Relationship between pitch and thrust?
03-06-2007 | 04:04 PM
#26
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RE: Relationship between pitch and thrust?
Fliton Extra @ 6lbs. give or take an ounce. Saito .82a running 25% Omega, regulated. I'm at sea level. I'm focused on IMAC/3d flying.
The reason I didn't simply ask which would be best is because I wanted to be able to calculate these things on later planes for myself. You know, teach me to fish rather than giving me a fish.
Besides, I want to have a greater understanding of the physics at work. I am not the kind of person who is happy that his plane flies the way he wants. I want to know precisely why it flies the way it does.
The reason I didn't simply ask which would be best is because I wanted to be able to calculate these things on later planes for myself. You know, teach me to fish rather than giving me a fish.
Besides, I want to have a greater understanding of the physics at work. I am not the kind of person who is happy that his plane flies the way he wants. I want to know precisely why it flies the way it does.
ORIGINAL: jaka
Hi!
As I said earlier ...Tell us what airplane you have and what engine you intend to use ...and we can tell you what prop is best! That simple ...because we/I have been there, done that! Simple ?! And no mumbo jambo calculations!
Hi!
As I said earlier ...Tell us what airplane you have and what engine you intend to use ...and we can tell you what prop is best! That simple ...because we/I have been there, done that! Simple ?! And no mumbo jambo calculations!
03-06-2007 | 04:32 PM
#27
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RE: Relationship between pitch and thrust?
At static, I was careful to not change the power load and rpm of the prop. That is the only way to be able to compare results.
As soon as the plane speeds enters the picture, results get interesting though. My calculator cannot handle that at the moment, so new rpm and prop load have to be calculated manually using i.e. Clark-Y lift-drag curve data.
The graphs show however clearly, that the higher pitched prop will ask more engine power at higher speeds, even though the heavily graded prop diameter is less.
As soon as the plane speeds enters the picture, results get interesting though. My calculator cannot handle that at the moment, so new rpm and prop load have to be calculated manually using i.e. Clark-Y lift-drag curve data.
The graphs show however clearly, that the higher pitched prop will ask more engine power at higher speeds, even though the heavily graded prop diameter is less.
03-06-2007 | 04:55 PM
#28
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From: Sacramento,
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RE: Relationship between pitch and thrust?
Are you saying that as speed increases, load on the engine increases as well? I would have thought that as speed increased, engine load would decrease proportionate to the amount of pitch in the prop and the speed with which it moved.
ORIGINAL: pe reivers
The graphs show however clearly, that the higher pitched prop will ask more engine power at higher speeds, even though the heavily graded prop diameter is less.
The graphs show however clearly, that the higher pitched prop will ask more engine power at higher speeds, even though the heavily graded prop diameter is less.
03-07-2007 | 05:52 AM
#29
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From: Floro, NORWAY
RE: Relationship between pitch and thrust?
Pe Reivers is right about that the a high pitch prop will take more load airborne at climb-out than static. However, when the plane get high airspeed e.g. in a dive, the load decreases and RPM increases. The reason is that in the center of the high pitch prop, angle of attack is above 18 degr. hence, that disc area is stalled static.
As the airspeed increases, effective angle of attack decreases and the prop is not stalled anymore, even i center. (the efficiency of the frop increases). The load on the engine will increase as long as the efficiency of the prop is increasing. At some point, the speed of the airplane exceeds the max efficiency of the prop. Further increase in airspeed from this point, the load on the engine will decrease.
This is why 3,75 is the lowest pitch you'll get on a prop, this is max pitch you can have static without ANY part of the prop beeing stalled at static speed.
As the airspeed increases, effective angle of attack decreases and the prop is not stalled anymore, even i center. (the efficiency of the frop increases). The load on the engine will increase as long as the efficiency of the prop is increasing. At some point, the speed of the airplane exceeds the max efficiency of the prop. Further increase in airspeed from this point, the load on the engine will decrease.
This is why 3,75 is the lowest pitch you'll get on a prop, this is max pitch you can have static without ANY part of the prop beeing stalled at static speed.
03-07-2007 | 07:56 AM
#30
Senior Member
RE: Relationship between pitch and thrust?
I better rephrase that. I meant relative to the low pitched prop.
As speed increases, the effective angle of attack is reduced, and so is the engine load of course.
