Variable Speed Rotors and Prop(s)
Thread Starter
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
Variable Speed Rotors and Prop(s)
The linked web page Variable Speed Rotors and Prop(s) presents a concept for varying the rotational speed of the rotors and the propeller(s) in a smooth and variable relationship.
It is posted to place the concept into the public domain and to solicit critique and suggestions.
Dave
It is posted to place the concept into the public domain and to solicit critique and suggestions.
Dave
Join Date: Jul 2003
Location: Port Townsend,WA. USA
Posts: 440
Likes: 0
Received 0 Likes
on
0 Posts
Dave,
I dont think you want a variable speed prop. Variable pitch is what you want.
The tip speed needs to remain somewhat in the normal range I think, so for all the flight speeds to be covered you need variable pitch. Probably the simpler way also.
Bill
I dont think you want a variable speed prop. Variable pitch is what you want.
The tip speed needs to remain somewhat in the normal range I think, so for all the flight speeds to be covered you need variable pitch. Probably the simpler way also.
Bill
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
Hmmmmm...
My initial though is that there is a lot of system complexity. Lots to develop, lots to go wrong. But...
On e-paper i think variable speed prop makes sense, since you can keep the blade closer to optimum AOA. The ideal would be active propeller twist so that each prop element could run at optimum AOA. I thought the Carter Copter variable pitch prop an interesting approach. The PRPM would be altered depending on required thrust, and likely mapped to reduce PRPM with speed.
In practice i can see many problems with V-PRPM. For a start if the prop (or twin props) is/are also being used for yaw control the system gets very complex very quickly. In this case altering PRPM with airspeed only (if at all) is likely the best solution.
I actually think this is an area where maybe the hybrid helicopter has a major advantage - as long as no individual motor failure puts the machine at instantaneous risk (rotor clash, pilot overload etc). An important rule with hybrid systems is don't compromise your prime mover power/weight (ie force it to deliver high power at low RPM).
Regarding the proposed system, i can see it working only in situations of perfectly balanced torque (ie first time you bank it stops working
). For automotive applications differentials are a necessary evil (but interesting to develop 
). They attempt to balance torque for potentially unknown velocity differences. This is the problem though: when the torque no longer balances the diff no longer works. In your case the servo motor would end up having to introduce/remove quite a lot of energy. I've done calcs for similar active differential concepts, and found that a 20kW motor/gen would be required for my own FWD 1 tonne car!
A better approach would be to drive rotor and prop through seperate gearboxes, and vary the ratios to suit. CVT units are out there now, with more in development...
Mart
[Edit: not yet, but i'll bet i think of somthing after a coffee..]
On e-paper i think variable speed prop makes sense, since you can keep the blade closer to optimum AOA. The ideal would be active propeller twist so that each prop element could run at optimum AOA. I thought the Carter Copter variable pitch prop an interesting approach. The PRPM would be altered depending on required thrust, and likely mapped to reduce PRPM with speed.
In practice i can see many problems with V-PRPM. For a start if the prop (or twin props) is/are also being used for yaw control the system gets very complex very quickly. In this case altering PRPM with airspeed only (if at all) is likely the best solution.
I actually think this is an area where maybe the hybrid helicopter has a major advantage - as long as no individual motor failure puts the machine at instantaneous risk (rotor clash, pilot overload etc). An important rule with hybrid systems is don't compromise your prime mover power/weight (ie force it to deliver high power at low RPM).
Regarding the proposed system, i can see it working only in situations of perfectly balanced torque (ie first time you bank it stops working
). For automotive applications differentials are a necessary evil (but interesting to develop ). They attempt to balance torque for potentially unknown velocity differences. This is the problem though: when the torque no longer balances the diff no longer works. In your case the servo motor would end up having to introduce/remove quite a lot of energy. I've done calcs for similar active differential concepts, and found that a 20kW motor/gen would be required for my own FWD 1 tonne car!A better approach would be to drive rotor and prop through seperate gearboxes, and vary the ratios to suit. CVT units are out there now, with more in development...
