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Is all thrust the same?
This is probably a stupid question but if a larger diameter turbine is putting out the same thrust as a smaller turbine but at a significantly lower rpm, is there a difference in performance? For example, a P-70 at 3.7" diameter needs 123,000 rpm to produce 17.5 lbs of thrust but a 4.4" P-120 needs only 105,000 rpm. Seems like the smaller turbine would have to put the air out faster to achieve the same thrust.
I've never heard mention of "exhaust gas speed" but if it's different does it affect performance (such as with prop pitch, length and rpm)? Would a same thrust, higher rpm turbine fly faster than a lower rpm one? I'm guessing not in a practical way but it's still an interesting question. Any thoughts? Craig |
RE: Is all thrust the same?
Exhaust gas speed of our turbines is around 400 MPH. Thrust is thrust is thrust. The little engines have to spin faster to achieve it, thereby theoretically wearing out faster.
The fuel has so many BTU's per ounce you're never gonna get more than that amount of energy no matter the size of the engine. |
RE: Is all thrust the same?
I have heard exhaust velocities do differ from the the bigger engines like a P-200 compared to a 120 or 160 with the 200 having a lower velocity, but this is second-hand. It was also explained as an example using a KingCat/P-200 vs. KC/P-160. I've seen the same exact KC flown with both engines and it sure was hard to tell a difference between the two even with the 200 putting out nearly 10 more pounds of thrust. Granted, I was not flying it, so I couldn't "feel" the difference, but it was hard to see a difference, if any. I'm sure a difference could be quantified with some data, but I doubt in the box we fly.
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RE: Is all thrust the same?
ORIGINAL: seanreit Thrust is thrust is thrust. Not too sure I agree with that. I know that you can have two different engines with the same power and one will fly the same plane faster than the other. One instance that springs to mind was when Larry Kramer was flying his Falcon powered(22lb I think) Bobcat at the same time as I was flying my Bobcat with a 19.0lb Supersport. The Supersport powered plane was by far the fastest of the two and anyone who has seen Larry fly will know he's not one to hang around! So I do think that exhaust gas speed can make a big difference. Jason |
RE: Is all thrust the same?
Hey Jason, I'm not going to disagree with you, and yes, I've seen Larry fly quite a bit, he's even taken a plane or two of mine for a spin, but I don't think I can agree that that flight or those flights are a scientific enough test.
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RE: Is all thrust the same?
ORIGINAL: seanreit Exhaust gas speed of our turbines is around 400 MPH. ORIGINAL: WrenTurbines The SuperSport is a high pressure engine and the exhaust velocity is high - over 1050mph. Sara Parish Wren Turbines |
RE: Is all thrust the same?
But I think Jason is correct (but I am not certain).
Remember when we were flying ducted fans? The exhaust speed was far slower than our modern day turbines. That exhaust speed is a limiting factor for speed. Even if you had been pushing 13 lbs of thrust with a two pound airframe, you cannot exceed the max exhaust velocity. When your 2 lb DF gets near its max exhaust speed, the thrust drops to zero. So it is limited. With turbines, some of the same thing is going on. We all know that the Wren Super Sport puts out its thrust at a very high (well, high for model turbines) exhaust velocity. Let's say it is putting out 18 lbs and we have a P-80 putting out 18 lbs too, but at a lower exhaust velocity. When the planes are going 250 mph, the SS will still have say 16 lbs coming out the back where the P-80 may only have 14 lbs. So I believe that Jason's Kramer example is very possible. It is almost as if the SS is in a higher gear and the P-80 has better low end torque (crappy analogy). |
RE: Is all thrust the same?
Yep, this is a topic that tends to crop up from time to time. In fact Sean and Jason are both right, thrust is thrust no matter what, But, the difference between 2 engines of similar Static thrust is the rate at which that thrust decays with speed and increased drag. This is because the stuff squirting out the back has 2 components..mass and velocity.
