exceeding powerband????
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exceeding powerband????
I have been advised by some of which rpm to prop a certain engine at. I have for example been told to aim at 11,000 to 12,000 rpm, now I know my engine can turn more RPM, within the reccommended prop range... is there any reason not to go beyond the power band
since I am looking for a little more speed is there anything wrong if I can tune my engine to run in the mid 13,000 range if it supposedly makes its nominated HP at about 12,000( P.S I know HP figures dont mean anything)
thanks Frank
since I am looking for a little more speed is there anything wrong if I can tune my engine to run in the mid 13,000 range if it supposedly makes its nominated HP at about 12,000( P.S I know HP figures dont mean anything)
thanks Frank
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RE: exceeding powerband????
thanks for the reply w8ye but how can you do that if few to none publish torque readings?... I'm pretty sure torque will be ok..
but really want to know if there is a down side to going past reccomended rpm band if the engine can willing do it... I'm under the impression the rpm means everything and don't know what I'm missing herewhen I know I can do better than where I've been advised to prop it for
well even for argument sake lets say that the additional rpm pushes it past both hp and torque peaks,
what effect does this have on wot flight?
thanks
Frank
but really want to know if there is a down side to going past reccomended rpm band if the engine can willing do it... I'm under the impression the rpm means everything and don't know what I'm missing herewhen I know I can do better than where I've been advised to prop it for
well even for argument sake lets say that the additional rpm pushes it past both hp and torque peaks,
what effect does this have on wot flight?
thanks
Frank
#4
RE: exceeding powerband????
G'day Frank,
What engine are we talking about, because the revs can vary greatly, a 46AX, can rev upwards of 13,000 RPM with a 12x4, but a 160FX is overreved above 10,000 RPM.
What engine are we talking about, because the revs can vary greatly, a 46AX, can rev upwards of 13,000 RPM with a 12x4, but a 160FX is overreved above 10,000 RPM.
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RE: exceeding powerband????
it's a G90 ST that turns an apc 12 X8 scimitar at 13,100 for launch (as peak is about 13,500-13,600) , and I still have to try the tuned pipe I have for it , I've been told to run it at around 12,000
#6
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RE: exceeding powerband????
The optimum prop size for most airplanes will always keep your engines peak rpm at safe levels. You can use a smaller prop, (like a 10-6 on a 60) turn more rpm, making more power and loose performance. You can also turn a bigger prop slower which at some point will lower horsepower enough to cause a performance loss. The minimum rpm is determined by the carb size unless a pump is used. Mufflers always lower the rpm that peak torque and horsepower occurs, open exhaust engines will rev much higher and a tuned pipe will allow the peak power to be placed at any rpm its tuned for. The Supertigre 90s peak horsepower occurs just below 13,000 with its stock muffler, according to Mike Billingtons review.
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RE: exceeding powerband????
ORIGINAL: propjets4me
thanks for the reply w8ye but how can you do that if few to none publish torque readings?... I'm pretty sure torque will be ok..
Frank
thanks for the reply w8ye but how can you do that if few to none publish torque readings?... I'm pretty sure torque will be ok..
Frank
Next time you are at the field, try the following trick, which might give you an idea as to where the torque curve peaks.
For this, I assume your engine carburates well in the mid range. Start your ST engine and idle it. Get your tach out and set the throttle so it's doing 4000 rpm. Then set the throttle where it's doing 5000 rpm. Do this for each 1000 rpm break up to peak.
I'll wager that there will be a certain point up around the 8000-10000 rpm breaks at which you will find the engine will be unwilling to run steadily. For example, we were doing this on a guy's .61 some time ago where it could do each step except 9000 rpm. It could do a steady 8000 rpm and 10,000 rpm quite happily but not 9000 rpm. There was a "torque hit" somewhere leading up to 9000 rpm on this engine, making it jump quickly from 8000 rpm to 10,000 rpm with very little throttle advance. This 8000-9000 zone, we believe, is where the torque peak occurred on this particular engine. This effect is even more pronounced with a tuned pipe and the point at which it does it is tunable with pipe length, as Kweasel noted.
Try it next time and let us know.
#10
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RE: exceeding powerband????
The torque peak for the G90 is down around 7200 rpm according to Dave Gierke. also the HP peak is at a little over 11,000
Dave recommends a 15-8 and I tried one. The engine didn't get hot. It turned the prop a little above 8000. But it was the sickest sounding thing ever on spool up.
I use a 14-6 on them and a stock one turns at 10,500. They seem just about right with that set up.
Bax at Great Planes recommends a 13-6 for them.
Dave recommends a 15-8 and I tried one. The engine didn't get hot. It turned the prop a little above 8000. But it was the sickest sounding thing ever on spool up.
I use a 14-6 on them and a stock one turns at 10,500. They seem just about right with that set up.
Bax at Great Planes recommends a 13-6 for them.
#11
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RE: exceeding powerband????
...But I really want to know if there is a down side to going past recommended rpm band if the engine can willing do it...