Mart
[Edit: not yet, but i'll bet i think of somthing after a coffee..]
Join Date: Feb 2005
Location: Québec, Canada
Age: 69
Posts: 39
Likes: 0
Received 0 Likes
on
0 Posts
Dick Degraw's Rhinogyro.
It seems to me that this set up is used by this Degraw's jump take off gyro, but has a "torque differential" (Torsen) instead of a "power differential"...
Thread Starter
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
Slowrotor,
I agree that a constant speed propeller (& constant speed engine) is best for an airplane. This is probably because there is the desire to maintain the engine speed at a specific location on its rpm/torque curve. Perhaps gyrocopters (CarterCopter, etc.) benefit from a constant speed propeller as well, for the same reason.
This proposed concept also utilizes a constant speed engine. However, the rpm of the rotor must be variable, to adapt to high-speed flight. The fixed gearing arraignment therefore dictates that the rpm of the propeller must be variable as well. Because we want the rpm of the rotor to decrease as the forward velocity of the craft increases, the rpm of the propeller has to increase.
Graviman,
Why do you believe that it "stops working"? This proposed concept is different from the wheels of a car 'breaking loose' from the road.
The servomotor does not contribute to the driving of the rotor or propeller. It is only required to overcome a small portion of the friction between the worm and wheel. The majority of the friction is handled by the angle of repose of the worm thread.
Quadrirotor;
WOW. Thanks for mentioning Dick Degraw's Rhinogyro. I looked up the Torsen differential . It also uses the feature of one way power transmission through a worm and wheel. The results of a preliminary search on the Torsen differential.
Now for some additional studying, so as to better understand the Torsen differential, and then how Dick DeGraw might have applied it to his gyrocopter.
Dave
I agree that a constant speed propeller (& constant speed engine) is best for an airplane. This is probably because there is the desire to maintain the engine speed at a specific location on its rpm/torque curve. Perhaps gyrocopters (CarterCopter, etc.) benefit from a constant speed propeller as well, for the same reason.
This proposed concept also utilizes a constant speed engine. However, the rpm of the rotor must be variable, to adapt to high-speed flight. The fixed gearing arraignment therefore dictates that the rpm of the propeller must be variable as well. Because we want the rpm of the rotor to decrease as the forward velocity of the craft increases, the rpm of the propeller has to increase.
Graviman,
"Regarding the proposed system, i can see it working only in situations of perfectly balanced torque (ie first time you bank it stops working ). "
"In your case the servo motor would end up having to introduce/remove quite a lot of energy"
Quadrirotor;
WOW. Thanks for mentioning Dick Degraw's Rhinogyro. I looked up the Torsen differential . It also uses the feature of one way power transmission through a worm and wheel. The results of a preliminary search on the Torsen differential.
Now for some additional studying, so as to better understand the Torsen differential, and then how Dick DeGraw might have applied it to his gyrocopter.
Dave
Last edited by Dave_Jackson; 11th Jul 2005 at 02:50.
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
"Why do you believe that it "stops working"? This situation is different from the wheels of a car 'breaking loose' from the road."
True, but if you pull collective (like take-off and landing) the torque becomes imbalanced, unless you want to constrain the pilot to have to accelerate every time he wants to manouvre...
"The servomotor does not contribute to the driving of the rotor or propeller. It is only required to overcome a small portion of the friction between the worm and wheel. The majority of the friction is handled by the angle of repose of the worm thread."
But for reasons given above system as drawn won't work, although a torsen diff will greatly improve matters. Don't forget that whenever the diff is locking up it is effectively wasting power, whether torsen or viscous. At any rate i don't understand why you want the prop to run slow in hover but fast at speed - compressibility/noise could become a problem. If any thing the higher mass fow rate through the prop at high speed suggests slowing it down would be more effective - ie direct drive from rotor to prop...
Mart
True, but if you pull collective (like take-off and landing) the torque becomes imbalanced, unless you want to constrain the pilot to have to accelerate every time he wants to manouvre...
