hpghost

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I was just asking myself...would a varaible exaust nozzle be effective on a remote control airplane or would it be only there for scale effect?

F-22-Raptor

Hi renaud,

a variable exhust nozzle on our turbines makes no sence at all. Reason is that the variable nozzle in real jets is used to compensate the gas extraction while the afterburner is launched and to keep the flame inside the turbine.

Doing this with our engines will cause a "block" of the hot air which will then reduce the thrust and raise the heat.


Andreas

hpghost

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thanks for the reply...

Any more pics of your jet in action?

F-22-Raptor

Your hunger of information never stops

Beware of nemo!!! Sometimes he strikes back! [:@]

F-22-Raptor

And here is a video of it:

[link=http://www.modelljets.com/4images/details.php?image_id=29]F22-In flight[/link]

Miniflyer

Andreas, i have to correct you on that.
On full scale airplanes, the nozzle is actuated mainly according to the EGT. The tighter you squeeze the more thrust you will get (same amount of air at higher velocity, pulse= mass x velocity²), however at higer EGT and with horrible spoolup capabilities. Variable nozzle gives optimum temperature, spoolup AND max thrust in all situations.
Same holds 100% true for model turbines. However, it is very difficult to actuate an appropriate mechanism in a jet. Then there is no ECU on the market that will allow an interface to actuate the nozzle according to current need. And last but not least: our turbines usually do not end where the real ones do: at the end of the plane. Most people utilise thrust tubes, in which the airflow has time to "adapt" to the pipe. Increasing exhaust velocity in the middle of the plane will not help much, as it will have slowed almost to the "normal" velocity by the time it exits the pipe...
A variable nozzle at the end of the pipe would help, but the losses due to the resulting high drag (=backpressure) in the pipe would probably eliminate all the advantages.

A variable nozzle would be great in terms of performance, but it would be a mechanical nightmare and an enormous effort, plus you would have to move the turbine right in the tailpipe...considering also the rather "small" speed envelope we have to cover compared to full scale aircraft, it is not "economical". It's cheaper, easier and a lot quicker to just simply put in a stronger turbine :-)

Regards
Hank

starwoes

sorry but i must disagree with some of what I am reading here regarding varying the exhaust nozzle....EGT may be factored into the equation but if the airplane will mostly spend its time below supersonic speeds, the exhaust nozzle can be "dialed in" ie, fixed straight from the factory and the airplane will perform quite well with enviable engine performance/economy. my simple understanding of the reasons why it is necessary to vary the exhaust nozzle has something to do with subsonic....transonic and supersonic regimes of flight. generally, while the airplane is at below supersonic speeds, the nozzle is markedly closed which greatly enhances the propulsive forces (trade pressure for velocity) needed to move or accelerate the airplane. in the supersonic regime of flight, pressure becomes more important than velocity (trade velocity for pressure) therefore, the nozzles open wider to propel or accelerate the airplane.....air at those speeds acts quite different. transonic speeds has it's special requirements also. in effect, engines are tuned to a particular airframe/airplane and a combination of nozzle size and power setting is ideal at different flight envelopes -- however, the exit portion of the nozzle remains mostly narrow during subsonic speeds as compared to supersonic flight where the nozzle is mostly dilated.

so....if you have a supersonic aircraft which must have a variable exhaust nozzle, why keep the nozzle fixed at narrow during subsonic flight and dilated during supersonic flight? you can of course vary it continuously to achieve optimum thrust at any power setting and flight regime....

my 2 cents...

bela

I agree totally with miniflyer. not impossible, but very problematic. in full scale engines, theoretically the core or spool should operate independently of the afterburner... that is to say core speed and temp is only marginally effected when throttle is advanced beyond military power in to the ab range. in a turbojet, there are systems to deal with (theoretically) ab backpressure to avoid compressor stag conditions....though they do happen sometimes. but more often you have( in turbofans) ab pressure spikes that can and often does backtravel the bypass duct and stall the fan.

