Been doing some research on propeller-pitch. And it seems that a variable-pitch-propeller seems superior, with a constant-speed-propeller(adjusting pitch for constant RPM) seems the most efficient of all. 

A quadcopter with a low RPM will generally spend less energy on air-drag so we have more efficiency. The problem is once you accelerate the RPM increases and efficiency is lost due to airdrag, similar to a car in a low gear driving fast, causing a too high RPM of the engine itself, which wastes energy.

I've seen a few variable pitch-quads, most seem to be in the development stage, there are even petrolium variable-quads being researched.

My question is, are there(other than added weight) some big mechanical drawbacks to having a variable-pitch propellers over a conventional propeller?

Also will all the professional high-speed quads be using variable-pitch like helicopters in the future?

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  • I actually have a patent pending on a drastic simplification on how to achieve controllable pitch propellers/fans:

    https://m.youtube.com/watch?v=P-eCXzImUq0

    The idea is to have a thread on the propellers shaft with a cylinder with a mating  thread that is ferromagnetic mounted on the propellers shaft thread.  Electromagnets on the stator system cause the cylinder to rotate and the resultant axial motion alters the pitch of the blades.  The threads pitch creates mechanical advantage.

    The idea is under license and development in the automotive sector for engine cooling fans - so it's practical and simple enough for mass production.

    I am looking for other licensees /partners.

  • I agree with the responses here, that the complexities of variable pitch are not worth the benefit, which for smaller quads are minimal.  

    I am designing a very large (gas powered) quad, and for this application I am planning to use variable pitch for several reasons, most of which relate to safety.  For safety, I am using one engine (if 1/4 fails - the loss of control is rapid and violent).  If power is lost, I want the quad to be able to descend in autorotation, which a fixed pitch prop cannot do.

    As for efficiency, you need to evaluate your flight conditions.  Helos in hover have less efficiency (require more power) due to the fact that they are trying to "swim upstream) in their own downwash.  Helos in forward flight gain efficiency as the rotor's are constantly flying into "fresh" air (still air, not part of the downwash).  The more I work on this project the greater respect I have for the complexity of both the aerodynamics and the mechanical dynamics of a rotary wing craft.

    There is no "perfect" design because every little maneuver affects all parts of the system.  Forward flight creates different airspeed for advancing and retreating blades.  It also changes the angle of attack of the blade.  If you look at an airfoil chart, you will see there is usually an "optimum" angle of attack (highest ratio of lift/drag), but once you start considering forward flight, climb/descend, pitched flight, it becomes clear that there are too many variables to "optimize" the blade conditions.  It would seem that for quads, much of their time will be in hover (filming, photo, etc), so if that's the mission, design for hover.  Others could be for filming moving targets with varying degrees of agility required.  In these situations, you will find that you might need to sacrifice hover efficiency to allow a wide enough operating window to satisfy your performance objectives.  

    I saw this quite quickly when I was looking at using a very efficient Gemini II airfoil which has an optimum AOA very near it's stall point.  That wouldn't give me any "headroom" to increase lift beyond the nominal hover, such as when climbing, or decelerating a descent, or just responding to gusts and downdrafts.  Additionally, I noticed that the sensitivity of that airfoil was very non-linear.  At low angles of attack, a small change in angle would give a large increase in lift, but at high angles of attack, the same increase in AOA would produce a much smaller increase in lift.  This can create tuning nightmares, and if you're not careful can cause the system to go unstable when the aerodynamics enter a "sensitive" zone.

    Finally, the real variable for efficiency is the rotor diameter.  Lift is created by transferring the force of gravity, to the momentum of air.  A larger rotor has a larger area, and therefore doesn't need to speed up the column of (downwash) air as much to get the same amount of lift.  The energy (power) required to do this is related to the velocity of the air ^2 (squared), so a larger rotor, with low downwash flow is most efficient.  Then you need to look at size, weight, stresses, maneuverability etc etc etc.  Designing any aircraft, but certainly rotory winged aircraft is a massive exercise in compromise.

  • quads, unlike fixed wing aircraft, tend to operate with one thrust value (ie, in hover, one prop supplies some constant percentage of the weight). Now, this is not true in maneuvering, but in pure hover, it certainly is. so at that design point, for a given RPM and prop diameter, there's something close to an optimal twist distribution for the prop. so to maximize endurance for a quad with a fixed weight, you can get some pretty amazing performance out of a fixed pitch propeller if you do your homework. now, if the payload weight changed and required more or less thrust, that optimum twist distribution would change and something like a variable pitch prop might get you a few % of your endurance back.

    fixed wing craft can get a lot out of a variable pitch prop if they spend significant portions of their missions at different thrusts, rpms, and prop loadings. something that spends 99% of its time at cruise, however, can be optimized for a fixed pitch prop.

    typically, at our normal scales, we tend to see better efficiencies out of larger diameter props which, in turn, calls for lower Kv value motors to get the peak motor efficiency and peak propeller efficiency (as function of rpm) to lie on top of each other. 

    this is all said with the proviso that I normally work with fixed wing aircraft, not multirotors, so I might be off base as multirotors are essentially always doing static thrust which breaks my normal analysis methods :)

    and I was around when they were working on the variable pitch quad mentioned by @Sergey.... very responsive, indeed! takes much less time to move the blades than to slow or speed up a prop.

    i think the bottom line is that in general, fixed pitch gets you a lot for very little investment. variable pitch has a lot of nice attributes but the appeal of quads is their simplicity and variable pitch is nothing of the kind. (yet)

  • Moderator

    the big issue is slow response time to change, The ESC can change at 400Hz, and go from 0 to 100% in close to the same time,I dont see a mechanical system or servo system getting close to 20Hz and staying in one piece long enough to fly. 

    • Actually this is not true. 

      Studies show that variable-pitch copters are more responsive than fixed-pitch copters http://acl.mit.edu/papers/GNC11_Cutler_uber.pdf

      So, the only reason I see why they are not so popular is that they are more complex mechanically than fixed-pitch quads.

      http://acl.mit.edu/papers/GNC11_Cutler_uber.pdf
    • That makes perfect sense. Thanks.

  • Bad reliability, traditional helicopter is better if you are willing to have some mechanical points of failure.
    Reliable multirotor means 6 or 8 motors, then if you add servos and mechanical parts, it's just bad.
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