"The servomotor does not contribute to the driving of the rotor or propeller. It is only required to overcome a small portion of the friction between the worm and wheel. The majority of the friction is handled by the angle of repose of the worm thread."
But for reasons given above system as drawn won't work, although a torsen diff will greatly improve matters. Don't forget that whenever the diff is locking up it is effectively wasting power, whether torsen or viscous. At any rate i don't understand why you want the prop to run slow in hover but fast at speed - compressibility/noise could become a problem. If any thing the higher mass fow rate through the prop at high speed suggests slowing it down would be more effective - ie direct drive from rotor to prop...
Mart
Join Date: Jul 2004
Location: Huntsville AL
Age: 51
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
Dave,
While I'm still trying to fully grasp the concept of the servo motor and it's purpose in this application I can offer a small tidbit of experience concerning differentials. I once had a camaro with a typical "open" differential and it would do wonderful one sided burnouts. After deciding to upgrade to a true limited slip type diff I opened my unit to find the spider gears eaten away almost to the point of failure!! The moral of the story here is dont expect to use a typical automotive type differential for an application where there is can be a great difference in axle speeds as the internal gears are not meant to handle it long term. I suppose one could be developed for this purpose with spider and side gears that are bearing supported but most car diffs are not so. I can't speak for the torsen type diffs as I have no experience with them but I don plan on installing one in the new mustang I just bought so I'll let ya know!!
Max
While I'm still trying to fully grasp the concept of the servo motor and it's purpose in this application I can offer a small tidbit of experience concerning differentials. I once had a camaro with a typical "open" differential and it would do wonderful one sided burnouts. After deciding to upgrade to a true limited slip type diff I opened my unit to find the spider gears eaten away almost to the point of failure!! The moral of the story here is dont expect to use a typical automotive type differential for an application where there is can be a great difference in axle speeds as the internal gears are not meant to handle it long term. I suppose one could be developed for this purpose with spider and side gears that are bearing supported but most car diffs are not so. I can't speak for the torsen type diffs as I have no experience with them but I don plan on installing one in the new mustang I just bought so I'll let ya know!!
Max
Last edited by maxtork; 11th Jul 2005 at 03:46.
Thread Starter
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
This is long and detailed, but ....
Graviman,
Not so.
The rpm of the propeller has a fixed relationship with the forward velocity of the craft.
No forward velocity = no rotation of the propeller.
This is the job of the Speed Controller.
During hover and slow forward flight, the Speed Controller has the propeller 'locked'.
All power (rpm and torque) is going to the rotor.
During transition, the Speed Controller allows some rpm to be removed from the rotor and transferred to the propeller.
During cruise, the Speed Controller is allowing the propeller to take its half of the power.
The differential is sending half of the power to the rotor and half the power to the propeller.
The rotor is turning at half the speed it was during hover.
The propeller is turning infinitely faster then it was during hover.
The propeller does not have a variable pitch (constant-speed propeller) therefor the higher rpm is used, instead of the increased pitch, to provide increasing thrust during increasing forward velocity.
I beg to differ. Perhaps the action of the worm and gear is not fully understood.
Let's assume that we have a worm & gear that has a very large reduction ratio, say 70:1.
Let's also assume that the axle, which passes through the gear, is applying a large torque to the gear.
Now there is no way that this gear is going to turn because it is incapable, on its own, of turning the worm.
The force of the gear tooth on the worm is just about at 90º to the wall of the worm's thread.
All that the gear is doing is apply a lot of static friction against the wall of the worm's thread.
To allow the gear to turn, it will require a VERY large servomotor to TURN the worm.
Now let's assume that we have a worm & gear that has a very small reduction ratio, say 1:1. (Actual this would be called a pair of 90º helical gears)
Let's also assume that the axle, which passes through the gear, is applying a large torque to the gear.
At this ratio, it is extremely easy for the gear to turn the so-called worm.
There is very little static friction resisting the rotation of the gear.
