RPM = Thrust everytime?
07-10-2006, 04:38 AM
#1
Thread Starter
RPM = Thrust everytime?
If a turbine reaches it's rated maximum RPM does that mean it MUST produce it's rated thrust. For example my Wren is rated at about 8kg @ 160,000 RPM. If the engine is turning 160K does that mean it must be pushing the 8KG?
Reason I ask is a friend's Boomerang fitted with a Jet Joe was pumping out max RPM (also 160K) but the model didn't seem to accelerate as fast as I think it should & it failed to get airborne, it just seemed to reach a certain speed, not very fast, and would not continue accelerating. - John.
Reason I ask is a friend's Boomerang fitted with a Jet Joe was pumping out max RPM (also 160K) but the model didn't seem to accelerate as fast as I think it should & it failed to get airborne, it just seemed to reach a certain speed, not very fast, and would not continue accelerating. - John.
07-10-2006, 05:06 AM
#2
RE: RPM = Thrust everytime?
Boomerang, thrust is dependant on many things, mainly air pressure and density (somewhat relative to temperature). The higher the density, the higher the thrust will be at the same RPM.
On a hot day, a 8kg turbine might just barely be making 6 1/2 or 7 kg, on a very very cold day it might go up to 8 1/2 or 9kg.
On full scale airplanes, air density/temperature and other factors go into takeoff run calculation. In some southern high altitude fields, fully loaded takeoffs might not be allowed on hot days because the runway will get too short :-).
I would first try to get a thrust rating on the turbine (a fish scale on the nosewheel usually gives a good ideo of rough thrust). If it is not accellerating it would seem to me that there is too much friction. Check the lineup of the wheels (parallel, max. 1° toe-out!), and if they rotate freely (bearings?). Also pay close attention to the nosewheel. If it starts fluttering during takeoff run it will also cause too much friction to accellerate. Increase tension on the steering cables in this case.
Good luck
Hank
On a hot day, a 8kg turbine might just barely be making 6 1/2 or 7 kg, on a very very cold day it might go up to 8 1/2 or 9kg.
On full scale airplanes, air density/temperature and other factors go into takeoff run calculation. In some southern high altitude fields, fully loaded takeoffs might not be allowed on hot days because the runway will get too short :-).
I would first try to get a thrust rating on the turbine (a fish scale on the nosewheel usually gives a good ideo of rough thrust). If it is not accellerating it would seem to me that there is too much friction. Check the lineup of the wheels (parallel, max. 1° toe-out!), and if they rotate freely (bearings?). Also pay close attention to the nosewheel. If it starts fluttering during takeoff run it will also cause too much friction to accellerate. Increase tension on the steering cables in this case.
Good luck
Hank
07-10-2006, 05:31 AM
#3
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RE: RPM = Thrust everytime?
No, because turbine to turbine is different. For a turbine to produce the same thrust u have to consider 3 major things:a) compressor diameter and kind
b) turbine wheel diameter.
c)exhaust cone diameter.
For example let say you have 2 turbines with same compressor and turbine wheel of 54mm and you have different exhaust cone diameter then the one with smaller cone diameter is going to produce some more thrust than the one with bigger diameter but offcourse with some higher temp. Thats why the manufactures are using the optimum diameter cone for each turbine.
Now if you have all three a,b,c=a,b,c and both to the same Rpm then yes it should be approx the same thrust.If one of these 3 things is different then you don't have the same thrust.
But as well 160.000 Rpm does not mean 8kg of thrust. Some turbines with bigger compressors and turbine wheels can produce much more trust with lower Rpm for example 14-16kg@ 118.000 Rpm.
conclusion 160.000 Rpm is not constant or equal to 8kg.
Well is not the best explanation but i hope i kept it simple and been helpful.
Regards
Eftychios
b) turbine wheel diameter.
c)exhaust cone diameter.
For example let say you have 2 turbines with same compressor and turbine wheel of 54mm and you have different exhaust cone diameter then the one with smaller cone diameter is going to produce some more thrust than the one with bigger diameter but offcourse with some higher temp. Thats why the manufactures are using the optimum diameter cone for each turbine.
