RE: Is all thrust the same?
 

The jet engine simply throws molecules (mass) rearwards (velocity) and the reaction to this is what drives the engine in the opposite direction.

One popular way to describe this is this I learned at school: you stand on a wheeled cart that has a pile of bricks on it. As you start throwing some bricks rearward off the cart as hard as you can, the reaction of force required to throw them will slowly push the cart forward.
To maximize the obtained speed of the cart you can throw more bricks per second or increase your throw velocity or use bricks with more mass.
With this analogy we can clearly relate to a "top speed" being reached for the cart relative early as a person cannot throw that very fast.

So even for a reaction type propulsion there is a definitive top speed just as a DF or prop.

The rpm of the turbine can be left out of the equation, its just a result of other internal design parameters such as blade area and other geometrical dimensions, flow efficiency, sealing between the stages and EGT to name a few. The same goes for fuel consumption i.e efficiency of the total design

my two cents

RE: Is all thrust the same?
 

What would a plot of thrust vs. forward air speed look like for one our motors?

RE: Is all thrust the same?
 

The answer to your question is not so simple but look here for a good explanation, http://home.anadolu.edu.tr/~mcavcar/.../Jetengine.pdf

RE: Is all thrust the same?
 

It's not quite as easy as it sounds, though for the speeds at which we operate models it is fairly straightforward. The net thrust, exhaust velocity minus intake velocity falls in pretty much a straight line from static value at no airspeed to zero thrust where airspeed equals exhaust speed. So for example if static thrust is 20lbs and exhaust speed is 800mph, net thrust at 400mph would be expected to be 10lbs. But, there is another effect, that of the intake ram air effect which does much of the compressor's work thus freeing up much of the energy to be used as thrust instead of driving the compressor, thus increasing the thrust. This starts at zero with zero airspeed and rises exponentially as the airspeed rises. The net thrust is the combination of these two effects. Initially the thrust falls as forward speed reduces it and the ram air effect has little power. As speed rises the thrust contrinues to fall but the ram air effect becomes more powerful and eventually its increase matches the decrease caused by exhaust:airspeed difference and the thrust has reached its minimum point. Above that speed, the ram air effect rising exponentially is going up faster than the straight line decrease from exhaust speed is going down, and thrust rises again back up to and even over the static level.

At slow speeds therefore the reduction in thrust will be almost straight line towards the zero point because the ram air effect is negligible. For most of the speeds at which our models operate then, the reduction in thrust will be fairly straight line, barely moderated by the ram effect.
H

RE: Is all thrust the same?
 

Thanks Bob and Harry for the very useful info.

If ram effect is negligible at the speeds we operate, then it would seem from the above explanations that if motors A and B have the same static thrust, but A has a higher exhaust velocity, then A would have more available thrust than B at the upper end of our operating range - say 200 mph forward air speed (if you fly in the U.S.). Is that a generally true statement?

RE: Is all thrust the same?
 

Yes, but the difference is a small value of pounds, and though it will increase the aircraft speed, at high speed the total drag curve will be dominated by the exponentially rising friction drag curve so the increase in speed will be small, in the magnitude of a handful of mph. A disadvantage is that, all else being equal, the higher exhaust velocity engine will burn fuel more rapidly to get the same thrust, so you get a shorter flight or start with more fuel = more weight = slower top speed!
You will certainly get ram effects at 150 - 200mph, but due to model design they will be less than on full size. Our ducts do not provide 100% of their air to the engine and ram it into the engine! Sometimes the engine is not in a duct at all, just sitting open in the fuselage, the airflow into the intake will be fast but will slow down inside the diverging fuselage and the engine itself sits in a much slower airflow. Even fully ducted installations in models still have quite a bit of bypass air around the engine. Engines out in the open may get more ram effect than an engine which is simply sitting unducted in the fuselage but still at these speeds it is not going to make a marked difference.

RE: Is all thrust the same?
 

Thanks again Harry. Very enlightening.

RE: Is all thrust the same?
 

Well, it would appear that I am wrong to some degree.
There is some ‘true’ reactive force involved, I stand corrected on that point.
However it is still true that higher exhaust velocity will give a higher top speed.

Paul

RE: Is all thrust the same?
 

It is all reactive force Paul
H.

RE: Is all thrust the same?
 

Newton is STILL correct and he's from the UK! Action/Reaction nothing more. In fact the Active force is the propellant and the Reactive force is your plane moving in the other direction. Now it doesn't get any simpler than that :)

RE: Is all thrust the same?
 

Kevin, was that the Merc that I bought from you? It is going strong flying a Boomer Sprint and running great!