To NOT allow the gear to turn, it will require a VERY large servomotor to RESIST THE TURNING of the worm.
It should be apparent that as the ratio of 70:1 is reduced the size of the servomotor required to TURN the worm will be smaller.
It should be apparent that as the ratio of 1:1 is increased the size of the servomotor required to RESIST THE TURNING the worm will be smaller.
At a ratio of approximately 10:1 it become a 'flip of the coin' whether the servomotor is needed to TURN the worm or RESIST THE TURNING of the worm. In other words, at this ratio the servomotor can be quite small. This is why the ratio of 10:1 was picked for the Speed Controller.
maxtork,
Thanks for the very valid comment about the weakness of conventional spider gears when the differential is spending a lot of its time transmitting unequal RPM's to the two outputs.
Dave
True, but if you pull collective (like take-off and landing) the torque becomes imbalanced, unless you want to constrain the pilot to have to accelerate every time he wants to manouvre...
The rpm of the propeller has a fixed relationship with the forward velocity of the craft.
No forward velocity = no rotation of the propeller.
This is the job of the Speed Controller.
During hover and slow forward flight, the Speed Controller has the propeller 'locked'.
All power (rpm and torque) is going to the rotor.
During transition, the Speed Controller allows some rpm to be removed from the rotor and transferred to the propeller.
During cruise, the Speed Controller is allowing the propeller to take its half of the power.
The differential is sending half of the power to the rotor and half the power to the propeller.
The rotor is turning at half the speed it was during hover.
The propeller is turning infinitely faster then it was during hover.
At any rate i don't understand why you want the prop to run slow in hover but fast at speed
... for reasons given above system as drawn won't work
Let's assume that we have a worm & gear that has a very large reduction ratio, say 70:1.
Let's also assume that the axle, which passes through the gear, is applying a large torque to the gear.
Now there is no way that this gear is going to turn because it is incapable, on its own, of turning the worm.
The force of the gear tooth on the worm is just about at 90º to the wall of the worm's thread.
All that the gear is doing is apply a lot of static friction against the wall of the worm's thread.
To allow the gear to turn, it will require a VERY large servomotor to TURN the worm.
Now let's assume that we have a worm & gear that has a very small reduction ratio, say 1:1. (Actual this would be called a pair of 90º helical gears)
Let's also assume that the axle, which passes through the gear, is applying a large torque to the gear.
At this ratio, it is extremely easy for the gear to turn the so-called worm.
There is very little static friction resisting the rotation of the gear.
To NOT allow the gear to turn, it will require a VERY large servomotor to RESIST THE TURNING of the worm.
It should be apparent that as the ratio of 70:1 is reduced the size of the servomotor required to TURN the worm will be smaller.
It should be apparent that as the ratio of 1:1 is increased the size of the servomotor required to RESIST THE TURNING the worm will be smaller.
At a ratio of approximately 10:1 it become a 'flip of the coin' whether the servomotor is needed to TURN the worm or RESIST THE TURNING of the worm. In other words, at this ratio the servomotor can be quite small. This is why the ratio of 10:1 was picked for the Speed Controller.
maxtork,
Thanks for the very valid comment about the weakness of conventional spider gears when the differential is spending a lot of its time transmitting unequal RPM's to the two outputs.
Dave
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
"Perhaps the action of the worm and gear is not fully understood."
Dave, i think you're misunderstanding where i'm coming from. The point is that to control the prop velocity the worm gear has to react large torques. Since Power=Torque x Rot_Vel this means that whenever the worm is "controlling" the speed a lot of engine power is wasted.
I agree with all you are saying, but you have just found another method to avoid the servo motor itself having to dissipate all this power. We have a lot of problems when drivers think the limited slip we fit is traction control - it doesn't take much Torque x Rot_Vel to cause a diff clutch burn out.
Since this is effectively a type of shunt transmission, i suggest you do a power flow calculation for different flight regimes. You'll soon begin to understand my concerns. Don't forget that wasted power in a heli is lost payload...