So of our 2 engines the one with the greater exhaust speed will continue to produce useful thrust at higher speeds and ultimately give us a higher top end. It would be nice if manufacturers quoted dynamic thrust figures also so that we could better match engines to airframe types. Rob. |
RE: Is all thrust the same?
Much better way of saying it Rob!!!
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RE: Is all thrust the same?
If you took the comparison to an extreme, with say a turbine 2 feet in diameter and turning very slowly to generate the same thrust as our little turbines, surely the exhaust gas speeds there would be enough affect performance (i.e. top speed).
Still not sure it would be a noticable difference between the various turbines we fly. Nevertheless, wouldn't it at least theoretically be different? |
RE: Is all thrust the same?
I wish I could quote my source, but I believe Eddie Weeks measured exhaust velocity of an Olympus at around 400mph. If Wren is getting twice that, that is amazing. I'm not sure that I would want that in a go around though........
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RE: Is all thrust the same?
ORIGINAL: jason ORIGINAL: seanreit Thrust is thrust is thrust. Not too sure I agree with that. I know that you can have two different engines with the same power and one will fly the same plane faster than the other. One instance that springs to mind was when Larry Kramer was flying his Falcon powered(22lb I think) Bobcat at the same time as I was flying my Bobcat with a 19.0lb Supersport. The Supersport powered plane was by far the fastest of the two and anyone who has seen Larry fly will know he's not one to hang around! So I do think that exhaust gas speed can make a big difference. Jason The super sport is smaller in size and light too, correct? So I may say your bobcat was lighter too, so less lift is needed less lift means less drag...therefore you go faster ???? C4F |
RE: Is all thrust the same?
The analogy works the same no matter what the engine size.
Look at the difference between a prop engine and turbine engine with the same static thrust figures. Rob. |
RE: Is all thrust the same?
Tam has spent a lot of time and money on this topic in the EDF world. The success of the TJ100 is this very issue. The exhaust air speed in the EDF world is everything, anyone can get enough static thrust to get a jet off the ground but the winner is the guy who can have a significant amount of static thrust once the jet is doing over 150mph. With an exhaust velocity of over 220mph out of the TJ100, Tam was able to successfully fly a 21.5 pound BVM MIG 15 with only 12 pounds of static thrust.
What this means...thrust is thrust is thrust, yes...but exhaust speed is EVERYTHING in the EDF world! For turbines it would be the same although the mesurable differences would be much smaller when all of the turbines produce extremely high exhaust speeds. Russ |
RE: Is all thrust the same?
According to my copy of Rolls Royce's book about the jet engine, the limiting speed of the engine is its exhaust velocity because the velocity term in the momentum equation is the difference between exhaust gas velocity and free stream velocity. Therefore in effect a jet engine does have a zero thrust speed and thrust reduces with forward speed, somewhat like it does with a prop or d/f though it is not as simple because the intake effects considerably alter the rate of loss compared to the fairly simple straight line loss of a prop/fan . A major difference is that the jet's exhaust velocity is vastly higher than that of a prop or fan, and thus its zero thrust speed is commensurately much higher. At full throttle it is not unusual for the exhaust to be at the local speed of sound, which being at a very high temperature is much higher than the speed of sound of the surrounding air. Since the zero thrust speed and rate of loss of thrust is related to the exhaust velocity, it follows that two engines with identical static thrust but with different exhaust velocities will have different dynamic thrusts and therefore different top speeds. How much this translates to different speeds for the aircraft is limited though, bearing in mind that at the speeds at which model aircraft operate the increase in speed will be heavily affected by the square rule in the drag equation so it will take a big increase in available thrust to generate any noticeable change in speed.
The actual rpm of the jet engine is not particularly relevant, it is dictated by the need to generate optimum turbine blade speed and in a small diameter it requires a higher rpm to get the same blade speed in what may be an exhaust velocity of the same speed between a large and small engine. |
RE: Is all thrust the same?
