5
$\begingroup$

I came across this company E-Props that builds propellers and it seems like all their propellers are narrower, at least narrower than traditional ones I'm used to see, and I see on their picture gallery many aircraft using them, almost like a trend. Are these props more efficient, cost effective, or is just a trend or something?

What's the deal with it?

props
(source)

$\endgroup$
1
  • $\begingroup$ Same as the wings, aspect ratio. $\endgroup$ Commented Jul 11 at 3:37

2 Answers 2

5
$\begingroup$

Propeller chord is determined by the power which needs to be absorbed by the prop. As with wings, propeller blades perform best in a small band of lift coefficient values: Too high, and drag will become excessive, too low, and you need to spin more surface area through a viscous medium (air) than necessary.

One further consideration is Mach effects: For that reason, the lift coefficient values of the Junkers VS-9 propeller were kept low by widening the chord of the propeller blades. It worked indeed better than conventional propellers at high speed, but that line of development was quickly eclipsed by jet engines.

VS-9 Propeller

FW-190 / Ta-152 with Jumo 213 and VS-9 propeller.

$\endgroup$
2
$\begingroup$

I think here there's both a clever use of the structural characteristics given by carbon fiber and some marketing at work.

We can try to answer this question using one of their documents (PDF) to better understand their choices.

The E-PROPS propellers have a thin chord (chord = width of the blade). A thin chord generates less drag than a wide chord and leads to a better efficiency of the propeller, a better thrust, makes less noise and reduces fuel consumption

That's the marketing department speaking: lower chord means lower Reynolds number which, contrary to this claim, is actually connected with slightly higher drag and lower $C_{l_{max}}$.

With E-Props designs and profiles, for the same diameter, more blades = more efficiency. A 3-blade has a best efficiency (= a best thrust) than a 2-blade.

That can be true. Normally it's preferred to have a number of blades as low as possible: this gives a lower cost and weight overall. Anyway if the number of blades is incremented then they get a higher aspect ratio and therefore lower induced drag which (more than?) compensates for the higher drag due to the lower Reynolds number. More blades also give lower vibrations and a "more uniform" wake. Obviously a slender blade deforms more than a stubby one but this is where the clever use of carbon fibre I was mentioning comes into play.

The increase of the diameter is better for the efficiency, because of the improvement of the pusher efficiency. More static thrust is obtained with a propeller with a diameter of 180 cm than with a propeller with a diameter of 155 cm. With the same number of blades, the thrust gap depends on the diameters and on the engine + reducer.

Obviously a bigger blade delivers more thrust! Anyway that must be combined with a slower rotating speed otherwise the tip can reach transonic speeds with a big increase in drag and noise.

Are these props more efficient, cost effective, or is just a trend or something?

As seen, blade design is a compromise among contrasting requirements and it can be that more than one "shape" is able to deliver good aerodynamic characteristics.

It would be nice to compare their data with the ones of the propellers they replace in order to give a more solid answer.

$\endgroup$
3
  • $\begingroup$ Excellent! I may have an historical perspective regarding thinner blades and thrust, and why fat bladed propellers are used. The observed marketing and performance aspect of these new composite blades was also interesting. Superbly discussed! $\endgroup$ Commented Jul 12 at 20:05
  • $\begingroup$ @ThomasPerry: thanks 🤗 $\endgroup$
    – sophit
    Commented Jul 12 at 20:32
  • $\begingroup$ @sophit Interesting the first point you bring from their PDF document I also thought it was that way. Like in wings (like in a glider) it is supposedly more efficient. In the end I think it is as has been said in other things in aerodynamics: you gain from one area and lose from another area. But yeah as you put it in your last sentence it would've been nice some data to compare to. $\endgroup$
    – Gabe
    Commented Jul 12 at 21:34

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .