Active Blade Twist, the Tail Rotor, and Future Rotorcraft
Joined: Jul 2003
Posts: 440
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From: Port Townsend,WA. USA
Good points.
I think there is some desire to find an alternative to the tail rotor but the thinking is usually for preventing tail strikes to passengers and other objects. The alternatives are less efficient but have a reason for use.
If Dave is concerned with power efficiency (and he should be with piston engine) then he could improve the efficiency by a much greater amount with low disc loading.
Back in the 1980's the state of Alaska put out a RFP (request for proposals) for a
"Tailrotorless" helicopter. Preventing strikes was the reason stated in the RFP.
No bids were received from any major helicopter company. Only two proposals came in, one was from me. The state decided to abandon the scheme after a news reporter determined that the real purpose of the RFP was to transfer state funds to some friend of a legislator.
I think there is some desire to find an alternative to the tail rotor but the thinking is usually for preventing tail strikes to passengers and other objects. The alternatives are less efficient but have a reason for use.
If Dave is concerned with power efficiency (and he should be with piston engine) then he could improve the efficiency by a much greater amount with low disc loading.
Back in the 1980's the state of Alaska put out a RFP (request for proposals) for a
"Tailrotorless" helicopter. Preventing strikes was the reason stated in the RFP.
No bids were received from any major helicopter company. Only two proposals came in, one was from me. The state decided to abandon the scheme after a news reporter determined that the real purpose of the RFP was to transfer state funds to some friend of a legislator.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Nick,
"...active on-blade control is the future of helicopter technology."
Absolutely agreed absolutely.
"...the perennial loser, the syncropter."
For the normal speed range of military helis i agree, although i suspect the synchropter to be more efficient in hover (but perhaps heavier). In truth for the lower disk loadings of civil helis, which are still fuel guzzlers, i can believe the figure gets to 10% (lots of left foot on an R22).
My interest stems from the work done by both Lockheed in the 186/AH-56 and your old workhorse Sikorsky's ABC - both had the posibility of seriously pushing up top speeds. To avoid retreating blade stall drag my gut feeling is that synchropter (outboard forward) offers less drag than coaxial. My frustration is partly that Dave will open up a discussion, but won't give particulars of the application - as a specification this makes life very difficult...
---
Slowrotor,
"...the real purpose of the RFP was to transfer state funds to some friend of a legislator."
Oh the joys of high tech engineering!
I like the construction equipment (truck) industry for this sort of reason. There is less b*ll!!!!! than the UK auto industry, and the customer just wants a machine that effectively does a job. I wouldn't mind the transition to aerospace, but not if it means starving! 
Mart
[Edit: typos]
"...active on-blade control is the future of helicopter technology."
Absolutely agreed absolutely.
"...the perennial loser, the syncropter."
For the normal speed range of military helis i agree, although i suspect the synchropter to be more efficient in hover (but perhaps heavier). In truth for the lower disk loadings of civil helis, which are still fuel guzzlers, i can believe the figure gets to 10% (lots of left foot on an R22).
My interest stems from the work done by both Lockheed in the 186/AH-56 and your old workhorse Sikorsky's ABC - both had the posibility of seriously pushing up top speeds. To avoid retreating blade stall drag my gut feeling is that synchropter (outboard forward) offers less drag than coaxial. My frustration is partly that Dave will open up a discussion, but won't give particulars of the application - as a specification this makes life very difficult...
---
Slowrotor,
"...the real purpose of the RFP was to transfer state funds to some friend of a legislator."
Oh the joys of high tech engineering!
I like the construction equipment (truck) industry for this sort of reason. There is less b*ll!!!!! than the UK auto industry, and the customer just wants a machine that effectively does a job. I wouldn't mind the transition to aerospace, but not if it means starving! Mart
[Edit: typos]
Last edited by Graviman; 28th May 2005 at 16:13.
Thread Starter
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
So this thread still has legs.
Nick, I think you just slipped on your own bar of soap. 
Excerpts from previous threads, which you participated in;
Graviman,
This is my 'top down'. Others may differ.