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ORIGINAL: HarryC According to my copy of Rolls Royce's book about the jet engine, the limiting speed of the engine its exhaust velocity because the velocity term in the momentum equation is the difference between exhaust gas velocity and free stream velocity. Therefore in effect a jet engine does have a zero thrust speed and thrust reduces with forward speed, somewhat like it does with a prop or d/f though it is not as simple because the intake effects considerably alter the rate of loss compared to the fairly simple straight line loss of a prop/fan . A major difference is that the jet's exhaust velocity is vastly higher than that of a prop or fan, and thus its zero thrust speed is commensurately much higher. At full throttle it is not unusual for the exhaust to be at the local speed of sound, which being at a very high temperature is much higher than the speed of sound of the surrounding air. Since the zero thrust speed and rate of loss of thrust is related to the exhaust velocity, it follows that two engines with identical static thrust but with different exhaust velocities will have different dynamic thrusts and therefore different top speeds. How much this translates to different speeds for the aircraft is limited though, bearing in mind that at the speeds at which model aircraft operate the increase in speed will be heavily affected by the square rule in the drag equation so it will take a big increase in available thrust to generate any noticeable change in speed. The actual rpm of the jet engine is not particularly relevant, it is dictated by the need to generate optimum turbine blade speed and in a small diameter it requires a higher rpm to get the same blade speed in what may be an exhaust velocity of the same speed between a large and small engine. I think you just put this to bed! |
RE: Is all thrust the same?
ORIGINAL: CraigG If you took the comparison to an extreme, with say a turbine 2 feet in diameter and turning very slowly to generate the same thrust as our little turbines, surely the exhaust gas speeds there would be enough affect performance (i.e. top speed). If you take two fans or props of same pitch but different diameter and turn them at the same speed, you get the same exhaust velocity, the difference in thrust is the difference in mass flow due to different diameter. In two jet engines of different diameter you get different thrust due to mass flows, same or similar exhaust speeds, so those parameters can be the same as the fan/prop, but the turbines will turn at very different speeds unlike the d/f or props which may turn at same rpm,becase the turbine is not generating the movement of the air but instead is windmilling in it and its pitch will be designed to get the optimum tip speed. |
RE: Is all thrust the same?
I like your analogy Woketman ;)
Sean actually touches on something interesting which can also influence our choice....spool times and how quickly our engines produce that all important thrust, smaller engines in general do tend to require higher rpm to achieve similar thrust levels to larger dia. donkeys. Though we are now starting to see the next generation of smaller engine emerge which is starting to address that with compressor wheels better designed for our application. Rob. |
RE: Is all thrust the same?
Hi all, Exhaust speed depends on pressure ratio of the engine. Some numbers: KJ66 and derivatives: (RAm750, P80, SJ1700, etc): Exhaust gas speed around 340m/s , around 765mph. M90: 450m/s. around 980mph. So, on a plane flying at 200mph, the exhaust speed difference is high enough for that the KJ66 powered doesn't loose too much push. I remember to have read that the propulsive efficiency is better if the exhaust speed and vehicle speed are closer, this is the reason why a turbofan is more efficient than a pure jet, so, looking from this perspective, the engine with lower exhaust speed win... My opinion is that, at our speeds, thrust is thrust. GAspar |
RE: Is all thrust the same?
ORIGINAL: Gaspar this is the reason why a turbofan is more efficient than a pure jet, so, looking from this perspective, the engine with lower exhaust speed win... Thrust is the reaction to momentum imparted to the air and is simply mass * velocity. So a large mass at slow speed and a small mass at high speed can have the same velocity. If you double the mass you can halve the velocity and still get the same thrust. The kinetic energy of the exhaust can only come from energy supplied by the engine, i.e burning fuel. But the energy is related to the square of the velocity, whereas the thrust is related to the straight velocity. If you move twice as much air at half the speed then the mass term doubles but the velocity term is cut to a mere quarter, so the energy of the exhaust is cut in half and the energy required to be put in is thus cut in half, for the same thrust. That is why turboprops, and fan engines which are simply props in a duct driven by a jet engine, are so popular at subsonic speeds - it takes less fuel to get the same thrust. |
RE: Is all thrust the same?