Back to our model motors u, have the additional requirement of a flameholder to increase local turbulence and allow for more burning before exiting nozzle. now a nozzle for effect with no ab function, is far less complicated

In fact, nozzles do operate even when not in the reheat state... so if u found a way to mechanically build an nozzle... the nozzle off state would be wide open.. nozzle on state would be closed....for a few seconds at a time just for effect then heat probably wouldnt be an issue...but again this would not be an afterburner. just a variable nozzle. but i would probably expect points on scale look and not bet on increased power. would be majorly cool though!!!!!!!!!!!

bela

Andreas, Sorry 2 c wat happened 2 ur raptor! these kind of pics are never fun to look at!

jetace11

so whats it called when you can move the entire nozzel around and actually direct your thrust??

causeitflies

Vectored thrust.

col_tomb

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For model jets, maybe not. Unless you plan for supersonics flight...
Regards,
Col Tomb

Terry Holston

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Vectored thrust nozzle.
Rookies have them

hpghost

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Andreas....when are u planning to put bifurcated pipe in order to keep nice looking from the back?

Miniflyer

Starwoes,
no engine control system has an input "airspeed"....they all operate on engine-internal data, mostly being EGT. Speed has no effect on it whatsoever, since full scale Jets have supersonic exhaust velocities at the nozzle anyway....over 800m/s static in case of the J-79 (F-4 engine). So wether or not the airplane is traveling above or below the speed auf sound does not matter much for nozzle position (The fact that it opens further the faster you go has totally different reasons).
The exhaust leave the turbine hot section (mostly) at (locally!!!Not to confuse with absolute subsonic at atmospheric conditions!) subsonic speeds, passes through the AB chamber and is then accellerated through the nozzle to supersonic velocity, regardless of aircraft speed. How this is done can be seen excellent on F-15 nozzles (contraction and augmentation following in row).

#

Of course the nozzle factor can be "dialed in", (it could be on full scale airplanes as well for that matter), the point is you just won't get optimum performance throughout the turbine performance envelope. This just does not matter much on our turbines, as we can just put in a bigger turbine....the performance factor is simply not worth the extra complicity for hobby use.

Regards
Hank

starwoes

Miniflyer --

The concept of varying the nozzle of jet streams is very simple. You may wish to ask yourself why space rockets have fixed nozzles of the shape most often seen....that convergent-divergent thingy. It absolutely has everything to do with speed. what do you think would happen if the exhaust is exiting the nozzle at mach 1 while the aircraft flies at mach 1? what about wanting to accelerate? there is a point of diminishing returns and just as you ask for more power from your equipment, you have to keep an eye on its vital stats (like the egt) so you don't toast things.

Perhaps this equation may be of help:

An approximate equation for calculating the net thrust of a jet engine is:

Fnet = m(vjfe - va )
where:

m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity

"While the m·vjfe term represents the gross thrust of the nozzle, the m·va term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag. Conventional rocket motors, however, do not have an air intake, the oxidizer being carried within the airframe. Consequently, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are completely different from that of an air breathing jet engine"

Cullled from: http://en.wikipedia.org/wiki/Jet_engine

Supersonic Nozzle
If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient Pressure) is very high, to maximize thrust it may be worthwhile, despite the additional weight, to fit a convergent-divergent (de Laval) nozzle. As the name suggests, initially this type of nozzle is convergent, but beyond the throat (smallest flow area), the flow area starts to increase to form the divergent portion. The expansion to atmospheric pressure and supersonic gas velocity continues downstream of the throat, whereas in a convergent nozzle the expansion beyond sonic velocity occurs externally, in the exhaust plume. The former process is more efficient.

So you see, that's why your f-15 traveling at supersonic speed needs a convergent-divergent nozzle. for model purposes, it's USELESS!!....well it'd look cool....and perhaps increase performance by a smidgen but at what cost to weight?

afterburning is also cool and purely has flameholders, spray nozzles and a variable nozzle for matching to engine etc to add addition energy to the hot stream of gases...., i'd leave it out of nozzle design for now (simpler i think). i think it'd work on model airplanes tho....

causeitflies

starwoes, listen to miniflyer on this one. Aircraft speed does not determine nozzle position.

F-22-Raptor

Hi Reanaud,

I had a bifurcated pipe in my plane. I agree that it looks "smoother" but the negative points where too much. More noise (horrible sound) lost power, more heat in the airframe, more weight in the back of the plane. So all the positive nice looking did not compensate the negative things. But what makes it useless was the lost of speed!