Top level; The Configuration.
Dave
Excerpts from previous threads, which you participated in;
In opposition to the western perspective ~ " .... the single-rotor helicopter's tail rotor power consumption accounts for 10-12% of total power." ~ Eduard Petrosyan, Deputy Chief Designer of the Kamov Company
Nick, are you putting a little western spin on your percentages? ........
Would you agree that the torque of the main rotor must be offset in both hover and forward flight? In forward flight the 'angular drag' of the vertical stabilizer 'takes over' for the tail rotor?
Would you agree that the torque of the main rotor must be offset in both hover and forward flight? In forward flight the 'angular drag' of the vertical stabilizer 'takes over' for the tail rotor?
.... In addition, the latest aerodynamic text (year 2000), by Leishman (University of Maryland) concurs with Kamov's position.
This is my 'top down'. Others may differ.
Top level; The Configuration.
- Future commercial and military VTOL craft may involve concepts that are currently no more than dreams.
- Rotorcraft is subset of VTOL. Rotorcraft's near term future WILL be twin counterrotating main-rotors (w/o tail-rotor).
- The intermeshing, the interleaving, the side-by-side, or all of them may be part of the future. IMO, the intermeshing is probably the best prospect for small, high performance rotorcraft.
- The rotor is the essence of rotorcraft. Therefore, my current project is to develop and build the blade.
- This blade, with minor modifications, should be applicable to all of the above twin main-rotor configurations.
- In addition, If the current blade design does not work, the production methodology and equipment is still appropriate for the three alternative blade designs.
Dave
Last edited by Dave_Jackson; 28th May 2005 at 19:55.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Lets start over...
Dave,
"Next level; The Rotor...."
OK, I see where you are coming from now. You have to forgive me for being thouroughly confused, since it was not clear to me what the actual heli project was. There have been quite a few of them...
Can you give a summary of the control system you intend to use with this rotor? I am particularly interested in how you intend to control the tip and root seperately, past the blade hub interface. Indeed, it's due to this system complexity that i suggest fixed tip tabs as a starting point. My concern is how you intend a practical control system that will cyclically alter twist with forward velocity. I presume the average root/tip twist is pilot controlled (or gyro or whatever), then a servo system alters the differential pitch? Is there any way this system can be implemented purely mechanically, for application in an inexpensive light helicopter?
I am not convinced about reverse velocity utilisation because:
1. There is a region (zero velocity circle) where upwash may still occur.
2. It does not answer the drag problems from retreating blade stall at high speed.
3. In an intermesher, use of the retreating blade to produce lift will likely be detrimental on advancing blade.
4. Although improving downwash distribution, it does not allow the optimum downwash that would come from (say) a fixed wing.
5. The reverse flow aerofoil will be at best a compromise, and at worse will suffer structural divergence.
That said, i don't see why active blade twist can't optimise advancing blade, and allow optimum feathering of retreating blade.
How about this system for an intermesher: Cyclic alters tip pitch only, with nominal tip AOA 0deg to airflow (minimise tip losses). Collective alters root pitch, but swash plate arranged to force retreating root to feathered in downwash.
Mart
"Next level; The Rotor...."
OK, I see where you are coming from now. You have to forgive me for being thouroughly confused, since it was not clear to me what the actual heli project was. There have been quite a few of them...
Can you give a summary of the control system you intend to use with this rotor? I am particularly interested in how you intend to control the tip and root seperately, past the blade hub interface. Indeed, it's due to this system complexity that i suggest fixed tip tabs as a starting point. My concern is how you intend a practical control system that will cyclically alter twist with forward velocity. I presume the average root/tip twist is pilot controlled (or gyro or whatever), then a servo system alters the differential pitch? Is there any way this system can be implemented purely mechanically, for application in an inexpensive light helicopter?
I am not convinced about reverse velocity utilisation because:
1. There is a region (zero velocity circle) where upwash may still occur.
2. It does not answer the drag problems from retreating blade stall at high speed.
3. In an intermesher, use of the retreating blade to produce lift will likely be detrimental on advancing blade.
4. Although improving downwash distribution, it does not allow the optimum downwash that would come from (say) a fixed wing.
