highhorse

My Feedback: (2)

I see that a lot of ARFs meant for engines of X lbs to Y lbs thrust range come with a pipe which is presumably useable for engines within that range.

Is there a substantial tradeoff in not having a custom pipe built for a specific engine?

Specifically, will a pipe that is meant for a 21 lb Merlin also suffice for a 13 lb P-60 ?

It's obvious that a pipe can be too small, but can it be too large?

I'm asking because I have need for a P-60 pipe NOW, but this plane may be re-engined with an M-90 later.....................

So If I plan for the future and buy a bigger pipe for the Merlin now, can I use it for the smaller engine until I do the engine swap?



I asked a pipe vendor, and he said NO. But then again, he is in the business of selling pipes. right?



Thanks, Don.

Dave Wilshere

Don

The pipe can be too small and too big! Both lead to low static thrust, the smaller dia high temps too!

A P60 with a 90-100mm (3 1/2-4") pipe designed for the G-Booster 80-90 produces very little static thrust, even throttling off and opening up involves a long wait for thrust to come back.

Overal pipe length will make a difference, the Wren designed MW54 pipe is the best we have used with a P60.

Dave Wilshere

PS Really he is right...

highhorse

My Feedback: (2)

I completely understand the temps issue, but the concept that the pipe can be too big baffles me.

Is there an easy explanation for that?

sidgates

My Feedback: (2)

I am surprised too. The ultimate large pipe is no pipe at all like the Boomerang. This doesn't seem to affect thrust or acceleration or decel. that much.

Rush!

is it the same principle that the full scale turkey feathers work on?  Since they have a variable exhaust opening allowing them to maintain a certain range of thrust/efflux over a wider range of speeds, also allowing the turbine to run in a relatively high loaded state....  if that makes sense. 

On the ARFs i'd think that the pipes to be designed to be a compromise between thrust/efflux velocity for a certain type of engine.   so a certain engines pipe could be improved by customizing it depending on the engine and at what range the turbine would work best at (I.e. Throttle range for optimum spool up, thrust vs efflux velocity, etc...)? 

so a to big pipe would be efficient at high throttle settings, but inefficient as you'd loose thrust in the low end/mid range? 

highhorse

My Feedback: (2)

My thoughts exactly. An engine on the test stand has, in effect, an infinitely large pipe. Hmmmmmmmm.

I hope Dave can check back soon to fill in the blanks (between my ears)

Dave Wilshere

Sorry, missed I needed to look again!

I guess it's that the tube needs to fill with hot fast moving gas, to get the good stuff coming out the back! The time taken to fill this affects acceleration. I'm no tail pipe expert, but we fitted a P60 on the end of a 2' long 3 3/4" tube and the thing has no acceleration! It takes two circuits to get on the step, thottle back and slow down and it is slow to get going again.
A no pipe set up has no real drag/losses.
Dave