ESR Effect: EXTENDED SPEED RANGE
At E-Props, the term "ESR effect" refers to a fixed-pitch (or ground-adjustable pitch) propeller whose behavior is very close to that of a variable-pitch propeller, i.e. a strong thrust at all engine RPM. ESR stands for Extended Speed Range.
This page details that feature in two parts, to expand below: what the ESR effect is, which E-Props propellers have it and how it is technically achieved; then the measured performance and real flight results it delivers.
1 — What is the ESR effect?definition, propellers concerned (Durandal, Excalibur, Ascalon), thrust rather than engine RPM, technical means to achieve it.
E-Props propellers with ESR effect are the ground-adjustable pitch models DURANDAL, EXCALIBUR and ASCALON.
Thrust above all, not engine RPM
This ESR effect induces very little gap between the static RPM and the flight RPM, which makes it possible to maintain high power at take-off. Thanks to hollow airfoils capable of more lift, the blades do not stall, resulting in a very strong take-off thrust.
With an ESR-effect propeller, the full-throttle engine speed stays almost constant. But beware: just because a propeller keeps the same RPM all the time doesn't necessarily mean it has the right efficiency — a propeller can be "Constant Speed" without having this ESR effect, whereas a propeller with ESR effect is necessarily "Constant Speed".
| Flight phase | Full-throttle engine speed |
|---|---|
| Take-off | 5400 RPM |
| Max speed, 200 km/h | 5500 RPM |
RPM gap between 0 and 200 km/h: only 100 RPM, versus up to 1000 RPM on a standard (non-ESR) propeller between take-off and max cruise.
How the ESR effect is technically achieved
From a technical point of view, there are several ways to obtain this ESR effect:
- a "narrow" chord with a classic airfoil (nearly flat lower surface), which generally implies some degree of blade stall;
- pitch reduction at low speed through blade torsional deformation (correct mechanical strength of these blades is then complex to achieve);
- a "narrow" chord with a hollow airfoil (hollow lower surface) to avoid blade stall — this is the case for E-Props propellers.
Increasing the diameter reduces the ESR effect; reducing the number of blades helps restore it.
2 — Measured performance and real flight resultsmeasured thrust gain, why thrust between 40 and 80 km/h matters most, real flight results (VL3, Virus), comparison with variable-pitch propellers.
Measured thrust gain
Example for a Rotax 912 engine, pitch set for 5800 RPM at 120 km/h:
| Without ESR effect | With ESR effect (E-Props) | |
|---|---|---|
| RPM gap at take-off | – 600 to – 1000 RPM | – 200 RPM (5600 RPM) |
| Static thrust | < 185 kg | 210 kg (+10%) |
When trying to reduce an aircraft's take-off distance, thrust between 40 and 80 km/h matters far more than thrust between 0 and 40 km/h: while the aircraft accelerates from 0 to 40 km/h, its average speed is only 20 km/h, so the distance covered stays small (e.g. 16 m in 3 s); between 40 and 80 km/h, average speed is 60 km/h — this is where runway length is really "consumed" (e.g. 60 m in 3.5 s). In the case of stalled blades, the efficiency loss can sometimes exceed the power gain: this is why optimizing the "max thrust versus speed" curve is essential when designing a propeller.
Real flight results
Thanks to their geometries and airfoils, E-Props propellers show a very strong ESR effect, considerably improving aircraft performance at take-off and in cruise compared with a standard fixed-pitch propeller, and even compared with some variable-pitch propellers.
See many more test reports showing the value of the ESR effect: E-Props: comparative flight tests.
Any questions about this ESR effect? Contact our design office: helices@e-props.fr.
