Today, the E-PROPS design department is composed of 12 aeronautical engineers and technicians.
They are doing theoretical calculations, modelling, prototypes development, then experiments on ground and in flight.
They have a great experience, recognized by their peers, and are regularly requested for specialized conferences and symposia.
The design department establishs the specifications of each propeller by taking into account :
- The characteristics of the engine (power, troque, thrust, RPM)
- The airframe which is going to be equipped with this optimized engine + propeller
(puller or pusher configuration, aerodynamic characteristics, wings / fuselage interaction...)
- The required performances of the aircraft
- The conditions of use and the missions of the aircraft
To optimize a propeller, for a specific engine and a specific aircraft, is a complex task because :
- flight speed, engine RPM and power are compulsory
- propeller diameter is limited either by aircraft geometry (ground clearance or fuselage clearance) or by peripheral speed (supersonic issues).
Propulsion efficiency factor is calculated from propeller diameter and engine power. This efficiency factor is the max achievable propeller efficiency.
Then, it is up to the propeller designer to come closer to this limit.
The available optimization parameters are :
- number of blades
- blade loading distribution vs span
- chord distribution
- pitch distribution
- airfoil vs span
To increase number of blades allows reducing lift of each blade.
So the induced drag of each blade is reduced. But, with a constant chord, this increases the friction drag.
And if chord is reduced, Reynolds number decreases and airfoils characteristics are degraded. Use of small chords also leads to mechanical strength issues.
When looking for the optimum load distribution, induced drag must be taken into account. For example, blade tip cannot generate high lift without high induced drag.
Chord optimization leads to use each airfoil at best lift/drag ratio, without forgetting Reynolds variation effects and checking airfoil matching to CL conditions (Reynolds and Mach).
Pitch distribution is used to maintain an optimum lift coefficient to each airfoil in order to get the chosen lift distribution with the optimized chord and airfoil distribution.
Linked to this complex process, propeller design is an iterative calculation. The modification of one parameter leads to change the others.
Since the beginning of aviation, propellers did not stop evolving.
In leisure aviation, three main periods can be distinguished :
The 1st generation of propellers for the light aviation was contituted by wooden or metal propellers.
In the 1940s - 1950, those fixed-pitch propellers were adapted more or less well to direct drive engines (as Continental, Lycoming, Volkswagen).
They were mostly certified. To have a little better efficiency, the only solution was to use some rare variable-pitch, heavy and expensive.
In the 1980s - 1990, some composite propellers come on the market. Those propellers were lighter and showed a better efficiency.
The ground adjustable pitch system marks a significant step forward for leisure aviation.
The 2000s - 2010 have discover the 3rd generation of propellers.
Due to mechanical performances of the carbon fiber, new aerodynamic designs become possible : high CL profiles, narrow chords, very big diameters, positions of the blades...
The numerical modelling studies allow to optimize propeller's performances on all speed's range of the aircraft.
It is possible to obtain the best thrust during all the flight with the same pitch (what is called "ESR effect" on E-PROPS propellers).
It is not necessary to choose between "take-off" and "cruise" performances.
The constant advances in innovative technologies, design's tools and tests systems let envisage in the next years new progress on propellers.
VIDEO : E-PROPS test campaign V20 range (April 2020)
Test campaign : E-PROPS propellers for Aircraft (LSA) & Ultralights, 2020 range. Tensile, bending and torsion tests. Carbon parts as strong as the Rotax gearbox shaft !
E-PROPS has developed many test and measurement benches, for example:
- Instrumented propeller test bench for functional tests, with traction measurement system. An electric motor allows comparative tests of propeller noise.
- Fatigue test bench. This bench allows the carbon propeller to be stressed by alternating bending, to reproduce the engine torque and thus demonstrate a verified potential of the hub-blade assembly.
- Mechanical tensile test bench: 40-tonne hydraulic jack for tensile and compression tests of blades and hubs. Instrumented assembly for data recording.
In 2015, the E-PROPS team has acquired a SKYRANGER ultralight with a 100 hp Rotax 912S.
This investment has been made in order to continue testing the E-Props for aircraft and ultralights.
This simple airplane is equipped with innovative instruments (DAU : Data Acquisition System) to record all propellers parameters in all flight configurations.
This DAU (Data Acquisition System), called MERLIN, designed and manufactured by the E-PROPS design department, allows to obtain the following parameters at each moment of the flight :
propeller traction and torque, T°, static et dynamic pressures, engine RPM, engine T° and manifold pressure, angle of attack and angle of sideslip, etc.
In order to understand the propeller operation, it is simpler to perform the analysis at propeller level rather than at airfoil level.
First, the third Newton law assess : "If a part A applies a force FA on a part B, The part B applies a force FB on the part A.
FB has the same value than FA, the same line of action but the opposite direction". This law is summarized by "action = reaction" principle.