Regarding prop velocity, let me clarify:
Prop RPM should be proportional to SQRT(Prop_Thrust), so that dynamic pressure controls thrust at fixed blade AOA. As aircraft flight speed increases the blades will twist/pitch, so that more of the lift/thrust component becomes torque. This just means that for a given input torque Prop RPM will drop off with airspeed, resulting in a a reduction in thrust. Alternately increase torque linearly with speed, to maintain fixed prop RPM hence thrust. For ideal blade twist, design a torsionally soft blade with trailing edge trim (or servo flaps).
Mart
[Edit: hopefully to clear up last paragraph...]
Dave, i think you're misunderstanding where i'm coming from. The point is that to control the prop velocity the worm gear has to react large torques. Since Power=Torque x Rot_Vel this means that whenever the worm is "controlling" the speed a lot of engine power is wasted.
I agree with all you are saying, but you have just found another method to avoid the servo motor itself having to dissipate all this power. We have a lot of problems when drivers think the limited slip we fit is traction control - it doesn't take much Torque x Rot_Vel to cause a diff clutch burn out.
Since this is effectively a type of shunt transmission, i suggest you do a power flow calculation for different flight regimes. You'll soon begin to understand my concerns. Don't forget that wasted power in a heli is lost payload...
Regarding prop velocity, let me clarify:
Prop RPM should be proportional to SQRT(Prop_Thrust), so that dynamic pressure controls thrust at fixed blade AOA. As aircraft flight speed increases the blades will twist/pitch, so that more of the lift/thrust component becomes torque. This just means that for a given input torque Prop RPM will drop off with airspeed, resulting in a a reduction in thrust. Alternately increase torque linearly with speed, to maintain fixed prop RPM hence thrust. For ideal blade twist, design a torsionally soft blade with trailing edge trim (or servo flaps).
Mart
[Edit: hopefully to clear up last paragraph...]
Last edited by Graviman; 13th Jul 2005 at 19:53.
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 419 Likes
on
221 Posts
Is this Eng-Tips or PPRuNe or what? 
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
Hehehe. I gave up on Eng-Tips a long time ago - aparently using my experience as an engineer to speculate about future developments wasn't seen as profesional 
. Guess i'm just not happy debating wing-nuts...
I appreciate that i am to a large extent a guest on PPRUNE, so will reduce tech content if requested.
Mart
. Guess i'm just not happy debating wing-nuts...I appreciate that i am to a large extent a guest on PPRUNE, so will reduce tech content if requested.
Mart
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 419 Likes
on
221 Posts
Did you upset Pat Primmer? I hope you used the search engine properly before daring to post, to avoid wasting the valuable time of professionals! 
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
No, I upset Dave Murphy (moderator) himself! Apparently my not regarding his site as a learned academic establishment, and posting this sort of thing really upset him. I noticed a lot of prof engineers disappear from that one - wonder why? 
How many engineers does it take to change a light bulb?
Only one, but he has to develop a prototype.
How many heli pilots does it take to change a light bulb?
Again only one, but he has to safely ground the failing one first.
Mart
How many engineers does it take to change a light bulb?
Only one, but he has to develop a prototype.
How many heli pilots does it take to change a light bulb?
Again only one, but he has to safely ground the failing one first.
Mart
Join Date: Jan 2001
Location: Dallas, TX USA
Posts: 739
Likes: 0
Received 0 Likes
on
0 Posts
Dave, it seems to me that for any of this to work, you have to get the weight off of the rotors during high speed cruise. In a conventional helo, the rotor has the provide both forward thrust and lift so at least theoretically, the rotor speed has to be maintained.
In a compound helocopter, you have some other device providing the thrust. So likewise it seems the only way you can slow the rotor down in cruise flight, is to have some other surface providing the lift (at least a major part of it). This seems the only way to slow the rotor down and still maintain a decent low drag AOA for the rotor blades.
I'm often amazed by what was accomplished with the Fairey Rotodyne for its time. I know it used autogyro principles, but this is in the end, a variable speed rotor.
