Model turbojet characteristics

On 18 Nov 2006 10:47:47 -0800, "Adam Chapman" wrote in :

Adam,

I can't do the search right now--I'm in DC at a conference--but there is an RC jet organization that probably has all the answers.

I can't think of the name of it right now.

Sorry to be such a dry well, but I wish you well on your project. It sounds like fun.

You might browse through AT's links. I'll bet he's got links to pages that may help:

Marty

There is a jets forum on RCU:

There is also an e-mail list that can be accessed by sending a message, snipped-for-privacy@kidsource.com and typing "subscribe" in the subject line. The listmaster for this list is a professional engineer who designs/builds UAVs for a living, and did design work on the F-117 stealth fighter, so he knows his business!

Also you can contact the Gas Turbine Builders Association in the UK. Google it for the URL.

Ooops, mistake on the e-mail list:

What in the world was your powerplant selection criteria, then? Frankly, you haven't come across as part of university group that is designing "anything."

Abel

Thanks guys. Your help really is appreciated.

Abel: The selection criteria was based on the thrust-weight ratio neededto meet the performance specification. The jetcat p-180 was chosen because it had tfavourable thrust and fuel consumption, and a fairly small diameter so we can use a complete flying wing. My job now is to suppress the IR and acoustic signatures, which is why i need to get hold of this information.

Thanks guys. Your help really is appreciated.

Abel: The selection criteria was based on the thrust-weight ratio neededto meet the performance specification. The jetcat p-180 was chosen because it had tfavourable thrust and fuel consumption, and a fairly small diameter so we can use a complete flying wing. My job now is to suppress the IR and acoustic signatures, which is why i need to get hold of this information.

Sorry for the skepticism in my reply, if unwarranted. Your brief description of the UA you are designing seemed to indicate an intention to focus on performance goals that are suggestively military (or paramilitary) in nature, i.e., speed and stealth. I hope your project is legit and you don't wear a towel around your head.

The engine parameters you seek (i.e., exhaust gas temperature and velocity) should be quite easy for the member of your design team charged with propulsion to measure or derive. Re taipipe gas velocity for example, simple momentum theory will readily yield the answer. Thrust (a given) is by definition the axial rate of change of momentum of the ingested air mass, the tailpipe cross-section area is known, ejecta density can be accurately known after measuring the temperature, so exhaust velocity can be determined for any flight velocity where the the thrust is known - including zero flight velocity if that is all you have available.

Hey, if this is a design team effort, it only makes sense to get the responsibilites for design data assigned to the proper member of the team.

Abel

I appreciate what you area saying. I also know how to work out the basic variables like the velocity change across the engine, although this is useless if the mass flow rate is unknown. Myself and the propulsion guy are both becoming desperate to find the information so we are both looking everywhere we can think of at the moment.

My primary concern is the noise levels and directivity of that noise. Even if anyone has noise data for similar sized turbojet engines (The diameter of this engine is 130mm), we are very thankful for any help that can be offered. I have researched methods of reducing the acoustic signature but I would really like to be able to predict it real strength.

Adam

Adam, besides the sites I previously suggested, you might contact the BMFA

I gather that you are not a modeler, so you might not be aware that the BMFA is the governing body for model aircraft in the UK. They might have, or can direct you to the data you request.

Geoff

Yo are in luck, because its mainly a high frequency hiss with very little LF roar.

Suggest you blag a spectrum analyzer and toddle down to a club where the things are actually flying. They are loud close up, but in the sky are remarkably quite.

I'd say its more 'blue noise' with a spectrum in the KHz only region.

You should be able to compute efflux velocity from the diameter and thrust. My guess is its well over 100mph./.probably nearer 200mph Dunno what exhaust temps are, but a bit of bypass may be possible to drop that and the efflux velocity as well. Wren make a geared turboprop, so they may be worth contacting about a turbofan setup.

I think simply adding in ram air to the tailpipe would be a start..get the temperature down and the efflux velocity and noise down.