@ Yallaair
Prop stalling is of no importance here as long as the effective blade angle is smaller than 25 degrees at 75%D. Note the word effective, which means after correcting for diameter pitch twist. Below this angle, the prop operates with the airfoil lift curve in the linear part. When a prop approaches square values like 20x20, the prop thrust will no longer be linear, and rotational drag will increase. This is not yet stalled! Stall sets in when at the same rotational speed, the thrust values decrease with increased pitch.
As speed increases, the effective angle of attack is reduced, and so is the engine load of course.
@ Yallaair
Prop stalling is of no importance here as long as the effective blade angle is smaller than 25 degrees at 75%D. Note the word effective, which means after correcting for diameter pitch twist. Below this angle, the prop operates with the airfoil lift curve in the linear part. When a prop approaches square values like 20x20, the prop thrust will no longer be linear, and rotational drag will increase. This is not yet stalled! Stall sets in when at the same rotational speed, the thrust values decrease with increased pitch.
03-07-2007 | 08:13 AM
#31
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RE: Relationship between pitch and thrust?
ORIGINAL: yallaair
...The reason is that in the center of the high pitch prop, angle of attack is above 18ΒΊ. hence, that disc area is stalled static.
...The reason is that in the center of the high pitch prop, angle of attack is above 18ΒΊ. hence, that disc area is stalled static.
Once a prop, any prop, has formed a vortex, the air flowing through it is at roughly pitch speed and thus the effective AoA of the prop's blades is close to 0ΒΊ, i.e. not stalled.
Before a vortex is formed, or if it cannot be formed; like if the prop is in ground effect, most of the blade will be stalled and more of it, if the prop's pitch is higher.
03-07-2007 | 09:53 AM
#32
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My Feedback: (19)
RE: Relationship between pitch and thrust?
Martin Hepperle's java Prop will show the prop partially stalled at very low airspeeds once a certain pitch is reached. I've certainly seen square props as used in F5B models that are partially stalled on launch they make a god awful howling sound for maybe 10 feet (launched vertically), then go dead silent, and accelerate like mad. The prop is not completely stalled as there is thrust. An airfoil creates lift even if it's stalled, just not efficiently.
03-07-2007 | 11:14 AM
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From: Floro, NORWAY
RE: Relationship between pitch and thrust?
This is more or less why we don't set up the heli rotor with a pitch more than 11 degree in any cases. Above 11 dregree, the efficiency goes down and you'll risk rotor stall. This applies for all RC helies. If the rotor / prop thust was linear above 11 degree, everyone would be using shorter blades with better 3D performance....
03-07-2007 | 11:42 AM
#34
RE: Relationship between pitch and thrust?
Hi!
Even in pylonracing we don't calculate anything ...we go from what we have (long winding road of cut and try)and use our knowledge to modify the prop so it works as good as possible.
Don't belive for a minute that you can calculate anything if you want outmost performance.
Use simple knowledge that says large prop disc : good climbing performance at slow speed.
Small prop disc and higher pitch: Higher air speed and not so good climbing performance.
These are the basic prop rules!
For your airplane that weights around 2,6-3kg and is powered by a .82 four stroke I would say 13x7-13x8 APC. Or a 14x4W -14x5N APC. Why APC ...?? because these are the best props at these prop sizes. There are some other good prop brands too like Bolly, RAM and Graupner Cam prop. All of these are 2-blade props. 2-blade props are always better than 3-blade props at these small sizes.
Also remember that all props, plastic as well as wood , could be modified to perform better. Less blade surface means higher rpm and sometimes better climbing performance and speed.Less diameter could mean better speed (but not always).
Remember to keep the rpm of any four stroke lower than 10000rpm for best power and reliablility.
Even in pylonracing we don't calculate anything ...we go from what we have (long winding road of cut and try)and use our knowledge to modify the prop so it works as good as possible.
Don't belive for a minute that you can calculate anything if you want outmost performance.
Use simple knowledge that says large prop disc : good climbing performance at slow speed.
Small prop disc and higher pitch: Higher air speed and not so good climbing performance.
These are the basic prop rules!
For your airplane that weights around 2,6-3kg and is powered by a .82 four stroke I would say 13x7-13x8 APC. Or a 14x4W -14x5N APC. Why APC ...?? because these are the best props at these prop sizes. There are some other good prop brands too like Bolly, RAM and Graupner Cam prop. All of these are 2-blade props. 2-blade props are always better than 3-blade props at these small sizes.
Also remember that all props, plastic as well as wood , could be modified to perform better. Less blade surface means higher rpm and sometimes better climbing performance and speed.Less diameter could mean better speed (but not always).