Now if you have all three a,b,c=a,b,c and both to the same Rpm then yes it should be approx the same thrust.If one of these 3 things is different then you don't have the same thrust.
But as well 160.000 Rpm does not mean 8kg of thrust. Some turbines with bigger compressors and turbine wheels can produce much more trust with lower Rpm for example 14-16kg@ 118.000 Rpm.
conclusion 160.000 Rpm is not constant or equal to 8kg.
Well is not the best explanation but i hope i kept it simple and been helpful.
Regards
Eftychios
ORIGINAL: Boomerang1
If a turbine reaches it's rated maximum RPM does that mean it MUST produce it's rated thrust. For example my Wren is rated at about 8kg @ 160,000 RPM. If the engine is turning 160K does that mean it must be pushing the 8KG?
Reason I ask is a friend's Boomerang fitted with a Jet Joe was pumping out max RPM (also 160K) but the model didn't seem to accelerate as fast as I think it should & it failed to get airborne, it just seemed to reach a certain speed, not very fast, and would not continue accelerating. - John.
If a turbine reaches it's rated maximum RPM does that mean it MUST produce it's rated thrust. For example my Wren is rated at about 8kg @ 160,000 RPM. If the engine is turning 160K does that mean it must be pushing the 8KG?
Reason I ask is a friend's Boomerang fitted with a Jet Joe was pumping out max RPM (also 160K) but the model didn't seem to accelerate as fast as I think it should & it failed to get airborne, it just seemed to reach a certain speed, not very fast, and would not continue accelerating. - John.
07-10-2006, 05:31 AM
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RE: RPM = Thrust everytime?
The amount of thrust depends on the efficiency of the engine. The early Mk 1 MW54s produced 10-11 lbs of thrust, the Mk 2 did 12lb+ and the Mk3's are 14lb+, the SuperSport is 18lb and all of them are the same size and have the same top rpm of 160,000.
The efficiency of the engine depends on how well it has been designed and manufactured.
"If a turbine reaches it's rated maximum RPM does that mean it MUST produce it's rated thrust. For example my Wren is rated at about 8kg @ 160,000 RPM. If the engine is turning 160K does that mean it must be pushing the 8KG? "
In the case of your Wren Supersport, it should be pushing the 8kg at 160,000, because we test all our engines and they don't go out if they aren't up to spec. But thrust ratings are a notoriously difficult area because manufacturers can claim anything they like and unless an engine has been independently reviewed you may not be able to get an accurate figure.
Whilst I was writing the above two other posts were submitted. I had written a sentence or two about the effect of air pressure/temperature on thrust, but have deleted it because Miniflyer has explained it in much more detail. With regard to the other post about size of compressor, turbine wheel and tailcone, this is correct but just getting them all the same size will not guarantee efficiency. There are many more thngs which affect performance, especially the combustion chamber and diffuser, and even with the turbine wheel there are many other things rather than size overall which can make a difference - for example the number of blades, their shape and size. Wren have done a lot of work on engine efficiency but we know there are still more things to discover.
Sara Parish
Wren Turbines
The efficiency of the engine depends on how well it has been designed and manufactured.
"If a turbine reaches it's rated maximum RPM does that mean it MUST produce it's rated thrust. For example my Wren is rated at about 8kg @ 160,000 RPM. If the engine is turning 160K does that mean it must be pushing the 8KG? "
In the case of your Wren Supersport, it should be pushing the 8kg at 160,000, because we test all our engines and they don't go out if they aren't up to spec. But thrust ratings are a notoriously difficult area because manufacturers can claim anything they like and unless an engine has been independently reviewed you may not be able to get an accurate figure.
Whilst I was writing the above two other posts were submitted. I had written a sentence or two about the effect of air pressure/temperature on thrust, but have deleted it because Miniflyer has explained it in much more detail. With regard to the other post about size of compressor, turbine wheel and tailcone, this is correct but just getting them all the same size will not guarantee efficiency. There are many more thngs which affect performance, especially the combustion chamber and diffuser, and even with the turbine wheel there are many other things rather than size overall which can make a difference - for example the number of blades, their shape and size. Wren have done a lot of work on engine efficiency but we know there are still more things to discover.
Sara Parish
Wren Turbines
07-10-2006, 05:59 AM
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RE: RPM = Thrust everytime?