Fairey Rotodyne
In a compound helocopter, you have some other device providing the thrust. So likewise it seems the only way you can slow the rotor down in cruise flight, is to have some other surface providing the lift (at least a major part of it). This seems the only way to slow the rotor down and still maintain a decent low drag AOA for the rotor blades.
I'm often amazed by what was accomplished with the Fairey Rotodyne for its time. I know it used autogyro principles, but this is in the end, a variable speed rotor.
Fairey Rotodyne
Thread Starter
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
Graviman,
I partially agree with what you are saying. In addition, I think that you will agree with the following;
For the power to be wasted there must be sliding friction. The only location of sliding friction in the powertrain is between the worm & gear, in the Speed Controller.
During slow flight and hover the worm & gear are not turning, therefor there is no power loss due to friction.
During cruise, the propeller is consuming all the power on that side of the differential. The worm is not resisting the gear, therefor there is no power loss due to friction.
IMO, the power loss due to friction will only take place during forward speeds between transition and cruise. I am currently trying to calculate the power loss here. It is looking like the power loss may be a curve; from 0% at transition, up to 13%, and then back down to 0% at cruise. This is not particularly attractive and requires more thinking.
Flight Safety,
What you say is correct, however there is an alternative to the wings of a compound helicopter. This alternative is the use of large chord rotor blades. Here is an example and here is an explanation
Dave
The point is that to control the prop velocity the worm gear has to react large torques. Since Power=Torque x Rot_Vel this means that whenever the worm is "controlling" the speed a lot of engine power is wasted
For the power to be wasted there must be sliding friction. The only location of sliding friction in the powertrain is between the worm & gear, in the Speed Controller.
During slow flight and hover the worm & gear are not turning, therefor there is no power loss due to friction.
During cruise, the propeller is consuming all the power on that side of the differential. The worm is not resisting the gear, therefor there is no power loss due to friction.
IMO, the power loss due to friction will only take place during forward speeds between transition and cruise. I am currently trying to calculate the power loss here. It is looking like the power loss may be a curve; from 0% at transition, up to 13%, and then back down to 0% at cruise. This is not particularly attractive and requires more thinking.
Flight Safety,
What you say is correct, however there is an alternative to the wings of a compound helicopter. This alternative is the use of large chord rotor blades. Here is an example and here is an explanation
Dave
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
"The only location of sliding friction in the powertrain is between the worm & gear, in the Speed Controller"
But replacing worm gear with a Motor/Gen helps to recapture this power. Dig up some papers on the Toyota Motor/Gen shunt transmission system used on the "Prius" hybrid.
"It is looking like the power loss may be a curve; from 0% at transition, up to 13%, and then back down to 0% at cruise"
Sounds about right. Trouble with shunt systems is that by the time you've minimised either the losses or recirculated power you end up with an inline transmission (or 2 in this case). Don't forget a pilot in an emergency may not follow the calculated flight envelope, but the heli has to deliver without trans failure.
What were your thoughts regarding proposed method of linking prop thrust with prop RPM? I agree with variable rotor rpm being far preferable to compounding.
Mart
But replacing worm gear with a Motor/Gen helps to recapture this power. Dig up some papers on the Toyota Motor/Gen shunt transmission system used on the "Prius" hybrid.
"It is looking like the power loss may be a curve; from 0% at transition, up to 13%, and then back down to 0% at cruise"
Sounds about right. Trouble with shunt systems is that by the time you've minimised either the losses or recirculated power you end up with an inline transmission (or 2 in this case). Don't forget a pilot in an emergency may not follow the calculated flight envelope, but the heli has to deliver without trans failure.
What were your thoughts regarding proposed method of linking prop thrust with prop RPM? I agree with variable rotor rpm being far preferable to compounding.
Mart
Thread Starter
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
Graviman,
I am having difficulty understanding the paragraph in your earlier posting. Does your use of the word 'speed' mean 'rotational speed'?
