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Contra Rotating Propellers

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Old 02-12-2003, 06:57 PM
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venividivici
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Default Contra Rotating Propellers

What are the advantages and disadvantages of aircrafts using counter rotating propellers to provide lift?

Is its thrust or lifting power the same as to a single propeller setup?

Thanks!
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Old 02-12-2003, 07:14 PM
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Default Contra Rotating Propellers

The main advantage is the torque is offset by each propeller, producing little or no torque to the fuse. I saw a helicopter show a few nights ago and the heli had counter rotating blades and it did not require and tail rotor.

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Old 02-13-2003, 11:53 AM
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Default Contra Rotating Propellers

mahoo is right concerning the canceling out of torque. To answer the second part of your question, counter rotating propellers are less efficient than a single propeller if they are "coaxial".
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Old 02-13-2003, 08:33 PM
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Default Contra Rotating Propellers

thanks!

so there is no gain in thrust when they are placed on one axis.

what if the propellers are placed side by side (prop1 )|(prop2 ), each prop are enclosed individually and they are still contra rotating?

they would produce more power against a single prop (right?)? and they won't require having a tail rotor?

thanks in advance!
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Old 05-15-2003, 01:56 AM
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Default Contra Rotating Propellers

After I got out of WWII I went to work in experimental at Northrop Aviation on the XB35 and the YB49. As you may know the XB35 had counter rotating pusher props, 4 sets of them. The shafts and gear boxes were almost 35' long. This airplane had unequaled range and load carrying capacity for its day. The wingspan of the B2 is within a few inches of the XB35. The insistence of the then Sec'y of Defense that the airplane be converted to jet power as the YB49 ruined a fine aircraft. Apparently the four sets of counter rotating props were efficient enough even if they were on single shafts. The effects of torque were not a consideration on the 172' airplane. Later models of the P38 had props that turned in opposite directions that canceled out the effects of torque but there were on opposite sides of the center line. The difficulty with the Northrop wing was that it tended to hunt directionally which made it an unstable bombing platform. Edwards Air Force Base is named for a test pilot who was killed in the crash of a YB49.
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Old 05-30-2003, 06:53 AM
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Default Contra Rotating Propellers


on full-size aircraft the usual reason for contra-rotating props is not so much to cancel out torque, its to avoid having a long undercarriage necessary for a single large prop on a high powered engine.They were frequently used on early turbo-prop aircraft, for example the Westland "Wyvern" and Fairey "Gannet" naval aircraft, and the russian "Bear" reconnaisance aircraft. They are less efficient than a single prop, not least because of the frictional losses and extra weight of the necessary gearing.
Another way of avoiding torque problems is "push-pull" power from two engines, example the Dornier 335 heavy fighter.Of course the snag there is to keep the rear prop from striking the ground.
I have flown a few "push-pull" surveillance RPVs and can confirm that they are very nice to fly.
By the way, dont confuse the effects of torque, (usually manifested as a rolling tendency, especially at low forward speeds, or a strong yaw, example during take-off) with the effects of assymetric disc loading, which can result in a strong yaw/roll moment.
To clarify ; when you pull the nose up,(assuming a counter clockwise rotation of the prop) and especially during climb-out from take-off, the right(starboard)) side blade, going downwards, is taking a bigger bite at the air than the port, up going side, so the aircraft tries to turn/roll to the left (port).
If the airspeed is insufficient for the rudder/ailerons to hold the plane straight, the result is sometimes a strong left turn culminating in a roll and dive into the ground. Remedy, back off on the throttle damn quick!
Saw just this happen to a .90 powered Mustang a couple of weeks ago! A black bag job!
Another point is that the spiral airflow coming off the prop will "blow harder" on the left side of the fin than on the right,, also causing a left turn tendency. Which is why we often use some side thrust on the engine mount to counteract the effect.
Here endeth the lecture. Heh heh heh.
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Old 05-30-2003, 01:43 PM
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Default Contra Rotating Propellers

The ultimate attempt to clear air for prop blades was made about 1947 buy someone who tried a single counter-balanced blade on a J3. It was certainly more efficient but had a large drawback. The uneven pull on the crankshaft of the 65hp Continental wore out bearings at a surprising rate. The usual, at least two bladed prop, had none of these deficiencies. Counter-rotating props on a single shaft didn't either. The DO335 was surprisingly fine fighter that fortunately, for us combat aircrewmen. They had a mechanism to blow off the rear prop so the pilot could bail out fi necessary.
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Old 05-31-2003, 12:30 PM
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Default Contra Rotating Propellers

Continuing macwing's lecture, the spiral airflow coming from the propeller is the only reason for the left turn tendency in slow climbing flight. Assymetric thrust does exist as described, however the rotating propeller is a gyroscope and responds to this left turning force on the blades as a pitch force at the hub. When an airplane pitches up the effect of assymetric thrust is a tendency to pitch up further, not to yaw. This is one of those things that people have copied out of each others books so long that it seems it must be true, but it isn't. Engineers treat this assymetric thrust as a destabilizing factor in "longitudunal" stability, and it is why an airplane under power is less stable than when it's gliding power off.
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Old 06-26-2003, 04:57 PM
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Default Contra Rotating Propellers

Regarding the Northorp XB35, i thought that the main reason for substituting the counter rotating props with jets, was constant troubles with the gearboxes, and lack of power/efficiency? The yaw hunting problem (due to lack of (big enough) vertical stabilizers) was solved with a yaw-damper (gyro), that solved the bomb-sight problem. Im am very interested in flying wings, and really have a desire to build a model of the XB35. Do you know about a source to model drawings of this fantastic air plane?

