Practicality of pulsejet made from turbo

[imperium2600]

im finding a lot of relatively simple plans for jet and pulsejet engines using a turbocharger. what I am wondering is how practical would one (or multiple jets) be in a small aircraft.

the aircraft I am building is a model jet I am converting into a homebuilt uav. its about 14 feet long and weighs only a few hundred pounds. what i would like to know is if one of these engines is practical and what would i have to do to get enough power for the jet.

 

[Mech_Engineer]

You're going to need a lot of thrust to get that thing flying, and I don't think a home-made turbo/pulse jet would fit the bill. I would look into a 4-stroke engine and prop.

 

[Danger]

I agree with Mech. While it's certainly possible to build your own pulse-jet, it wouldn't likely be practical for your purposes. If you want the appearance of a jet, without the hassle, go with a ducted fan powered by the aforementioned 4-stroke. With the scale that you're looking at, it would be more efficient (and a lot quieter) than a 2-stroke.
If you insist upon using a jet, check out jetzilla.com to see what's what.

 

[FredGarvin]

Pulse jets are finicky beasts. They're also loud as all get out. Granted, they are relatively simple, but I don't think they are practical for what you want to do. A few hundred pounds is an order of magnitude greater than you would most likely be able to produce in terms of thrust.

 

[brewnog]

Agree with Fred, the home made ones are crude, incredibly noisy (seriously) and the specific thrust is pretty crap compared with something proper.

 

[Danger]

Noisy... yeah, in about the same context as the 'noisy cricket' gun that Will Smith used in 'Men In Black'.
Think of lying around with your head in a 200hp wood-chipper.

 

[imperium2600]

thanks for the help, every site i found on pulsejets never applied them to an actual vehicle. people just had them bolted into a metal frame for their own personal pyrotechnic pleasures.

 

[brewnog]

I've seen pulsejets propelling things like shopping trolleys and go-karts, (and even some scary contraption which got a very brave man up to 70mph on a runway) but I really doubt the practicality of trying to get airborne with one.

 

[NateTG]

Pulse jet and turbo don't belong in the same contraption, and quite possibly, not in the same sentence.

The V-2 did fly with a pulse-jet engine, and had a ~500 lb payload or something like that. Pulse jets are also popular for model airplanes.

 

[Ivan Seeking]

I had a buddy with a fairly large pulse jet - I would say about a foot in diameter and six feet long - and we took it up to the Oregon Museum of Science and Industry for a demo. The museum received complaints about the noise from people living on the other side of the Columbia River! And yes, for all the noise and fury, the thrust was a joke. This was a resonance type with no moving parts.

 

[ray b]

Pulse jet and turbo don't belong in the same contraption, and quite possibly, not in the same sentence.

The V-2 did fly with a pulse-jet engine, and had a ~500 lb payload or something like that. Pulse jets are also popular for model airplanes.

V-1 was a Pulse jet buzz bomb, V-2 was rocket named the A-4

I agree a pulse is not a turbo, as a turbo spins and a pulse uses a flap
also a pulse need a running start as the flow at rest is = to zero

V-1 used an air drop or RATO to get going

 

[Danger]

I think that you're confusing a pulse jet with a ram jet, ray. It doesn't need a running start because the combustion chamber is closed at the time of pressurization. The V-1's were launched from ramps resembling ski-jumps, and usually (AFAIK) started with a compressed air blast the same as some turbojets.
An interesting side-note is that the pulse jets (at least the V-1 versions) actually drew something like 85-90% of the new intake charge in through the tailpipe rather than the valve box.

 

[ray b]

as I understand the pulse jet is a kind of ram
as there is no compresser to suck in air
and unless they force air in [by moving] there is no power
so ''As the pulsejet requires a high volume of incoming air for full thrust, the V-1 was launched from a ramp through the use of a chemical/steam catapult.''

Operating speed: 375-400 mph
Launching speed: 220 mph
Range: 150 miles
Operating altitude: 2,000 to 4,000 ft.

now how they get air to flow in a tail pipe againts the open front vanes
inflow at 390mph I do not understand some maybe but > 80%?

 

[Danger]

I acknowledge that catapults were used, but for much the same reason that they are on an aircraft carrier. The V-1 simply couldn't build up enough speed in that distance to become airborne under its own power. The engine was already running at launch. As I said, most of the new intake came through the tailpipe, so forward speed wouldn't have too much to do with the power generation.

 

[Danger]

now how they get air to flow in a tail pipe againts the open front vanes
inflow at 390mph I do not understand some maybe but > 80%?

It doesn't burn like a normal jet or rocket; it's actually a very rapid series of individual explosions, more like a car engine. As with any explosion, a vacuum is created inside the blast area as a result of the initial overpressure. The tailpipe has less resistance to airflow than the valve box does, so that's where it gets sucked in the most.

 

[russ_watters]

[edit...looked into it] According to Wik:

The inertia of the traveling exhaust gas causes a low pressure in the combustion chamber. This pressure is less than the inlet pressure (upstream of the one-way valve), and so the induction phase of the cycle begins.

http://en.wikipedia.org/wiki/Pulse_jet_engine

I'm sure that's true, but I suspect that the pressure difference isn't enough to do much more than keep the engine running at low speed. I doubt most can produce much usable thust without a running start.

Fred would be the expert, though...

 

[FredGarvin]

We haven't studied pulse jets in my day. However, I have worked with some "old timers" that had played with them in the test cells. The ones that were run did use a compressor to provide the start conditions for the static tests. I can't say yeah or nay about the 80-90% flow through the tail pipe that Danger has mentioned. That seems like an awful lot of mass flow to be going against the inlet flow.

I have seen video of the German launches of the V1 and come to think of it, I have not seen how they initiated a start. I have always just seen them launch from ramps on the coast. I have read that a lot of them were launched from He-111's.

If anyone is interested, here's a pretty cut and dry theory for pulse jets: http://www.pulse-jets.com/count/pulse.php?ID=32

 

[ray b]

while I see a danger is doughting Danger
none of the on-line links say anything about high rear fill rates
in fact the burn is not sparked after starting but steady depending on pulse/fuel ratio
and it is hard to see how that works with a high % backflow
in fact why use a front scoop and valves if 85% is coming in the back

computer control of both air valve and fuel injection may get modern version to work better as would a form of stratifed or swirl
fuel/air mix and timing

other notes, the USA copys used the RATO launch not the germans
but has any type of pulse jet been flown without a kick of some kind to start the event?