5. The reverse flow aerofoil will be at best a compromise, and at worse will suffer structural divergence.
That said, i don't see why active blade twist can't optimise advancing blade, and allow optimum feathering of retreating blade.
How about this system for an intermesher: Cyclic alters tip pitch only, with nominal tip AOA 0deg to airflow (minimise tip losses). Collective alters root pitch, but swash plate arranged to force retreating root to feathered in downwash.
Mart
Thread Starter
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Graviman,
This particular thread was intended to be a general one about future rotorcraft and the features needed to finally take rotorcraft into its 2nd generation.
The UniCopter is a specific attempt to achieve the above and this is not the place to go into its details. To answer the first of your many points, the dry and boring work on this craft's flight-controls can be found here.
Want to get involved in CNC?
Dave
This particular thread was intended to be a general one about future rotorcraft and the features needed to finally take rotorcraft into its 2nd generation.
The UniCopter is a specific attempt to achieve the above and this is not the place to go into its details. To answer the first of your many points, the dry and boring work on this craft's flight-controls can be found here.
Want to get involved in CNC?
Dave
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Dave,
Of course the Kamov people exaggerate the tail rotor losses, they make coaxials! Quote them all you wish, the real tail rotor losses for a single rotor helo are about 5% in hover at MGW and about 1% in cruise.
The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines. You do the math, I am too tired of arguing with you!
Of course the Kamov people exaggerate the tail rotor losses, they make coaxials! Quote them all you wish, the real tail rotor losses for a single rotor helo are about 5% in hover at MGW and about 1% in cruise.
The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines. You do the math, I am too tired of arguing with you!
Sikorsky spin.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Dave,
As usual, you think this is a debate of words and theories, and somehow words mean nothing or everything, depending on how they agree with you.
As Lenin said, "Facts are stubborn things."
As the data I provided shows, the S-76B in a steady hover uses less than 5% of its total power for the tail rotor (and it has a particularly high thrust tail rotor). That is a fact, it can and has been measured. Your poor confused mind might think it is "spin" and your clever little icons look cute, but it is a fact, and you are simply not correct.
You are perhaps the ppruner who can least deal with facts that disagree with your opinions. You have several times accused me of "spin" which infers that somehow what I have posted is not true, or is incomplete and misleading. In all threads here and elsewhere, I try to post the facts as I know them, based on hard data and true experience. You glean half-truths from statements and try to make them into proofs for your strange and unproven assertions.
The fact is that single rotor helos use less than 6% of their hover power for anti-torque. Deal with this fact.
As usual, you think this is a debate of words and theories, and somehow words mean nothing or everything, depending on how they agree with you.
As Lenin said, "Facts are stubborn things."
As the data I provided shows, the S-76B in a steady hover uses less than 5% of its total power for the tail rotor (and it has a particularly high thrust tail rotor). That is a fact, it can and has been measured. Your poor confused mind might think it is "spin" and your clever little icons look cute, but it is a fact, and you are simply not correct.
You are perhaps the ppruner who can least deal with facts that disagree with your opinions. You have several times accused me of "spin" which infers that somehow what I have posted is not true, or is incomplete and misleading. In all threads here and elsewhere, I try to post the facts as I know them, based on hard data and true experience. You glean half-truths from statements and try to make them into proofs for your strange and unproven assertions.
The fact is that single rotor helos use less than 6% of their hover power for anti-torque. Deal with this fact.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
My brain hurts, make it stop!
Nick,
Please feel very free to criticise these thoughts - I am extremely interested in both your facts and opinions on this subject:
"The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines."
In cruise the vertical stabiliser takes over from tail rotor for anti-torque. Tail rotor loss becomes vertical stabiliser trim drag, but total anti-torque power requirement stays at the same 5%. Result is main rotor power requirement goes up, but tail rotor power will go down to 1%. Additionally in cruise main rotor hub presents a worse aerodynamic profile, unless the horizontal stabilisers produce more trim drag to keep same attitude.