Remember to keep the rpm of any four stroke lower than 10000rpm for best power and reliablility.
03-07-2007 | 01:02 PM
#35
Senior Member
RE: Relationship between pitch and thrust?
ORIGINAL: yallaair
This is more or less why we don't set up the heli rotor with a pitch more than 11 degree in any cases. Above 11 dregree, the efficiency goes down and you'll risk rotor stall.
This is more or less why we don't set up the heli rotor with a pitch more than 11 degree in any cases. Above 11 dregree, the efficiency goes down and you'll risk rotor stall.
Helicpters fly in ground effect, otherwise most of the rotor will not be stalled at much greater AoA than 11ΒΊ.
If the rotor is stalled, its blades will not make enough lift to take-off.
03-07-2007 | 01:46 PM
#36
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From: Sacramento,
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RE: Relationship between pitch and thrust?
I'm currently using an APC 15x4w. I wish APC had a 14x5N as I feel this might be the best compromise.
ORIGINAL: jaka
Hi!
Even in pylonracing we don't calculate anything ...we go from what we have (long winding road of cut and try)and use our knowledge to modify the prop so it works as good as possible.
Don't belive for a minute that you can calculate anything if you want outmost performance.
Use simple knowledge that says large prop disc : good climbing performance at slow speed.
Small prop disc and higher pitch: Higher air speed and not so good climbing performance.
These are the basic prop rules!
For your airplane that weights around 2,6-3kg and is powered by a .82 four stroke I would say 13x7-13x8 APC. Or a 14x4W -14x5N APC. Why APC ...?? because these are the best props at these prop sizes. There are some other good prop brands too like Bolly, RAM and Graupner Cam prop. All of these are 2-blade props. 2-blade props are always better than 3-blade props at these small sizes.
Also remember that all props, plastic as well as wood , could be modified to perform better. Less blade surface means higher rpm and sometimes better climbing performance and speed.Less diameter could mean better speed (but not always).
Remember to keep the rpm of any four stroke lower than 10000rpm for best power and reliablility.
Hi!
Even in pylonracing we don't calculate anything ...we go from what we have (long winding road of cut and try)and use our knowledge to modify the prop so it works as good as possible.
Don't belive for a minute that you can calculate anything if you want outmost performance.
Use simple knowledge that says large prop disc : good climbing performance at slow speed.
Small prop disc and higher pitch: Higher air speed and not so good climbing performance.
These are the basic prop rules!
For your airplane that weights around 2,6-3kg and is powered by a .82 four stroke I would say 13x7-13x8 APC. Or a 14x4W -14x5N APC. Why APC ...?? because these are the best props at these prop sizes. There are some other good prop brands too like Bolly, RAM and Graupner Cam prop. All of these are 2-blade props. 2-blade props are always better than 3-blade props at these small sizes.
Also remember that all props, plastic as well as wood , could be modified to perform better. Less blade surface means higher rpm and sometimes better climbing performance and speed.Less diameter could mean better speed (but not always).
Remember to keep the rpm of any four stroke lower than 10000rpm for best power and reliablility.
03-07-2007 | 01:49 PM
#37
RE: Relationship between pitch and thrust?
Dar is correct about the ground effect, it reduces the speed the air will go through the helicoptor and on reason they will often fly to higher altitudes than they can take off from. I may be wrong but a prop or wing create no lift when stalled, you are talking about a near stalled condition. The scream from the prop may be the blades cutting near or above mach 1, you can hear it when tangent to the prop but the volume goes down greatly as you go away from tangent. Though being near stalled condition would be less efficient I'm not sure this makes more sound. Seems like on varible pitched fans there is no additional sound when stalled, but they operate at much slower speeds.
03-07-2007 | 02:48 PM
#38
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From: Jonkoping, SWEDEN
RE: Relationship between pitch and thrust?
ORIGINAL: Sport_Pilot
Dar is correct about the ground effect, it reduces the speed the air will go through the helicoptor and on reason they will often fly to higher altitudes than they can take off from.
Dar is correct about the ground effect, it reduces the speed the air will go through the helicoptor and on reason they will often fly to higher altitudes than they can take off from.
Basically the ground effect is made up of two components:
- The vortices coming off the rotor blade tips are reduced in strength thus reducing induced drag.
- Air passing through the rotor disk from above is accelerated downwards towards the ground below. Because the ground to some extent prevents the air from getting out of the way quickly, the air can't accelerate as rapidly as it does when the helicopter hovers at a higher altitude. This reduction in induced flow velocity doesn't reduce the angle of attack of the rotor blades as much as it does when hovering out of ground effect.