I know Sara i agree with you thats why i said '3 major things' and 'is not the best explanation but i hope i kept it simple'
Hope the best for your further plans and discovers as ive seen you try the best for the support of your customers. Keep the good work!
Regards
Eftychios
[quote]ORIGINAL: WrenTurbines
With regard to the other post about size of compressor, turbine wheel and tailcone, this is correct but just getting them all the same size will not guarantee efficiency. There are many more thngs which affect performance, especially the combustion chamber and diffuser, and even with the turbine wheel there are many other things rather than size overall which can make a difference - for example the number of blades, their shape and size. Wren have done a lot of work on engine efficiency but we know there are still more things to discover.
Hope the best for your further plans and discovers as ive seen you try the best for the support of your customers. Keep the good work!
Regards
Eftychios
[quote]ORIGINAL: WrenTurbines
With regard to the other post about size of compressor, turbine wheel and tailcone, this is correct but just getting them all the same size will not guarantee efficiency. There are many more thngs which affect performance, especially the combustion chamber and diffuser, and even with the turbine wheel there are many other things rather than size overall which can make a difference - for example the number of blades, their shape and size. Wren have done a lot of work on engine efficiency but we know there are still more things to discover.
07-10-2006, 10:07 AM
#7
RE: RPM = Thrust everytime?
Isn't the jet joe a lower rated thrust engine to begin with? if the Wren SS is giving 18lbs of thrust, that's about 4 more than the Jet Joe from what I remember assuming all is operating properly. I'm not sure on the JJ though so I'm just wondering.
07-10-2006, 02:27 PM
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RE: RPM = Thrust everytime?
ET, to be more accurate, it would be like comparing a Hongqi to a Jaguar.
Woody.
Woody.
ORIGINAL: EASYTIGER
You can't compare a JetJoe versus a Wren, it's like comparing a Yugo to a Jaguar!
You can't compare a JetJoe versus a Wren, it's like comparing a Yugo to a Jaguar!
07-10-2006, 02:36 PM
#10
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RE: RPM = Thrust everytime?
Well, I've never driven a Hongqui!
I have driven a Yugo!
Is the Hongqui as fine an example of Chinese motoring as the Yugo was a fine example of cutting edge Serbo-Croation(ex-Fiat Panda) automotive technology?
The JJ, well, everybody slags off on it, but seems like lots of guys have gotten theirs to run okay.
But to compare a Wren to it, well, Wren is pretty much at the exact OPPOSITE end of innovation, quality, and performance. Wren stands for top-end quality, Wren is an innovator, JJ is rock-bottom cheap, inconsistent quality, and makes copies of what Wren was making five years ago....
I have driven a Yugo!
Is the Hongqui as fine an example of Chinese motoring as the Yugo was a fine example of cutting edge Serbo-Croation(ex-Fiat Panda) automotive technology?
The JJ, well, everybody slags off on it, but seems like lots of guys have gotten theirs to run okay.
But to compare a Wren to it, well, Wren is pretty much at the exact OPPOSITE end of innovation, quality, and performance. Wren stands for top-end quality, Wren is an innovator, JJ is rock-bottom cheap, inconsistent quality, and makes copies of what Wren was making five years ago....
07-10-2006, 03:04 PM
#11
Thread Starter
RE: RPM = Thrust everytime?
Whoa, Whoa! This isn't a Wren versus a 'Wren copy' argument! I just used my Wren as an example. What I needed to know is if the JJ made, say, 5kg of thrust at 160K RPM on one day, if it achieved the same 160K the next day would it have to produce 5 KG thust? One day the poor innocent Boomerang takes off & flys o.k. with Joepower, at 160K, the next it staggers down the runway as if someone left the handbrake on but the engine is still putting out 160K according to the GCU. Does the 'inconsistent quality' mean JJ will have good days (5kgs) and bad days (3kgs) at the same 160K? The weather has not changed much from flight 1 to flight 2.
Thanks, Miniflyer, yes we did check the wheels. The best think I can say about them is they are free & aligned & the Kavan electric brake is not binding. Apart from that they are rather 'sub - optimal' (sub - optimal because I'm not sure if we can say crap here!)
