Pulsejet versus turbine

[RCModelReviews]

Some of the guys around here are having a go at the world speed record for jet-powered models (about 555kph) but I figured I'd see if I could beat them with a purpose-built pulsejet model.

To the best of my knowledge, the fastest pulsejet models flown to date have been about 400-430kph so I've got a lot of work to do if I'm to get an extra 150kph on top of that.

Shall I start a design/build/flight blog so everyone can laugh-along as I "give it a go"?

[jomarx]

OF COURSE!

nuff said

[RobC]

too right Bruce, we want to see the project from start to record!

[Joe]

Any progress?
Thanks, Joe

[RCModelReviews]

I've got the air for the tyres!

Actually, a new fuel pump for the engine turned up today and I've been doodling some airframe designs.

I'll start committing my thoughts and preliminary designs to the web shortly. I'm thinking that I'll go with a pulsejet with about 25lbs of thrust to start with so I've started the design calculations for that too.

[Joe]

25 lbs., thats a healthy size engine. How does that translate to combustion chamber diameter?

[RCModelReviews]

A 25-lb engine can be built with a combustion-chamber diameter of as little as 120mm (4.75 inches).

Life is a huge set of compromises with engine-design so you have to weigh up a number of factors when determining what the *best* size for an engine is. There are benefits to be had by going slightly larger diameter than the minimum.

[Joe]

4.75"/120mm should do the job nicely!
I was thinking if you kept the CC under 5" it would give good results. 700kph should be well within reach, a flight path that gives approx. 45-60 seconds of constant acceleration may be tricky to obtain, distances stretch into miles.

[Roger]

for this sort of attempt (speed run) then you are going to want to consider even more factors than your engine - a slippery airframe and tight control surfaces are going to be critical. I have a friend who has done (just) a bit of composite work on F1 parts while on his OE, then went in to building high performance RC planes... Brute force will get you most of the way there as I'm sure you know but the finer details may make all the difference.
Is there any way of calculating the engine thrust at various speeds for a pulsejet, or are they a less predictable beast? Concorde used a variable intake duct to reduce the airflow to below supersonic speeds so that an "normal" jet engine could be used for supersonic flight - is there an optimal airflow in / intake speed for pulse jet...?
The Dynamic soaring guys have broken the 400mph barrier (well & truely...!) A lot of them are using twin servos on the elevators and multiple servos on the ailerons. Also Airfoil selection will play a big part, especially if you want to be able to fly very fast, and yet still be able to glide in for a landing at less that a hundred miles an hour for a landing...!
I probably don't need to mention any of this to someone of your abilities... but I just thought I'd put a few more variables into the mix...

[RCModelReviews]

Yes, there are many variables and one of the key decisions when designing a pulsejet is establishing the speed at which maximum thrust will be generated. If you design an engine for maximum static thrust then the ram-effect will soon cause problems once you get any decent amount of speed on.

For example, the Argus engine used in the V1 flying bomb was said to produce about 500-600lbs of static thrust but at the V1's designed flying speed (around 360-380mph) the thrust increased to about 900lbs. This was achieved by the use of a carefully designed diffuser on the intake.

Airframe considerations are also very important. Structural integrity is one of the biggest problems -- mass balancing of control surfaces and ensuring that the combination of airframe stiffness and mass don't combine to produce unwanted resonances that can create levels of flutter that produce structural failure at certain speeds.

You can do "so much" with a calculator but eventually it comes down to refinement by empirical means -- that's where the fun begins