Articles of Interest
3-blade vs 2-blade propellor
written by Keith Pickersgill 17 June 2009
The best propellor has a single blade (with a heavy counter-weight on a small
radius), with a diameter of dozens of meters, with VERY low RPM (perhaps 300
RPM or lower going through several geardown ratios) and a tip-velocity under
160kph.
Obviously this is not possible on paramotors, so we are forced to use ridiculously small diameters (approx 1.3m is ideal in terms of performance in most cases), hence we are forced to increase the prop RPM to compensate. Note, we are speaking specifically of propeller RPM, not engine RPM.
Obviously due to weight contraints, a single blade (with a very heavy
counterweight) is not an option, so 2-blades or 3-blades are acceptable compromises.
Pros of 3-blades:
- MUCH, MUCH, MUCH, MUCH (!!!) easier to balance 100% (it is almost impossible to
get this level of balance on a 2-blader).
- More pleasant sound delivering more of a "humm" than a "RRRRR" sound due to the
exhaust pulses reflecting every 120 degrees instead of every 180 degrees, as
well as typically narrower blades used on 3-bladers improving the quality of
the engine/prop noise.
Down-sides of the 3-blader:
- More expensive.
- More difficult to transport/store (unless you dismantle the prop every time)
As regards to thrust comparison, it is difficult to forecast which will deliver
more thrust on a particular engine.
Consider the following:
Any wing (or airfoil) has an angle of Attack (AoA) at which it delivers the
highest ratio of Lift to Drag, the so-called Best L/D ratio. Most airfoils
deliver Best L/D at approx 4 degrees AoA.
A prop is simply 2 or more wings flying in a very tight circle, and "lift"
equates to "thrust" on a prop (except for Heli and Gyroplane rotors).
If you had a 2-blader and a 3-blader, both of the same diameter and width and
airfoil, etc... and both set to their respective pitch which delivers the
same perfect maximum RPM on the same engine, then that one where the AoA is
closest to best L/D will deliver more thrust, as less power is wasted on
overcomming drag and more of the available horsepower is available to convert
into thrust.
It goes without saying that the 3-blader will have a significantly lower AoA
than the 2-blader to deliver the same RPM. It is extremely difficult to predict
which prop would have the best AoA and operating closest to best L/D, hence it is difficult to predict which one would have more thrust than the other, without actually doing the test, setting the two prop's pitch until both deliver the same max ideal RPM on that particular engine and reduction ratio.
The actual AoA of a particular prop is easy to calculate if you know the Pitch
of the prop, the RPM and the airspeed. Change any one of these, and you change
the AoA. There exist many spreadsheets to claculate this for you.
Ask if you want a copy of my own spreadsheets developed with George Killey's input and assistance.
So.... think about this:
Which of the following options is more important to you?
- max thrust at full power while standing still on the ground,
i.e. Static Thrust, or
- max thrust at full power at take-off airspeed
(for easier launching in still air), or
- max thrust in-flight at trimspeed and at max power setting
(max climb thrust), or
- the lowest RPM to deliver the required thrust to keep your aircraft in level
flight at cruise speed
(cruising economy prop), or
- the lowest RPM to deliver the required level of thrust to keep your wing in
level flight at max airspeed (trims wide open and gull speedbar applied),
i.e. racing prop...?
If your engine is short of power, then you have only one option, to work
towards option 2.
However, if you have some surplus power, then you can work towards
either option 3, 4 or 5, according to your specific needs and preference.
Option 1 is pretty useless and yet this is how all thrust-bed tests are
done!
Your comment would be most welcome.
Address comment to Keith Pickersgill at keith@xplorer.co.za
