Modifying Ducted Fan with Afterburners

A true after burner is going to involve a lot of fire out the tail pipe. That would be one major issue to consider when building.

How to simulate it without some sort of flame would be a good trick. I can't imagine how to go about that without considerable risk of fire! Both to the model and everything else around it. But it would be interesting!

Now to just point the tail towards the judges! Gotta let them have a good look at the After Burner scale function! :)

Chuck

TROLL!

Yeah right. Whatever. I think it is out of your league.

But true, safety and hazards are a concern. I don't know of the designer's original intent was to make the RC jet more realistic or it was modified for performance purposes.

Thanks anyways. Nick

Paul Mc> TROLL!

I would think that getting a ducted fan unit to spit fire on command would be quite a trick. I could be mistaken, but isn't a ducted fan just a regular piston engine with a small propeller in a round shell?

Inducing a mini-turbine engine to spit fire would be much simpler, although probably not the wisest thing to do. Fire hazard and all, both to the plane and *anything* around it.

TomC

Okay, let's put it this way. There would be no benefit in injecting and igniting raw fuel into the exhaust stream of a DF model. The exhaust gas velocity and temperature is too low for it to provide any additional thrust.

A model turbine could, in theory, gain similar benefits as full-scale jets, and at a comparably increased fuel consumption rate, which is already quite high. Remember, an F16 that can fly at full mil power for a couple of hours has only about 15 minutes in 'burner.

It sounds to me like someone was dreaming.

As far as a true DF plane, no way.

Honestly, the question did look like a troll.

--- Rich

Thanks Rich. It is clear now there are no performance advantage even if the feat was possible. I've only seen a few DF jets fly and looks to me like they go fast enough already.

I think any modifications to simulate a real full scale jet would only be to make it more authentic visually.

I am an aerospace engineer, although a newbie to the RC world, was interested in seeing how this could have been achieved. Thanks for the great insight. I guess this newsgroup really has intelligent people here.

Thanks, Nick

Rich Lockyer wrote:

Thanks Rich. It is clear now there are no performance advantage even if the feat was possible. I've only seen a few DF jets fly and looks to me like they go fast enough already.

I think any modifications to simulate a real full scale jet would only be to make it more authentic visually.

I am an aerospace engineer, although a newbie to the RC world, was interested in seeing how this could have been achieved. Thanks for the great insight. I guess this newsgroup really has intelligent people here.

Thanks, Nick

Rich Lockyer wrote:

Thanks Tom. Please see my previous posting to Rich.

Nick

Tom Crabtree wrote:

Hi Nick,

Others have posted the fact that the DF won't give enough heat to ignite additional fuel, but I remember something similar that might be worth looking into...

Back when DF was in it's infancy, I remember seeing an image in MAN (I believe) where the pilot had put a handful of high-intensity red LED's around the inside of the tailpipe that would light up at

95% power. The visual effect was pretty good, the whole inside of the tailpipe would start to glow when the throttle was brought up to power.

Steve

Actually putting an AB on a ducted fan would be far easier than on a turbojet. All an AB does is raise the temp/pressure/velocity of the exhaust gases which in a normal turbine only uses up about 5% of the available oxygen in the core engine if it's not a turbofan.

On a jet engine, the nozzle area is critical in that if it can't be opened when in AB mode then the extra back pressure will stall the compressor. Not really a problem with a ducted fan except the engine would slow down slightly.

Heat in the tailpipe for full sized engines is only necessary to vapourise the liquid fuel which runs through small diameter pipes and exits through tiny holes around the fuel ring as a gas. For a cold airstream then LPG or BBQ fuel would be necessary. Upstream of the fuel line (and very close to it) is what's called a flameholder. This just provides an area of stagnant air so the full airstream doesn't blow out the flame. A source of ignition must also be used even in a full sized after burning engine, the temp of the exhaust isn't enough for ignition. Most use what's called a "hot streak" ignition where maybe 50cc of fuel is injected through the turbine and as the name implies gives a raw flame which then lights up the vapour from the AB feed line.

So it's doable but you'd need to make sure the tailpipe was well insulated :)

Brian Hampt> Thanks Rich. It is clear now there are no performance advantage even if

This argument assumes that the fixed nozzle gets chocked with AB on. If the quantity of AB fuel remains low, one could run the engine w/o fan stability issues. I don't know whether anyone has ever taken measurements of dry engine model nozzle flow, pressure & jet velocities to estimate the choke margin but already I can see a small science project for some of the young keen enthusiasts out there.