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With contrarotative propeller (or not, if twin engine), cyclic and collective blade control.

Would such an aircraft be safely flyable, and how efficient would it be, assuming it can have perfectly smooth wings (and tailplane)?

In case of engine stop (allowing axle free spin and control on blades), could it control its glide and land on the runway, with the propeller's blades in an almost feathered autorotation configuration, allowing attitude control and minimum disk drag?

(like autorotating one reversed Kamov on its rotor-head, in a skydiving wind tunnel blowing a bit slower than terminal velocity)

Edit: If it goes twin engine and tailless (and still controlesurfaceless) how active cyclic pitch control would be necessary to allow use of non-reflex wing's airfoil?

enter image description here

Edit2: Switch from thrust + attitude control mode, to no-thrust + attitude control "reversed autorotation" mode. Reversed rotation allows most efficient use of blade's airfoil camber. enter image description here

user721108
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3 Answers3

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An interesting thought. Control the aeroplane through its propeller(s), like a helicopter does.

  • Propeller torque differential would control roll
  • Propeller cyclic would control both pitch and yaw
  • Propeller collective would control engine thrust, like already done in constant speed propellers.

The thing that immediately comes to mind is moment arm for pitch and yaw. Helicopter blades are relatively long, and the rotor is mounted about halfway along the fuselage. The prop is limited in its blade length due to ground clearance.

enter image description here

With a configuration like above, longer propeller blades can be mounted so the pitch and yaw moment arms can be extended. As @Sanchises points out, placing of the propeller like this creates a coupling between pitch and thrust - not a bad thing, increasing pitch controllability by controlling thrust. A strong nose wheel might be required for take-off.

Not sure about your reference to the skydiving Kamov, but the aircraft could glide down after engine failure while keeping enough RPM to control cyclic. It would be a bit draggy though, with the drag comparable with a parachute of the same diameter as the propeller. Autorotation works best with a large blade moment of inertia, and the propeller would definitely not have the optimal blade length for that.

Koyovis
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  • If you glide straight forward at best L/D, blade's pitch will be close to feathered, at minimal drag. Eventually propeller will stop spinning, unless you want to roll, and therefore apply differential pitch between contrarotatives, increasing rpm, inducing torque, (and roll) Same for pitch and yaw. Each input increases blades rpm and authority to do whatever change in attitude. Low inertia due to relative small prop diameter helps these rpm variations. Most of the time, blades are aligned with relative wind. It could be less draggy than one stadard C172 prop idling. – user721108 Dec 04 '17 at 14:15
  • That would mean that in order to pitch or yaw the aircraft you would have to spin up the propellers first in autorotation, causing a time delay in the response. Roll response would be immediate because the spinning up is roll response already. – Koyovis Dec 04 '17 at 14:19
  • Exact, spin stop idea was to illustrate close to zero angle of attack while gliding. One minimum rpm should be set in order to minimise delay and authority on pitch & yaw, relative to drag produced by spinning. – user721108 Dec 04 '17 at 14:24
  • Except for increased propeller blade length, offsetting the propeller does not change anything; control forces are almost a pure torque on the propeller shaft, and a given amount of torque has the same effect on a body regardless of position. Only a force benefits from a longer arm, but the only significant force is thrust, which now creates a large moment around the c.o.g. which needs to be counteracted by your cyclic. – Sanchises Dec 04 '17 at 14:25
  • @Sanchises axis of rotation has an effect on moment of inertia. – Koyovis Dec 04 '17 at 14:32
  • One issue of this system may actually be roll control at high speeds. Throttling down a standard C172 at Vne does not induce that much roll. – user721108 Dec 04 '17 at 14:34
  • Yes, plus the wings have a large roll inertia and high aero damping. The blade length would have to be much higher than that of a C172 for sure. – Koyovis Dec 04 '17 at 14:38
  • @Koyovis Other solution is to go twin engine, collective controls speed and yaw, cyclic controls pitch and roll. New question could be : Why Osprey has rudder ailerons and elevator? – user721108 Dec 04 '17 at 14:43
  • Good point. Prpobably for continued flight with one failed engine. – Koyovis Dec 04 '17 at 14:45
  • Osprey's rotors are mechanically linked, one engine failure only means half power. – user721108 Dec 04 '17 at 14:47
  • Yes. But if acted upon by a pure torque, a body will rotate around the c.o.g., regardless of the location where the torque is applied. The control forces in a cyclic are transferred through the blades to the propeller shaft as torque, not a force (a propeller does not impart in-plane forces on the shaft) – Sanchises Dec 04 '17 at 18:51
  • @Sanchises Yep! I've amended the answer. By the way, helicopter blades don't impart torque due to the flapping hinges. – Koyovis Dec 04 '17 at 23:30
  • Hmmm I hadn't considered blade flapping at all, sorry! Am I correct if I say that that would change the thrust vector rather than impart torque? Then pitch and yaw are done by thrust vector changes (in which case you really need a nose or tail mounted propeller to get any arm for your torque moment) and roll is still done by pure torque, which means that a regular nose mounted prop (probably a tail dragger to get sufficiently large prop diameter) is the best option. – Sanchises Dec 05 '17 at 11:15
  • @Sanchises for this aircraft the propeller wouldn't flap, so yeah torque in all axes. – Koyovis Dec 05 '17 at 14:41
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This will work as long as the propellers produce enough thrust and blade pitch can be adjusted fast enough to outrun all eigenmodes.