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Old 06-27-2003, 02:15 AM
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Default Contra Rotating Propellers

The XB35 and the YB49 had the same yaw characteristics regardless of the means of propulsion. Electronics at the time were not far enough advanced to provide the directional stability necessary for a stable bombing platrform. Sec'y of Defense at the time was Stuart Symington who unilaterally decided that as long as jets were the coming thing that all new aircraft would be so powered. In the presence of a General Officer he cursed one of the finest gentlemen in aviation and threatened to give the contract for the YB49 to General Dynamics because he had a friend in the business and didn't think Northrop had the capability for the job. When he stormed out of the room Jack Northrop asked the General if Symington was serious and was assured that he was. We had to witness the destruction of two XB35s and twelve YB49s by cutting torch at the Hawthorne CA plant of Northrop Aircraft. By the by the F-16 is a rip off of the Northrop J5 which is still used as the Agressor aircraft for training of top guns. The XB 35 didn't have any vertical stabilizers but the YB49 had four. You might try one of the aircraft documentation companies that advertise in the model magazines for three views and pictures. You could scale up the three views. While I think of it there were no gearbox failures on the contra-rotating props in spite of being powered by the largest radial engine built until that time.
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Old 06-27-2003, 07:30 PM
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Default Contra Rotating Propellers

A lot of things contributed to the failure of counter rotating propellers to become common in full scale airplanes. The foremost of which was the mechanical complexity. Even if the machinery were to operate flawlessly it is still far more complex than driving a single propeller. Complexity is expensive. Although some of the advantages mentioned in this post are real, efficiency is not one of them. The counter rotation of the two propellers in close proximity guarantees a lot of turbulence, which is just lost energy. If you want to tackle a coaxial setup, and can figure out the gearing required, bear in mind that the rear propeller must have greater pitch as itís operating in the higher velocity slipstream of the forward prop.

I believe the original question related to helicopters. The counter rotating rotors do eliminate requirement for a tail rotor, but there still must be something to provide directional control. This could be a rudder in forward flight but would require something like differential torque control for each propeller in hover flight. There would also be the problem of providing cyclic pitch control for the upper propeller. It quickly gets pretty mechanically complex. There was one fairly successful full scale helicopter (Kellet XR-10) in the fifties that used meshed counter rotating rotors on different shafts (and rudders for directional control). The Piasacki (Vertol) H-21 has rotors on each end and had a very wide cg range for a helicopter. It used differential torque for directional control. The gearboxes were a continuing problem.

The fact that after years of experimentation, the majority of actual helicopters use the single rotor, tail rotor configuration is testimony to the efficiency and relative simplicity of the concept. That is not to say some radical new breakthrough might not occur in the future, but it probably wonít involve coaxial counter rotating rotors.
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Old 07-07-2003, 09:45 PM
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Default Contra Rotating Propellers

thanks for the replies.

are there any mathematical evidences or theories that will show the disadvantages of a coaxial rotor setup?
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Old 07-09-2003, 06:41 AM
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Default Contra Rotating Propellers

Mathematics doesnít provide evidence. It is just a way of expressing relationships discovered during observations of the physical world. As to theories, I thought that was what was expressed in all the answers in this thread. Let me try again.

Most basic aerodynamic texts devote a section to propeller theory but to my knowledge none cover coaxial propellers. If you want to research coaxial propellers, and rotors, your best bet would be to search the NACA reports, especially for the period of the 40,s through the 60,s for some test reports that may exist.

As far as theory, whether coaxial propellers or rotors, the downstream rotor operates in the rotating wake of the upstream rotor. This requires a reversal of the motion of the air in a very short distance. The result is a lot of turbulence. To create this turbulence requires energy that comes from the power source driving the rotors. This energy produces no thrust and thus represents a loss. This loss reduces the efficiency of the coaxial configuration to less than two rotors operating separately.

Though significant, this drop in efficiency is not the major problem with the coaxial configuration. The primary disadvantage is mechanical as noted in the previous response. There is simply no way to control cyclic pitch of the top rotor, and all other ways to control the lift vector are relatively weak. Yes coaxial helicopters have been built and flown, but not very well. None have made it into production. The disadvantages can be summed up as:

1. Mechanical complexity
2. High cost
3. Lack of performance

Refering back to your original question, for the same total disc area, a larger diameter single rotor is considerably more efficient than two smaller diameter rotors whether coaxial or seperate.
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Old 07-09-2003, 10:41 AM
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Default Contra Rotating Propellers

thanks! the replies are well noted and will be presented to our panelists.

here's some background about our project which is also the reason that im seeking more help about the difference of single rotor to coaxial setups.

Unmanned Levitating Object
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Old 08-17-2003, 05:46 PM
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Richard Wagner
 
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Default Contra Rotating Propellers

Does anyone know the pros and cons of coaxial contrarotating rotors on autogyros?
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Old 08-20-2003, 09:04 AM
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Default Contra Rotating Propellers

The main incentive for contra rotating rotors is to cancel out torque. In an autogyro the rotors are not driven by a shaft, but by aerodynamic forces so there is no resulting torque. I canít imagine what advantage would justify the complexity of contra rotating rotors in the absence of torque.
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Old 06-07-2007, 01:52 PM
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Default RE: Contra Rotating Propellers

Consul:
I just found a place that has drawings for the XB-35. It is called "Bob's Aircraft Documentation" out of Costa Mesa, California. The drawings consist of 6 11X17 pages and are advertized as exceptional detail with cross-sections and a fact sheet with history, dimensions etc. Price is $13.00. I just ordered these today, June 7, 2007. I am also in the process of designing and hopefully building my own XB-35. To find Bob's Aircraft Documentation go to Google and search for the name. It should pop up at the to of the list. If you can find one AMT also produced a 1/72nd scale plastic model of this plane which I have built and is hanging over my drawing board.
Kind regards and good luck. yankeeone
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