the limiters. All the internals on the mags have been replaced with brand new bits so they should work properly.
The pump is connected by a flexible pipe to the motor with a bypass allowing oil to go straight back to the tank controlled by a valve in the cockpit. The idea was to be able to run the engine without the prop spinning at all if required by by passing oil back to the tank without passing through the Hyd motor.
I would go along with Stuarts idea that two much pressure caused the pump to fail. The Hyd motor has got a built in pressure relief so that should have vented excess pressure, not sure why this didn’t happen.
You can see from the video the pump failed when I pushed the revs up not from throttling back.
It was never my intention to drive the prop any where near that fast just got a bit carried away.:o
Once the pump is fixed (unlikely) or replaced I will put another pressure relief valve between the pump and the motor and open up the by pass valve to allow more oil to by pass the motor, and keep the revs below 2500 RPM.
Once I ascertain the settings for a prop speed of about 100RPM I will retain those settings.
From a quick calculation I estimate the prop in the video to be running from 4 to 500 rpm when the pump failed.
The Hyd motor does not drive the prop direct there is a torque hub which gives a 5 to 1 reduction from motor to prop.
I have tried electric motors to drive the prop and they just end up burning out or turning two slowly, it’s a big prop to spin.
The engine certainly runs allot smoother under load and I am sure I am on the right track.
Building from scratch is always trial and error, I did my home work matching up the pump, hyd motor and torque hub but you never know until you try it. I am sure that if I had not driven it so hard it would have worked fine.

Looks pretty damn good with that prop wizzing round though!

Yes, I have to echo that, the overall impression is very realistic…..no wonder people ask if it can fly! 😀

From the video it seems you have a bit of unbalance in the prop spinner as well. This could work its way back to the driveshaft and put a vibration on the pump/motor.

Presumably you have a pressure relief valve in the hydraulic circuit? normally hydraulic pumps are directly driven from the crankshaft, so i would have thought the pump splitting into two, was excess pressure, maybe caused by rapid deceleration of the engine (throttling back) but a relatively constant speed at the hyd motor, due to prop blade inertia; don’t forget a pump is a motor and viceversa, depending on which one is being driven.

Looks pretty damn good with that prop wizzing round though! I’m not sure we would get away with that here in the states!!
I do have an article somewhere on the perils of ground running a Merlin at high speed then chopping the throttle Or prop drive in this case! the big end bolts will fail on a Merlin on deceleration not under load. Royces wern’t really sure why, but just said they did…………..Paul……………..

Lemme get this straight.. instead of a big prop to absorb all the torque you used a hydraulic pump.. & then ran the engine to 3500rpm????????????:eek::eek:

I think I’d be tempted to run the propeller from a car (or truck) starter-motor for the next attempt (assuming you do try again).

Starter-motors come in all sizes and selecting the right ring-gear should give you a fairly realistic speed, balanced against the air resistance of the propeller, plus the system could be run independently of the engine (and wouldn’t risk damaging it). I know it wouldn’t look quite right with the propeller and engine speeds not synchronised but at least you shouldn’t be able to hear the starter-motor over the sound of the Meteor! 😀

Hi Graham

Never mind the rev-limit for the pump. . . . . the rev-limit of that engine is 2750rpm. I thought it had centrifugal limiters built into the magnetos? 3500 will not do it much good, even though the con-rod bolts should take it, the lubrication is poor at these speeds.

Another vital point is that because hydraulic fluid cannot be compressed, if you have a sudden failure in the system that does not cause an immediate leak from the feeds to the motor, be aware that the prop could in theory try and stop dead in the same way as a mechanical failure, which in turn tries to strip the propeller from the airframe. Have you got a flexible coupling between the engine and pump? Engine vibration causes all sorts of nasty things to happen, the keyway can be overloaded by more than just the torque being taken by the pump.

I am not trying to make light of what you are doing, as I said on a previous occasion, I am not qualified to design a hydraulic system for that installation, and I hope you get it working reliably.

Pete