As soon as you need to throttle back (and eventually you must, to come down again), the control effectivity of the propellers will be greatly reduced. Granted, you can float down in autorotation like an autogyro, but the landing will be more a crash than anything else.

Note that the Boeing V-22 Osprey is not capable of power-off landings because the propeller inertia is too small to support the landing deceleration. It can glide down in autorotation but cannot perform a soft landing. Your configuration looks quite similar and will similarly not be capable of autorotation landings.

If you want to control the plane with propeller forces, the propeller must spin at high speed all the times to have sufficient thrust potential available when it is needed for stabilization. For artificial stability you have no time to spin up the prop first! Thrust is controlled by pitch only, but the higher-than-normal prop speed will cause its own inefficiency.

Peter Kämpf
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  • This actually isn't autorotation like an autogyro or helicopter, since it's not providing lift + attitude control. It only provides attitude control. Each attitude correction slows you down. When gliding straight, blades are close to feathered. Wings provide lift, propellers provide best control/glide ratio – user721108 Dec 04 '17 at 16:18
  • autogiro should be autogyro. – user Dec 04 '17 at 16:33
  • @Peter Kämpf, since blade's asymmetric airfoil usage should be optimal during non powered flight, shaft rotation may be reversed in this case. Think HAWT windturbine mode, having positive AoA even if not providing thrust. – user721108 Dec 04 '17 at 18:53
  • @qqjkztd: How do you think the feathered props will contribute to stability? Without a significant blade loading they cannot do much in terms of pitch or roll control. The best you can do is to let them run at windmilling angle but at full RPM, so loads can be created quickly. And for approach and landing you cannot afford to have much thrust or even reverse thrust. – Peter Kämpf Dec 05 '17 at 15:43
  • @PeterKämpf by reversing rotation direction of the blades, compared to rotation direction while powered by the engine. Airfoil's camber should be used in its best lift producing possibility, which means reverse spin direction when unpowered, free rotating. – user721108 Dec 05 '17 at 16:47
  • AoA positive to relative wind in order to maximise L/D, instead of negative AoA ,like fixed pitch props spin when unpowered. (which requires wide pitch range, let's say -5 to 110 deg) 105deg pitch may mean : 4deg positive AoA in invert spin direction. – user721108 Dec 05 '17 at 16:49
  • @PeterKämpf illustrated in edit2 – user721108 Dec 07 '17 at 10:34
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from an engineering design standpoint, the performance of (for example) a small aircraft like the seaplane pictured above is not in any practical sense limited by the presence of an empennage carrying an elevator and rudder. For this reason, alternatives like cyclic pitch changes on the propulsion propellors have not been actively researched for pitch and yaw control.

niels nielsen
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  • Cyclic control on propellers is not meant to suppress the empennage. It may allow its suppression anyway, but primary goal is to have clean airfoils with no control surfaces, and get rid of the parasite drag they provide, on wings and stabilizers. – user721108 Dec 07 '17 at 12:39