Some food for thought: Have a look at the graphs below. They display results of static thrust measurements as a function of power input. In the left graph pitch is kept constant, in the right graph the diameter is kept constant.
As you can see, for "normal" model aircraft propellers, static thrust is almost independent of pitch, but depends strongly on the diameter.
/Red B.
03-07-2007 | 05:19 PM
#39
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From: Floro, NORWAY
RE: Relationship between pitch and thrust?
When hovering in ground effect, the helicopter experiences an INCREASE in performance, i.e., the helicopter can lift a heavier load at a given power setting than what is possible if hovering out of ground effect.
This is what I experience every time I fly the heli. Coming down to hover at waist level, the power can be slightly reduced to maintain constant altitude.
BTW
Have you checked the amps / RPMS at different airspeeds?? Just want to know how much extra amps can be pulled during flight on my 3D foamies. To calculate a buffer without burning the speed-controller
.There is a post on this forum regarding "unloading engine with leaf-blower" He ended up with the same RPM with and without simulated airspeed. This implies that the load on the motor is more or less constant no matter if the airspeed changes within a given value.
03-07-2007 | 06:45 PM
#40
Senior Member
RE: Relationship between pitch and thrust?
As you can see, for "normal" model aircraft propellers, static thrust is almost independent of pitch, but depends strongly on the diameter.
With same diameter and same watts, the disk loading is the same, and so is the generated static thrust. RPM will be different for all props though. Higher for the low pitch, and lowest for the high pitch.
03-07-2007 | 07:31 PM
#41
Senior Member
My Feedback: (19)
RE: Relationship between pitch and thrust?
I may be wrong but a prop or wing create no lift when stalled, you are talking about a near stalled condition.
http://en.wikipedia.org/wiki/Airfoil
RPM at speed is dependent on many things engine power curve, the prop characterisitcs, and airframe drag. We normally see a drop in current and power in electric models as airspeed increases. In the more pitch there is the less it usually unloaded, but it depended on how clean the airframe was. And in an electric model if RPM goes up, power goes down.
The below video shows the launch of F5B models. The models in the video are turning ~18x18 props at ~2100W.
http://www.bayrc.com/videos/f5bteamtrials.wmv
03-08-2007 | 02:38 AM
#42
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From: Sacramento,
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RE: Relationship between pitch and thrust?
I think I may have gotten in over my head just a bit. Eh, whatever! I'm really enjoying the conversation and am learning a lot!
03-08-2007 | 02:46 PM
#43
RE: Relationship between pitch and thrust?
I suppose that while it is possible in some cases for a stalled wing to produce some lift, most are so small to be considered zero lift. I think the graph on that site only showed a partially stalled wing, where the lift coeficient has started to go in the other direction. The graph in this site shows that a fully stalled wing has virtually no lift.
.
http://en.wikipedia.org/wiki/Stall_%28flight%29
.
http://en.wikipedia.org/wiki/Stall_%28flight%29
03-08-2007 | 04:09 PM
#44
Senior Member
RE: Relationship between pitch and thrust?
Hugh,
I believe that Wikipedia graph is exaggerated.
After all, any wing makes lift as a flat plate and a flat plate at any positive angle of attack will make lift.
If you put your hand out of a window of a moving vehicle and change its AoA, you will notice it will try to rise, even if it is at at 45 degrees. Back too, showing the drag is high, but there is always an 'up component'...
Even a hand does make lift, unlike a cylindrical object.
So will a wing, even at a fully stalled AoA.
I believe that Wikipedia graph is exaggerated.
After all, any wing makes lift as a flat plate and a flat plate at any positive angle of attack will make lift.
If you put your hand out of a window of a moving vehicle and change its AoA, you will notice it will try to rise, even if it is at at 45 degrees. Back too, showing the drag is high, but there is always an 'up component'...
Even a hand does make lift, unlike a cylindrical object.
So will a wing, even at a fully stalled AoA.
03-08-2007 | 04:20 PM
#45
RE: Relationship between pitch and thrust?
ORIGINAL: DarZeelon
Hugh,
I believe that Wikipedia graph is exaggerated.
Hugh,
I believe that Wikipedia graph is exaggerated.
The professor in the University in Oslo (Universitatis Osloensis) told the Wikipedia are not 100% sure as encyclopaedia or as a learning stuff because all people can come in and make change in the stuff in the Wikipedia.
