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I enjoy taking the time to try and visualize how fictional aircraft might actually work in reality. Unfortunately there are some that seem highly impractical from an engineering student's point of view.

One in particular is the Bella Ciela from "The Place Promised in Our Early Days." It features a set of blade-like wings that protrude out front and deploy into contra-rotating propellers, sustaining flight after its jet-assisted takeoff. The blades spin much slower than a conventional propeller as well.

An oddball aircraft indeed

I can get by the closed-loop wing. The V-tail has no issues as far as I can see. My main concern is the large rotating blades that extend far beyond the wingspan of the plane. Taking the Bella Ciela's design out of the equation and focusing on concepts alone..

crude design

  • How feasible is it to use a propeller larger than the wingspan? Is there any law of physics that prevent this configuration?
  • What immediate consequences would be seen in the airflow behind such a large propeller? Would the wings aft of the flow still be able to produce stable lift?
  • The blades remain flat when stowed and act as an additional lifting surface; but I assume one could position them so that their loss of lift doesn't affect the balance of the plane (like in the sketch, with them centered)?
  • Without twist, can a propeller still provide efficient thrust? Could you reproduce the twist's effect by instead varying the length of the blade's chord from root to tip?
  • Are there standard equations for calculating thrust and drag of a propeller that does not have twist? Or of a propeller in general?

I can already see how drag would be a huge issue with spinning something so big. Is there a limit to how long propeller blades can get?

My apologies if I am asking [too many] questions that do not fit this board's criteria. This is just a concept that has had me Googling around for the past few hours. It's neat, and I hate dismissing things as just art without at least trying to justify them.

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    $\begingroup$ How would you land and take off in that thing? I guess I wouldn't want to board a plane that is sitting on a carriage longer than the wingspan. It would feel like boarding a flamingo. $\endgroup$ – Alexander Sep 16 '15 at 8:48
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    $\begingroup$ @Alexander If I'm reading it right the idea would be to take off and land with the blades in the "wing" configuration (top picture) with them being deployed into "propellor mode" once airborne. Obviously there would need to be an additional source of thrust for take-off and landing. $\endgroup$ – Nigel Harper Sep 16 '15 at 11:51
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    $\begingroup$ There would be some practical engineering problems with a design such as this. I recall talking to an engineer who worked on the Bell XV-15 who explained that one of their major issues was that steel wasn't strong enough to make the rotor hubs, and having to come up with something that would work. I suspect that in an aircraft such as the one you've described the hubs would have to be made from unobtanium - an ideal substance for such use whose only drawback is that it doesn't exist. $\endgroup$ – Bob Jarvis Sep 16 '15 at 12:10
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    $\begingroup$ @abelenky while it may be about a fictional aircraft, I think it asks good specific questions, and Peter provides good answers that are certainly on topic here. $\endgroup$ – fooot Sep 16 '15 at 14:44
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    $\begingroup$ @abelenky the help center lists "Aerodynamics (related to aircraft)" so this appears to be on topic (I do submit that I'm just a HNQ browser and not familiar with the community here). If it is off topic here, it might be on topic for Engineering.SE (relevant help center page) in which case it might be appropriate to have the mods here talk to the mods there. It also might be something to bring up on meta to tweak the help center here to constrain the aerodynamics somehow to make it clear that fictional aircraft are off topic. $\endgroup$ – user2896 Sep 16 '15 at 14:54
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Short answer: This design will probably work, but it will not be very efficient. It can be tweaked into flying, but when you start tweaking, you would continue such that the outcome would look differently.

Now let's look at your questions one by one:

How feasible is it to use a propellor larger than the wingspan? Is there any law of physics that prevent this configuration?

There is no law which forbids such a large propeller. In order to create thrust, you need to accelerate a mass of air backwards. The larger the propeller, the smaller the acceleration needs to be for a given thrust, since a higher mass flow is available. This makes large propellers inherently more efficient, but larger blades are heavier and also produce more friction drag, so the sweet spot is at propellers which are quite a bit smaller than the wings of the aircraft they are attached to.

What immediate consequences would be seen in the airflow behind such a large propellor? Would the wings aft of the flow still be able to produce stable lift?

Since the acceleration provided by the propeller to the air mass is small, the wings behind them would fly in almost undisturbed air. The lift will wobble a little over time, because the boundary layer flowing off the propeller blades will produce a cyclic variation in dynamic pressure on the rear wings. This will, however, not impede their general capacity to create lift.

The blades remain flat when stowed and act as an additional lifting surface; but I assume one could position them so that their loss of lift doesn't affect the balance of the plane? (Like the sketch with them centered)

At slow speed it helps to have more wing area for lift creation. Note how much the fowler flaps of an airliner move backwards to increase not only the wings camber, but also its area. Using two wings flying in formation would allow to give the rear wing a much higher angle of attack and to use the gap between them to refresh the rear wing's boundary layer like it is done in slotted flaps, so in combination their lift would be higher than that of one wing of the same area. The long, narrow propeller blades of this fictional aircraft look too flimsy, however, to be of much use in adding lift: They would break off at a fraction of their potential lift if built with existing materials.