For what it's worth my model, which is about 3kg's heavier, accelerates down the same grass strip as if it has been shot out of a canon with the Supersport. - John.
Thanks, Miniflyer, yes we did check the wheels. The best think I can say about them is they are free & aligned & the Kavan electric brake is not binding. Apart from that they are rather 'sub - optimal' (sub - optimal because I'm not sure if we can say crap here!)
For what it's worth my model, which is about 3kg's heavier, accelerates down the same grass strip as if it has been shot out of a canon with the Supersport.
- John.
07-10-2006, 03:44 PM
#12
RE: RPM = Thrust everytime?
ORIGINAL: EASYTIGER
Well, I've never driven a Hongqui!
I have driven a Yugo!
Is the Hongqui as fine an example of Chinese motoring as the Yugo was a fine example of cutting edge Serbo-Croation(ex-Fiat Panda) automotive technology?
Well, I've never driven a Hongqui!
I have driven a Yugo!
Is the Hongqui as fine an example of Chinese motoring as the Yugo was a fine example of cutting edge Serbo-Croation(ex-Fiat Panda) automotive technology?
07-11-2006, 02:12 AM
#13
RE: RPM = Thrust everytime?
ORIGINAL: Boomerang1
Whoa, Whoa! This isn't a Wren versus a 'Wren copy' argument! I just used my Wren as an example. What I needed to know is if the JJ made, say, 5kg of thrust at 160K RPM on one day, if it achieved the same 160K the next day would it have to produce 5 KG thust? One day the poor innocent Boomerang takes off & flys o.k. with Joepower, at 160K, the next it staggers down the runway as if someone left the handbrake on but the engine is still putting out 160K according to the GCU. Does the 'inconsistent quality' mean JJ will have good days (5kgs) and bad days (3kgs) at the same 160K? The weather has not changed much from flight 1 to flight 2.
Thanks, Miniflyer, yes we did check the wheels. The best think I can say about them is they are free & aligned & the Kavan electric brake is not binding. Apart from that they are rather 'sub - optimal' (sub - optimal because I'm not sure if we can say crap here!)
For what it's worth my model, which is about 3kg's heavier, accelerates down the same grass strip as if it has been shot out of a canon with the Supersport. - John.
Whoa, Whoa! This isn't a Wren versus a 'Wren copy' argument! I just used my Wren as an example. What I needed to know is if the JJ made, say, 5kg of thrust at 160K RPM on one day, if it achieved the same 160K the next day would it have to produce 5 KG thust? One day the poor innocent Boomerang takes off & flys o.k. with Joepower, at 160K, the next it staggers down the runway as if someone left the handbrake on but the engine is still putting out 160K according to the GCU. Does the 'inconsistent quality' mean JJ will have good days (5kgs) and bad days (3kgs) at the same 160K? The weather has not changed much from flight 1 to flight 2.
Thanks, Miniflyer, yes we did check the wheels. The best think I can say about them is they are free & aligned & the Kavan electric brake is not binding. Apart from that they are rather 'sub - optimal' (sub - optimal because I'm not sure if we can say crap here!)
For what it's worth my model, which is about 3kg's heavier, accelerates down the same grass strip as if it has been shot out of a canon with the Supersport.
- John.
No, under the same atmospheric conditions it is impossible for a turbine to loose or gain thrust at the same rpm (unless it has sucked up a bird or something *g*). Given a temperature, air pressure and rpm the turbine will put out the same thrust every day, be that 3kg or 8kg.
Inconcistency with JJ means that one will put out 4kg at 160k while the other is putting out 7kg under the same conditions/at the same rpm.
If you are loosing this much performance from one flight to another then:
a) it has gotten terribly hot
b) you have an rpm sensor problem and are not spooling up all the way
c) you have something causing a lot of drag on the takeoff run
Best regards
Hank
07-11-2006, 04:05 AM
#14
Thread Starter
RE: RPM = Thrust everytime?
Thanks Hank and the other guys that chimed in, we will keep fiddling. The owner is in a bit of a panic & is talking about all sorts of mods, from bigger, better wheels to an engine transplant with a Jetcat P-80. I'll start by modifying the strip, with the lawn mower! [sm=lol.gif] - John.