If so, are you proposing that rotational aerodynamic drag of the propeller (enhanced profile drag due to extremely high blade pitch) be used as an alternative to the worm & gear, for controlling the rpm of the propeller?
_______________________________
Edited to add the following.
I have redone the calculations for power loss using the rotational speed of the wheel instead of that of the worm. This appears to be more logical. The maximum power lost to friction is now 0.83% of total power instead of the previous 13%. If correct, this is very acceptable.
I believe that the worm & gear is absorbing very little power (friction). It is redirecting the power to the rotor, as desired.
Dave
What were your thoughts regarding proposed method of linking prop thrust with prop RPM?
If so, are you proposing that rotational aerodynamic drag of the propeller (enhanced profile drag due to extremely high blade pitch) be used as an alternative to the worm & gear, for controlling the rpm of the propeller?
_______________________________
Edited to add the following.
I have redone the calculations for power loss using the rotational speed of the wheel instead of that of the worm. This appears to be more logical. The maximum power lost to friction is now 0.83% of total power instead of the previous 13%. If correct, this is very acceptable.
I believe that the worm & gear is absorbing very little power (friction). It is redirecting the power to the rotor, as desired.
Dave
Last edited by Dave_Jackson; 13th Jul 2005 at 06:57.
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
"I am having difficulty understanding the paragraph in your earlier posting."
Rewritten, hopefully to make it clearer.
"...are you proposing that rotational aerodynamic drag of the propeller be used as an alternative to the worm & gear...?"
Not really. I am trying to demonstrate that at a given airspeed the prop RPM should ideally vary with thrust. Your system doesn't cater for different thrust levels, since at a given airspeed the prop torque will vary (causing diff torque mismatch). Also for max engine power, assuming a "magic" variable ratio gearbox, prop will decrease RPM with airspeed (and acceleration will drop off), while torque goes up.
Since in practice varying prop RPM with thrust is impractical, prop RPM should decrease with speed while pitch controls instantaneous thrust. Rotor also ideally reduces RPM with speed, but unlikely to be along same curve as prop. This all seems to point to two seperate CVT gearboxes ideally. I am assuming that prop always provides horizontal thrust from hover to Vmax.
"I believe that the worm & gear is absorbing very little power (friction). It is redirecting the power to the rotor, as desired."
It is difficult for me to comment, since i am not sure how you believe the total rotor/prop/diff system to be working. The only way your statement can be true is if the prop torque always perfectly balances rotor torque, and i just don't see how this can be unless you are imposing a propeller thrust on the pilot (since rotor thrust is constant - in level flight only).
Mart
[Edit: 'cos my brain hurts
]
Rewritten, hopefully to make it clearer.
"...are you proposing that rotational aerodynamic drag of the propeller be used as an alternative to the worm & gear...?"
Not really. I am trying to demonstrate that at a given airspeed the prop RPM should ideally vary with thrust. Your system doesn't cater for different thrust levels, since at a given airspeed the prop torque will vary (causing diff torque mismatch). Also for max engine power, assuming a "magic" variable ratio gearbox, prop will decrease RPM with airspeed (and acceleration will drop off), while torque goes up.
Since in practice varying prop RPM with thrust is impractical, prop RPM should decrease with speed while pitch controls instantaneous thrust. Rotor also ideally reduces RPM with speed, but unlikely to be along same curve as prop. This all seems to point to two seperate CVT gearboxes ideally. I am assuming that prop always provides horizontal thrust from hover to Vmax.
"I believe that the worm & gear is absorbing very little power (friction). It is redirecting the power to the rotor, as desired."
It is difficult for me to comment, since i am not sure how you believe the total rotor/prop/diff system to be working. The only way your statement can be true is if the prop torque always perfectly balances rotor torque, and i just don't see how this can be unless you are imposing a propeller thrust on the pilot (since rotor thrust is constant - in level flight only).
Mart
[Edit: 'cos my brain hurts
]
Last edited by Graviman; 13th Jul 2005 at 21:57.