Without twist, can a propellor still provide efficient thrust? Could you reproduce the twist's effect by instead varying the length of the blade's chord from root to tip?

Good that you added "efficient"; this changes the answer from a "yes" to a "no". Only with twist would the local angle of attack be close to the optimum, but even without twist, thrust will be possible. Then the goal should be to pitch the whole propeller blade optimized for the outer 30% of its span. The thrust from this will create a strong root bending moment, however, and I doubt again that the slender prop blade will not break off. If you try to create thrust closer to the center, the outer part, which is flying at the highest dynamic pressure, will create substantial drag, requiring lots of torque, and again the propeller will break off, but in a different direction.

Are there standard equations for calculating thrust and drag of a propellor that does not have twist? Or of a propellor in general?

Yes. The first good ones were published by A. Betz and L. Prandtl in 1919, and the latest substantial improvements were added by Larabee. Incidence can be prescribed, and so can be set constant over the whole span. If you can run a copy of Mark Drela's XROTOR, you can try for yourself.

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    $\begingroup$ As an addendum: with such a long propeller, the tips will be moving much faster than the root. Props generally want to keep their tips subsonic; the changes in airflow and the structural stresses when they approach Mach speeds are... problematic. On a prop that long, either the tips will go supersonic, or the roots are very slow. This means that you'll have a lot of twist along a sizeable section of prop to produce any real thrust, which in turn may cause drag problems. $\endgroup$ – anaximander Sep 16 '15 at 12:05
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    $\begingroup$ How would long prop blades compare to the long blades of a helicopter? They can be very long and thin but don't break off. They provide both lift and propulsion. They have the same problem with avoiding supersonic tips but those problems have solutions. I assume there would be a big difference being vertical rather than horizontal. $\endgroup$ – TomMcW Sep 16 '15 at 17:43
  • $\begingroup$ @TomMcW: Good comparison! Helicopter blades have a very solid root and constant chord over their span. The props in the picture have very thin roots and more chord at mid-span, so they will produce higher root loads. Also, the rotation stretches helicopter blades horizontally. Here, when used as wings, no such help is available, and flutter and bending will take their toll. Also, the lift coefficient of a symmetrical or reflex helicopter blade produces much less lift per area than the slotted wing possible with the two blades here. $\endgroup$ – Peter Kämpf Sep 16 '15 at 18:24
  • $\begingroup$ @TomMcW 1) Helicopter blades suffer more loads (think of the air pushing on the prop tip as it rotates in front of the helicopter, creating an axial load that could potentially buckle it), 2) helicopter blades usually don't do too much with twist, because they use a cyclic to change the AoA of the blade to achieve control of the helicopter. Variable pitch props on planes also have significantly less twist than a static prop, but that has more to do with a static prop having different regions optimized for different speeds, whereas vpp are adjusted to get the best thrust for current airspeed. $\endgroup$ – MishaP Oct 3 '18 at 18:17
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Let's talk about the propeller size for a minute and ignore the aerodynamics of the rest of the vehicle as they have been covered in another answer.

Remember that the tips of a propeller spin faster than the roots. Even though the whole propeller spins at the RPM the tips must cover more distance the roots and are thus moving faster. This can create a supersonic tip situation which can be a problem in and of itself. You can find some coverage on that in this question. To keep the tips subsonic in this craft you would need to spin the propeller slowly which may not create enough thrust to fly the plane.

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It all boils down to physics. What we are looking at here should be a helicopter. Yes, absolutely, with contra-rotating rotors it would fly.

But now we must look at the demands of flight, how much force is needed to overcome gravity and how much is needed to overcome drag while producing velocity. It becomes clear the design is backwards, with tiny wings and an oversize propeller. If it flew as a helicopter, notice it would need only a slight forward tilt to move forwards.

This relationship was found while studying gliders. There is very little frontal area compared with area as viewed from the bottom. That, along with streamlining, allows the glider to move forwards through the air while only dropping slightly (flugzeug gefallen). Once in motion, the wing generates lift even more efficiently (segelflug).

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  • $\begingroup$ Now birds have it right. They swing their wings as large efficient propellers. $\endgroup$ – Robert DiGiovanni Jan 15 at 22:34
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Such aircraft already exist, with contra rotating propellers that extend beyond the wings:

V22 Osprey

V280 Valor

AW609

One non tilt rotor that never made it to production:

Vought XF5U

It showed great promise, especially for it's STOL characteristics, but had the misfortune to be in design when jet engines came out.

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  • $\begingroup$ -1; none of those aircraft have contra-rotating propellers - they each only have one propeller per shaft. "Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque." $\endgroup$ – T.J.L. Jan 15 at 15:19
  • $\begingroup$ Really? Aircraft such as the P38 and F82 are referred to as contra rotating, because the propellors rotate in opposite direction to cancel torque. The effect is the same, regardless of whether the props are stacked or not. $\endgroup$ – tj1000 Jan 15 at 19:29
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    $\begingroup$ Nope, P38 is counter rotatating. Look it up. $\endgroup$ – Robert DiGiovanni Jan 15 at 22:32

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