Aeronautical Engineering/Jet engines

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Question
Hi,

I am currently building a small jet engine for a radio-controlled aeroplane, and had a few questions.

1. What is the ratio of air/oxygen to fuel (a combustible liquid) in a small jet engine needed to ensure maximum thrust?

2. Where should the centre of gravity be in a jet powered aircraft?

3. How does the placement of the jet engine (above or below the wings) affect speed, handling, etc.

If you could share any light on any of these questions, it would be much appreciated.

Thanks,

From Rimpo.

Answer
Hi Rimpo
I am not familiar with jet fuel-to-air ratios, but if you go to this website you can read about it and run a little on-line design code that lets you change engine parameters and see performance changes:  http://www.grc.nasa.gov/WWW/k-12/BGP/PAT/Fuel_and_Air2_act.htm

The question of center of gravity (CG) relates to the more general question: how can I make this airplane stable and controllable? For example, an aircraft with the CG ahead of the center of lift is stable. But most aircraft have CG behind the center of lift and achieve stability with tail forces. I suggest you get a basic aerodynamics book and read about these things.

The jet engine placement also affects stability. A jet below the wing will generate a nose up moment that could contribute to stall unless the tail moments were able to handle it. A jet above the wing would do the opposite, but it would interfere with some of the important aerodynamics on the upper surface unless the jet were on the tail. But it could be done either way. I don't think speed is an issue relative to jet placement.

Best of luck.
Paul

Aeronautical Engineering

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Paul Soderman

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Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.

Experience

38 years as research engineer at NASA

Publications
AIAA, NASA

Education/Credentials
B.S. and M.S. Aeronautical Engineering - U. of Washington, Graduate work Standford U.

Awards and Honors
AIAA Associate Fellow (American Institute of Aeronautics and Astronautics)

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