Yes Gaspar puts it well, I know from personal experience he has done a great deal of development work in this area.
Rob. |
RE: Is all thrust the same?
ORIGINAL: seanreit Hey Jason, I'm not going to disagree with you, and yes, I've seen Larry fly quite a bit, he's even taken a plane or two of mine for a spin, but I don't think I can agree that that flight or those flights are a scientific enough test. Sean, I'm not saying it's a scientific test or asking you to agree it is. All I am saying is that in the real world of planes in the sky, it does seem to make a difference. Jason |
RE: Is all thrust the same?
ORIGINAL: HarryC Craig, you are partly confusing the way a jet works with the way a fan/prop works. The turning of the turbine does not create the thrust, it is a windmill, the turbine creates a heck of a lot of drag! The exhaust velocity of a jet comes from the expansion of the air caused by heating, the exhaust velocity of a prop or fan comes from mechanically pulling and pushing the air backwards. The rpm of the jet turbine is designed to make the turbine blade speed reach an optimum so on a large diameter turbine you have to turn it at a much lower rpm than on a small diameter turbine. Of course you're right and after I had thought about it after my post realized the rpm is not the issue with a smaller turbine, rather the volume or mass flow as you more accurately described it. If the exhaust velocities are as high as suggested in several of these posts, then I would conclude that drag is a much more limiting/equalizing factor on top speed than the diameter of equal thrust producing turbines. Thanks for the educated comments. It confirms what I suspected in a factual way. Craig |
RE: Is all thrust the same?
ORIGINAL: HarryC ORIGINAL: Gaspar this is the reason why a turbofan is more efficient than a pure jet, so, looking from this perspective, the engine with lower exhaust speed win... Thrust is the reaction to momentum imparted to the air and is simply mass * velocity. So a large mass at slow speed and a small mass at high speed can have the same velocity. If you double the mass you can halve the velocity and still get the same thrust. The kinetic energy of the exhaust can only come from energy supplied by the engine, i.e burning fuel. But the energy is related to the square of the velocity, whereas the thrust is related to the straight velocity. If you move twice as much air at half the speed then the mass term doubles but the velocity term is cut to a mere quarter, so the energy of the exhaust is cut in half and the energy required to be put in is thus cut in half, for the same thrust. That is why turboprops, and fan engines which are simply props in a duct driven by a jet engine, are so popular at subsonic speeds - it takes less fuel to get the same thrust. I dont want to a smart ass but I think you meant to say same momentum not velocity in your third sentance. Very interesting thread thanks HarryC for the informative posts....so in conclusion are we saying that gaspar is right and that at the sort of speeds we fly 200mph ish the residual thrust of an engine with higher EGV is not sufficient to accelerate the airframe any more than the engine with the same thrust but lower EGV and we would have to be flying at speeds approaching the exhaust gas velocity to see the differance? |
RE: Is all thrust the same?
Let’s look at a theoretical scenario here.
Say we have two planes which are both equipped with two different 20lb static thrust engines, one has a exhaust velocity of 400mph and the other has 1050mph (just to quote figures mentioned earlier in this thread) Surely with a air speed of say 200mph, the plane equipped with the engine with the higher exhaust gas speed would be trying the push the plane faster than the one with 400 mph EGS? Surly this is the point of turbines? Otherwise I might as well put a D.A 50 on the front of by Bandit?:) What we need is for some clever person to make a thrust gauge that fits between the engine and airframe which compares and logs the thrust to airspeed. Ok, maybe I'm going a little over the top here:eek: Jason |
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