Jens Eirik
03-09-2007 | 08:28 AM
#46
RE: Relationship between pitch and thrust?
While I agree the graph may be exagerated, I don't agree that a stalled wing creates lift at any positive angle of attack. I don't think any have any lift at say 89 degrees of attack. You are getting into nitnoids. Safer to assume no lift. Thinking there will be some seems almost dangerous. The lift will be small enough to think of it as zero.
03-09-2007 | 08:46 AM
#47
Senior Member
RE: Relationship between pitch and thrust?
ORIGINAL: Sport_Pilot
While I agree the graph may be exaggerated, I don't agree that a stalled wing creates lift at any positive angle of attack. I don't think any have any lift at say 89ΒΊ angle of attack. You are getting into nitnoids. Safer to assume no lift. Thinking there will be some seems almost dangerous. The lift will be small enough to think of it as zero.
While I agree the graph may be exaggerated, I don't agree that a stalled wing creates lift at any positive angle of attack. I don't think any have any lift at say 89ΒΊ angle of attack. You are getting into nitnoids. Safer to assume no lift. Thinking there will be some seems almost dangerous. The lift will be small enough to think of it as zero.
Maybe not at all angles, but an average wing stalls around 17ΒΊ and will make lift at any angle it is likely to reach in flight.
Even at angles beyond 45ΒΊ it will make some lift.
Only blown elevators can actually produce an AoA which is much higher than the stall AoA of the wing.
03-09-2007 | 09:06 AM
#48
RE: Relationship between pitch and thrust?
Well most varible pitched fans produce very little air flow at an aoa of 45 degrees which would be more like a relative pitch much greater, I did some research and once they stall the airflow goes down which only decreases the advance ratio raising the aoa further. So it would be for a prop. Once stalled the air flow would reduce and then the aoa would jump even further and possibly go well past 45 degrees. Also the noise of those fans is not that differant when stalled, but the airspeed is so much less I don't know it it matters. That is what this was about anyway the noise. I don't think those racer props are in a stalled condition, although their efficiency and thrust will improve greatly as they gain speed. But that is a function of the advance ratio and reduced aoa.
What is a blown elevator? Did you mean foam. Or were you refering to our aircraft elevators. That is a differant situation as the control force is not just the total lift but also the vector of the airflow that is diverted by the elevator. Lift that is 90 degrees relitive to the fuse does not have to be as large as lift that is 30 degrees.
What is a blown elevator? Did you mean foam. Or were you refering to our aircraft elevators. That is a differant situation as the control force is not just the total lift but also the vector of the airflow that is diverted by the elevator. Lift that is 90 degrees relitive to the fuse does not have to be as large as lift that is 30 degrees.
03-09-2007 | 09:19 AM
#49
RE: Relationship between pitch and thrust?
Air passing through the rotor disk from above is accelerated downwards towards the ground below. Because the ground to some extent prevents the air from getting out of the way quickly, the air can't accelerate as rapidly as it does when the helicopter hovers at a higher altitude. This reduction in induced flow velocity doesn't reduce the angle of attack of the rotor blades as much as it does when hovering out of ground effect.
This why a helicopter can fly higher than it can take off from. I said nothing about hover, at altitude it will hover close to the ground, but will not be able to gain enough altitude to gain foward flight. But the same helicoptor would be able to maintain foward flight at that altitude, but may crash if it tries to hover.
03-09-2007 | 09:24 AM
#50
Senior Member
RE: Relationship between pitch and thrust?
ORIGINAL: Sport_Pilot
What is a blown elevator? Did you mean foam. Or were you refering to our aircraft elevators. That is a differant situation as the control force is not just the total lift but also the vector of the airflow that is diverted by the elevator. Lift that is 90 degrees relative to the fuselage does not have to be as large as lift that is 30 degrees.
What is a blown elevator? Did you mean foam. Or were you refering to our aircraft elevators. That is a differant situation as the control force is not just the total lift but also the vector of the airflow that is diverted by the elevator. Lift that is 90 degrees relative to the fuselage does not have to be as large as lift that is 30 degrees.
Blown elevators are those that are in the direct flow path of the prop's wash.
The air thrown back by the prop always hits them chord-wise. So they produce 'up', or 'down' force even when they are at close to an AoA of 90ΒΊ relative to the actual direction of flight, because their AoA is relative to the prop-wash and not to the actual direction of flight.
The wing on the same aircraft can be at an AoA of 90ΒΊ and even more, making no lift at all, but the elevator will still be effective under power.
Foam? What foam?