The way i think about synchropters is that the tail rotor "migrates" up to where the main rotor is. The individual hub drag stays about the same, although there will be some adverse interference effects. In hover the synchropter beats conventional for power/weight requirement (better inflow to wake contraction and no swirl), although intermeshing complexity increases rotor mass. During transition to cruise, the synchropter will begin to suffer with a higher power requirement than the conventional (twin rotor/hub interference). By introducing variable blade twist and feathered retreating blade, this disadvantage can be lessened. Again the conventional can have active blade twist, but reverse velocity utilisation never allows ideal downwash distribution (retreating blade zero velocity circle).
The real decider comes when the conventional suffers retreating tip stall, at say >200 kts. If the intermesher is operating in outboard advancing ABC mode, this will start to beat the conventional for total power requirement since conventional can only use forward and rear rotor quadrants for lift. Eventually compressibility will limit top speed, but reduced synchropter rotor rpm will help attain a higher speed. Coaxial ABC is another alternative to the same ends, but will suffer far worse drag since hub fairing is virtually impossible.
Control is, agreed, not an issue. It is very easy by use of either gyro, large stabilisers or servo control to make either helicopter handle well (at a reasonable level of cost and reliability). For this reason breastroke rotation synchropters have, at best, a limited future.
Practical upshot: Only consider intermeshing for speeds > 200 kts.
Does this sound about right, or am i missing the plot somehow?
Mart
[Edit: to make point more clearly - and resolve headache
]
Please feel very free to criticise these thoughts - I am extremely interested in both your facts and opinions on this subject:
"The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines."
In cruise the vertical stabiliser takes over from tail rotor for anti-torque. Tail rotor loss becomes vertical stabiliser trim drag, but total anti-torque power requirement stays at the same 5%. Result is main rotor power requirement goes up, but tail rotor power will go down to 1%. Additionally in cruise main rotor hub presents a worse aerodynamic profile, unless the horizontal stabilisers produce more trim drag to keep same attitude.
The way i think about synchropters is that the tail rotor "migrates" up to where the main rotor is. The individual hub drag stays about the same, although there will be some adverse interference effects. In hover the synchropter beats conventional for power/weight requirement (better inflow to wake contraction and no swirl), although intermeshing complexity increases rotor mass. During transition to cruise, the synchropter will begin to suffer with a higher power requirement than the conventional (twin rotor/hub interference). By introducing variable blade twist and feathered retreating blade, this disadvantage can be lessened. Again the conventional can have active blade twist, but reverse velocity utilisation never allows ideal downwash distribution (retreating blade zero velocity circle).
The real decider comes when the conventional suffers retreating tip stall, at say >200 kts. If the intermesher is operating in outboard advancing ABC mode, this will start to beat the conventional for total power requirement since conventional can only use forward and rear rotor quadrants for lift. Eventually compressibility will limit top speed, but reduced synchropter rotor rpm will help attain a higher speed. Coaxial ABC is another alternative to the same ends, but will suffer far worse drag since hub fairing is virtually impossible.
Control is, agreed, not an issue. It is very easy by use of either gyro, large stabilisers or servo control to make either helicopter handle well (at a reasonable level of cost and reliability). For this reason breastroke rotation synchropters have, at best, a limited future.
Practical upshot: Only consider intermeshing for speeds > 200 kts.
Does this sound about right, or am i missing the plot somehow?
Mart
[Edit: to make point more clearly - and resolve headache
]
Last edited by Graviman; 1st June 2005 at 02:40.
Thread Starter
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Mia culpa
The math in the initial post on this thread was wrong.
The main-rotor improvements and the advantage of eliminating the tail-rotor are still valid. However, these improvements to the main-rotor do not proportionally increase the reasons for eliminating the tail-rotor.
Dave
The math in the initial post on this thread was wrong.
The main-rotor improvements and the advantage of eliminating the tail-rotor are still valid. However, these improvements to the main-rotor do not proportionally increase the reasons for eliminating the tail-rotor.
Dave