HP and torque are not just numbers...
The main disadvantage to exceeding the RPM of maximum HP is that from that point on, the HP output is reduced.
A given torque at a given RPM is by definition, horsepower.
Horsepower was defined as 33,000 lbs. ft. per minute; by James Watt in the middle of the 19th century.
It remains so, even though Mr. Watt miscalculated, over-estimating the ability of the Clydesdale he measured...
HP can be calculated from torque and RPM: HP = torque * RPM/5,252.1 .
Torque can be calculated from the reciprocal of the same formula.
As to your case; as RPM is increased from the point of maximum torque, torque declines gradually.
RPM is rising faster (percentage-wise) than torque declines, so the HP is increased.
At a certain RPM, which differs from engine to engine, the decline of the torque becomes equal to the rise of the RPM and the highest RPM is registered.
From this point on, torque declines faster than RPM rises and therefore, HP becomes smaller, despite the RPM rise.
The higher the HP output, the higher the speed attained!
If you use a smaller prop that will allow the engine to spin at an even higher RPM than peak HP, the flight-speed will be reduced, since HP is lower, despite the fact the engine is 'screaming' at a higher pitch. Drag at the previous, higher speed is too high for the 'lowly' current torque to evercome.
Torque * speed is HP.
So, there you go.
#12
RE: exceeding powerband????
ORIGINAL: Flyboy Dave
....there is no such thing as "exceeding the powerband" in a two stroke engine.
This is another Internet Myth....
....there is no such thing as "exceeding the powerband" in a two stroke engine.
This is another Internet Myth....
As w8ye says, the normal revs to run them is somewhere between the torque and HP peak depending on the type of model. The trouble is, who can believe the peak HP revs given by the advertising? And none of them give any mention of torque so we can only take an educated guess.
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RE: exceeding powerband????
PROP The ST 90 likes RPM -contrairy to belief -and that 12x8 MAS K is the perfect speed prop. The 12x8 scimitar is good also . 13x6 is close to 12x8. I am sure you'll find guys to tell you 10,000 is great but you'll fly circles around them.
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RE: exceeding powerband????
Speeds, what kind of rpm does the ST .90 trun the MAS. 12x8, my Fox .74s each turn a Graupner 12x8 at 11,200 regardless if equipped with a Fox EZ Just, Perry or SuperTigre carb.
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RE: exceeding powerband????
ORIGINAL: propjets4me
it's a G90 ST that turns an apc 12 X8 scimitar at 13,100 for launch (as peak is about 13,500-13,600) , and I still have to try the tuned pipe I have for it , I've been told to run it at around 12,000
it's a G90 ST that turns an apc 12 X8 scimitar at 13,100 for launch (as peak is about 13,500-13,600) , and I still have to try the tuned pipe I have for it , I've been told to run it at around 12,000
If you keep in mind that the G90 (and OS91 for that matter) is an over-grown .61 engine, the setup becomes a bit more intuitive. Porting is a good balance for performance and sport use, but it take full advantage of a tuned exhaust system.
My experience with the OS91FX and G90 when using the jett-stream muffler (similar to the pipe setup) is to prop the engine to run around 13,000-13,500 peak ground rpm. The 12x8 gets there easily. Although Ive also seen an 11x10 and 12x9 work very well also. You can set up a full length pipe to perform in a similar fashion. In the air the engine will unload and settle in to the sweet spot of the exhaust tuning range.
Do not get too confused about the rpm setup. 2c engines like to turn. The G90 does not mind (and quite enjoys) turning above 14,000 in flight. Set the engine up with a full pipe set around 14" (plug to fat part) to start with. Or go with an Ultra-Thrust or Jett-Stream muffler, and simply prop to the recommended RPM.
Ultimately the prop selection will depend on your application.
Bob
#18
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RE: exceeding powerband????
ORIGINAL: Flyboy Dave
....there is no such thing as "exceeding the powerband" in a two stroke engine.
This is another Internet Myth....
FBD.
....there is no such thing as "exceeding the powerband" in a two stroke engine.
This is another Internet Myth....
FBD.
---------------
Amen.
Making horsepower is important, but coupling that horsepower to the surrounding air is what makes the model goeth. <G>
Turning a lot of pitch in the prop while running the engine as fast as the results dictate is what you should be looking for. Prop pitch. Not only are the torque and horsepower peak points important, but so is the drag of your model. There is an interplay between all of these factors. The proper prop size, engine rpm, fuel (nitro%) are all intertwined/related. The best way to go as fast as you can is to keep records of the settings, equipment, local weather, etc. Then, unless you are just out for a psychological thrill of gaining the most Wow from your model, you will need some way to measure your performance as you progress through the changes. It is a lot of fun. BTDT.
Ed Cregger
#19
RE: exceeding powerband????
