A year or 2 ago RCJI had a article by Tom Wilkinson regarding some work he had done on thrust vs. pipe deminsions. I am trying to find out more about this, especially the deminsions. I beleive Wren also did some testing and had published the different dimensions for different size turbines. Does someone have this info they could share with me. Could you scan and email me?

Tailpipe from Tailcone
Pipe diameter
Convergence Diameter
Convergence Length

Thanks
Doug

I think it was John Wright who did the testing on the pipes and he is often on RCU.

Jason

Yes, John Wright RCJi Editor is the worlds expert on tail pipes...

Dave Wilshere

Doug,

I sent you a PM.

Thanks all. Yes Sara at Wren sent me the article. I am using them on my Jetcat's. Does anyone know what size turbine wheel is on the P-60 and P-70? Also the RAM500?

Thanks again.

Doug

Hi Doug

I see that you have the article but if there is anything else I can be of help to you just let me know. My P60 turbine wheel is approx 51mm and the Ram 500 is 55 mm as is the Wren MW54. Not sure about the P70.

John

I've always wondered about these pipes. Unfortunately I don't have the article but can anyone give me a very basic idea of how the tests were done?

Working on the basic premise that you don't get something for nothing, what do you lose for this thrust increase. Is it just a simple divergent duct with associated gas velocity decrease?

Thanks John. Would you think that all 3 would use the same pipe with roughly the same results? Or would you suggest a different pipe for the P60 since it is smaller? Also on the distance between the tailcone and the pipe mouth, is that a constant at 30mm for all? Seems that all I have read about they say 25-30mm for that distance.

Doug

I don't know of anyone using one of our pipes on a P70, but I know the 54-size pipe has been used on P60s with success.

We used to recommend 25mm as the distance but as our engines became more powerful (the Wren 44 went from 7.5 pounds to 10, the 54 from 12 pounds to 15 or more) we found that the pipe had to be moved further away and now recommend 30mm.

Sara Parish
Wren Turbines

According to Rolls Royce's book of the jet engine, the pressure and flow energy multiplied by the area at the outlet is greater than the pressure and flow energy multiplied by the area at the inlet, so there is a net forward force. At high throttle the flow is likely to be at the local speed of sound or above, leading to an increase rather than decrease in speed in a divergent duct, the increase in momentum being reacted as a pressure on the wall of the divergent duct which being angled has a forward pointing component.

Doug

The P60, Ram 500 could use the same pipe size that was developed for the MW54. The P70 will probably need a larger size pipe as for the KJ66 in the article. The MW54 pipe would be a bit small. The way to find out is to run it and measure the thrust and the temperature. If the thrust gain is low and the temperature goes up then it needs to be a larger size. The distance from the exhaust to the bellmouth was not that critical and is now usually set at 30mm. The divergent section only needs to be a short section and not too steeply divergent. It proved important that there was a straight section after the divergent section.

All I can say is that it works, use a digital fishing scale to measure the static thrust with a 'normal' parallel sided pipe, fit a divergent type pipe of the right size and you should be able to prove to yourself that you are gaining about 5% thrust.

I did the intial tests and then asked Tom Wilkinson to replicate them independently and publish the results.

John

Having thought about it after HarryC's explanation I can see why it would work. I didn't realise that the exhaust velocity in our little engines would be supersonic and therefore the duct would be working 'the other way round'.

How does the thrust vary with airspeed? Is it only static thrust that is increased or is this increase carried throughout the speed range?

p.s. Thanks Voodoodb for the article.

I can verify that they do work....and well!
Being the "doubting Thomas" type, and having to double check things before they are printed, I did a thrust (and loss) test with my MW 54 Mk 2.5 (a Mk II with a Mk III compressor and bell mouth) a few years ago.
My platted front porch is at 6000' msl. My "14.5 lb engine give exactly 10 lbs (several tests, calibrated thrust rig). Add the Wright Pipe (aka: Augmentor pipe) and we now get 10.5 lbs of thrust, and I was not able to get a zero friction set up...so posibbly a 6-7% gain, not just the 5 % claimed, and measured).
Soooo......we see a 45 % loss for altiitude (more than the tables say exists) and a 5+% gain from the tail pipe. YEH! on the gain....boo on the altitude loss!
Greg

I can verify that they do work....and well!
Being the "doubting Thomas" type, and having to double check things before they are printed, I did a thrust (and loss) test with my MW 54 Mk 2.5 (a Mk II with a Mk III compressor and bell mouth) a few years ago.
My platted front porch is at 6000' msl. My "14.5 lb engine give exactly 10 lbs (several tests, calibrated thrust rig). Add the Wright Pipe (aka: Augmentor pipe) and we now get 10.5 lbs of thrust, and I was not able to get a zero friction set up...so posibbly a 6-7% gain, not just the 5 % claimed, and measured).
Soooo......we see a 30% loss for altiitude and a 5+% gain from the tail pipe. YEH! on the gain....boo on the altitude loss!
Greg

It also works when the flow is subsonic and slows down in the divergent pipe. Although the flow speed falls its pressure rises and the total multiple of gas energy and pipe area is greater than the same sum at the start of the pipe due to the area rising more rapidly than the gas energy reduces. Thus the force at the back of the divergence is greater than at the front of the divergence and the result is a net forward force on the pipe.