On the 5th flight the engine lost its tail cone which stucks in one side of the pipe and generated so much heat that the frame was clos to get burned in flight. This gave me the feeling that a single tube is the better solution. For "Stand of scale" i can attach the middel "boom" so that it looks nice on the ground but its removed before flight.

joeflyer

My Feedback: (48)

A while back there was a servo controlled variable exhaust nozzle for ducted fans. Bob Fiorenze sold them. The idea was to open it up for maximum thrust on take off, then close it down for max. speed. I don't know how well they worked, but imagine they could be somewhat effective for a ducted fan. One doesn't seem practical or effective for a turbine model.

Joe

rongreg13

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I had the Bob Fiorenze variable outlets on my YA F-18 single (which was orginally Bob's plane). They worked really cool, although it either required a mix in my radio to use or else just manual actuation for show. I'm not sure if they made much of a difference in real world performance on the plane. They definitely looked cool, and I don't see why a similar setup wouldn't work for a turbine jet if the material was changed to something that could withstand the heat. I've attached some pics.

hpghost

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that looks very nicely made...could you give a pic showing better the machanism?

ShaneUSMC

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rongreg13,
What would you attach the nozzle to? I would think that it would get kind of hot as well. You would have to have whatever it mounts to heat resistant as well.

Nick C

starwoes,

you have just explained the need for a Con-di nozzle , and perhaps very well, obviously if you have NPR of over 3 then they become very efficeint,and obviously we must always have a jet exit velocity greater than the flight speed to provide forward thurst) but you don't make much mention of the need for a variable area nozzle.

As I understand it, its all to do with the ambient pressure and whether the jet is fully expanded or not. For maximum efficieincy you always want to fully expand your jet , so with a variable area geometry you can change the exit area of the divergent section such that the plume pressure matches the static pressure of the ambient air. And hence you do not end up with any shock system in the jet.

The control system controlling the final exit area then becomes a function of the ambient static pressure (altitude) and engine setting (nozzle pressure ratio) such that it dials in an area for each of these flight conditions.

The linear aerospike nozzle matches the exit plane pressure with the ambient pressure naturaly as part of its design , mainly as it was intended, (mounted on the X-33) to launch vertically and go up into space, i.e ambinet pressure is varying greatly. All anyone has to do is watch a rocket motor plume travel up into space and see the shape of it change enourmously from sea level to an almost vacumm. They choose not to mount variable area geometry on the nozzle for weight reasons and the fact that it is use once only (I believe, but sombody may correct me).


Nick

Miniflyer

Starwoes, simply pulling definitions from an enyclopedia does not really help a lot into understanding the principles involved.
While all you say is true, it does not matter at all in the "real world", as the exhaust velocities on modern jet engines is way way supersonic anyways (as i stated over 800m/s static WITHOUT reheat on the old F-4, thats over Mach 2 at standard atmospheric). No need to change anything when passing supersonic...it will work.
So the airspeed is no factor to be taken into the equation....(at higher airspeeds there are also a lot of changes in mass flow within the engine itself, thus creating a larger exhaust velocity automatically as you go faster....just a remark at the side).
Nozzle control is based solemly on engine internal data, mostly on EGT....
Rocket motors are a totally different chapter, but that is also a propulsion system on which i don't know my way around....

Nick, the "con-di nozzles" also work on the principle of variable area. The reason they are used is not to match ambient and plume pressure, but to increas exhaust velocity tho the most extent possible without pushing the turbine section out of their design envelope (=turbine surge, can be seen when bending the exhaust cone on your turbine real "tight"...homebuilders are always struggling to find a good compromise between high thrust and good running characteristics, eg. good spoolup and low EGT). The convergent-divergent design is neccessary because of the characteristic supersonic aerodynamics....a "simple" book concerning the basics is about 200 pages long, though....


Regards
Hank

Woketman

My Feedback: (6)

That is correct Nick, most rocket engine exhausts are not varable geometry primarily form a cost and weight standpoint. There have been some nozzles that had a two stage geometry where an extended "skirt" that was on the outside of the primary nozzle could be lowered into place at the appropriate point during the booster's ascent. But the vast majority of nozzles are simply sized for some point between the booster's liftoff ambient pressure and the ambient pressure at cut-off. That point being the one that yields maximum overall performance.