ORIGINAL: propjets4me
I have been advised by some of which rpm to prop a certain engine at. I have for example been told to aim at 11,000 to 12,000 rpm, now I know my engine can turn more RPM, within the reccommended prop range... is there any reason not to go beyond the power band
since I am looking for a little more speed is there anything wrong if I can tune my engine to run in the mid 13,000 range if it supposedly makes its nominated HP at about 12,000( P.S I know HP figures dont mean anything)
thanks Frank
I have been advised by some of which rpm to prop a certain engine at. I have for example been told to aim at 11,000 to 12,000 rpm, now I know my engine can turn more RPM, within the reccommended prop range... is there any reason not to go beyond the power band
since I am looking for a little more speed is there anything wrong if I can tune my engine to run in the mid 13,000 range if it supposedly makes its nominated HP at about 12,000( P.S I know HP figures dont mean anything)
thanks Frank
Quote"Several things happen when the engine runs out of port timing that are sometimes difficult to understand. It’s not the same as closing the throttle and restricting the air. Some of the symptoms are fuel leakage from the back plate gasket or the front bearing, the engine will go lean, and you will see excessive cyl. head temp. The reason for this is the crankcase takes a full charge but it can’t get it up through the ports to the top end. As the piston comes down crankcase pressure becomes excessive and it has to go somewhere. On occasion you will notice fuel that actually spits out the carb, or some of the other symptoms of reversion.
our engine are ported for a max RPM between 11,000 and 12,500 depending on the displacement. Short port timing allows the engine to produce more torque, but restricts RPM. They like larger diameter props."Quote
#20
RE: exceeding powerband????
I think of the power band as between the peak torque and peak HP points also. The fact that the airframe or prop efficiency dictates a smaller band, or even outside of it doesn't change the power band of the engine.
#21
Senior Member
RE: exceeding powerband????
Frank asked quite a basic question; if he can gain speed by propping his engine to spin at RPM which is beyond peak HP (this is in my own words).
The answer to this is a clear and unequivocal NO!.
Selecting the right prop that will also spin exactly at peak HP, will give a higher speed than a prop spinning at any other RPM.
The answer to this is a clear and unequivocal NO!.
Selecting the right prop that will also spin exactly at peak HP, will give a higher speed than a prop spinning at any other RPM.
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RE: exceeding powerband????
If you want to get an idea as to what your power curve looks like, try this.
Get together several props of the same brand/type and run these up on your engine, recording the peak rpm for each prop. The props should be of differing loads such that a 3000-4000 rpm spread is achieved around the rpm level you think maximum horsepower is achieved. For example, for an unpiped sport .90 two-stroke, try to cover the range of 10,000 - 13,000 rpm. Once the peak figures are in, use Mr. Reivers' Prop-Power.xls (or similar) to calculate the horsepower achieved and plot the results on a graph.
We did this a while back on a buddy's MVVS .77 with a tuned pipe, using different lengths of header. We used the APC 12x8, 12x12, 13x10 and 14x8 props.
Get together several props of the same brand/type and run these up on your engine, recording the peak rpm for each prop. The props should be of differing loads such that a 3000-4000 rpm spread is achieved around the rpm level you think maximum horsepower is achieved. For example, for an unpiped sport .90 two-stroke, try to cover the range of 10,000 - 13,000 rpm. Once the peak figures are in, use Mr. Reivers' Prop-Power.xls (or similar) to calculate the horsepower achieved and plot the results on a graph.
We did this a while back on a buddy's MVVS .77 with a tuned pipe, using different lengths of header. We used the APC 12x8, 12x12, 13x10 and 14x8 props.
#23
RE: exceeding powerband????
ORIGINAL: DarZeelon
Horsepower was defined as 33,000 lbs. ft. per minute; by James Watt in the middle of the 19th century.
It remains so, even though Mr. Watt miscalculated, over-estimating the ability of the Clydesdale he measured...
Horsepower was defined as 33,000 lbs. ft. per minute; by James Watt in the middle of the 19th century.
It remains so, even though Mr. Watt miscalculated, over-estimating the ability of the Clydesdale he measured...
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RE: exceeding powerband????
ORIGINAL: downunder
Actually it wasn't a Clydesdale (this is another Myth ) but one of the work horses used in grain mills that were attached to a long pole and walked around in a circle to drag the grinding wheel. From the length of that arm and knowing the load the horse pulled on it and the average time it took the horse to complete x many laps he came up with a figure close to the 33,000 but not exactly. Because another horse would have produced different figures it was rounded to 33,000 for convenience.
ORIGINAL: DarZeelon
Horsepower was defined as 33,000 lbs. ft. per minute; by James Watt in the middle of the 19th century.
It remains so, even though Mr. Watt miscalculated, over-estimating the ability of the Clydesdale he measured...
Horsepower was defined as 33,000 lbs. ft. per minute; by James Watt in the middle of the 19th century.
It remains so, even though Mr. Watt miscalculated, over-estimating the ability of the Clydesdale he measured...
#25
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RE: exceeding powerband????
My personal best in benchpress is 200 kilos (about 420-430 pounds) How many horsepower is that[sm=confused.gif][sm=lol.gif][sm=lol.gif]