If we want our propeller A use a forward force, it must apply on "B" a backward force.
For the aircraft, "B" is the air mass going though the blades swept disc.
It is not really a mass but a mass air flow. This "mass air flow" is equal to "disc surface" x "air speed" x " air density".
To apply a force on the mass air flow, blades are like wings.
Blade airfoils allow propeller to apply lift forces on air flow. The propeller applies a force on the air flow so the air flow speed is modified.
The difference between the upstream air speed and the downstream air speed is calculated as followed :
Delta Velocity (upstream/downstream) = pull / mass air flow DV = P / dm
from the second Newton law : F = d(m.v)/dt
This speed variation induced by the pull is applied half upstream and half downstream.
Mass air flow is so equal to : Dm = mvo x Sdisc x (Vflight + DV/2)
- mvo : air density (kg/m^3)
- S disc : blades swept disc (m²)
- Vflight : Flight speed
Some power calculations can be carried out :
- usefull power delivered by the propeller to the aircraft : Pu = Pull x Vflight
- absorbed power : Pa = Pull x (Vflight + DV/2)
So propulsion efficiency factor : rp = Pu / Pa
==> propulsion efficiency factor is an absolute limit which is the design goal for the propeller designer.
Choice of a small diameter for the propeller leads to mediocre performances. And this becomes worst with a low flight speed.
Number of blades may allow reducing the performance loss (see after in the text).
But this cannot be enough to reach the performances with an adapted diameter.
Propulsion phenomenon power losses cannot be decreased by the propeller designer.
But he must take care not to increase them with a bad pull distribution along the propeller disc.
So he must chose the right pitch, chord and airfoil distribution in order to get the optimum lift distribution.
Unfortunately, others energetic losses exist : losses linked to blade drag.
Blades are like wings and generate lift and drag. This drag consists of 2 parts : friction drag and lift induced drag.
A/ Friction drag on blade airfoils
Drag = 0.5 x Mvo x S x CD x V²
Blade case is more complex than wing one, because speed is variable from foot to tip of the blade.
At blade foot :
Low speed and small chord lead to ridiculous Reynolds number => airfoil performances are mediocre (high CD and low CLmax)
At blade tip :
High speed and very small chord => Reynolds number remains small.
But as the speed is close to sound one, Mach number is high.
High Mach leads to airfoil characteristics degradation.
With a small curvature or incidence defect, airflow may become supersonic and so generate noise and degrade performances.
B/ Lift induced drag
The wing has a finite span and so lift generate induced drag. Air speed is constant along the span. Induced drag can be calculated easily at wing level.
For the propeller blade, induced drag modeling is not easy because of the variable speed along the span.
For this drag assessment, Helices E-Props engineers don't find adapted calculation method in specialized press or in labs studies reports.
So the team has implemented a new and efficient calculation method.
Calculation duration is quite long : 90% of the airflow modeling duration is used to define induced effects on blades linked to iterative documentation.
This chapter has listed causes of propeller propulsion energetic losses.
Trigonometric aspects of the modeling have not been presented because they are out of scope of this simplified explanation of the modeling process.
The inertia of an object is its capacity to resist a variation of speed. The slowness is directly connected to the mass of the object and thus confronts in kg.
For rotating objects, the mass is not a sufficient information.
The mass of the object is associated with its distance by report the axis of rotation, in order to compare the capacity of resistance with a variation of angular speed.
It is the moment of inertia : MOI (in kg.cm²). The moment of inertia is a very important data for the propellers.
Indeed, the aeronautical engines are mostly piston motors.
The brace undergoes a push of the connecting rod in every tour in 2-strokes engines, and both tours in 4-strockes engines.
The brace is accelerated during an about-turn, and is slowed down during the rest of the cycle.
It is the inertia of all the rotary set which is going to allow to assure the rise of pistons and regularization of the rotation.
The propeller makes the biggest steering wheel of inertia.
If it is pulled by a reducer, the points of engine torque will be supported by the reducer.
If it is directly bound on the brace (for direct drive engines), this one will support all the efforts.
The efforts are besides passed on through all the system: the braces of redrive engines can also suffer if the moment of inertia of the propeller is too high.
And the screws of the propeller are submitted to the same efforts.
Using of a propeller with a moment of inertia upper to the values indicated by the engines manufacturers is going to decrease of the longevity, even to break the reducer or the screws of the propeller.
That's why the engines manufacturers indicate the maximum values of moment of inertia of the propellers which can be adapted to their engines.
Example for ROTAX engines, see the Rotax document : Max. Moment of Inertia Rotax 912 Serie
For example :
- Rotax 582 reducers A & B : 3000 kg.cm²
- Rotax 582 reducers C & E : 6000 kg.cm²
- Rotax 912, 912S, 912iS, 914 : minimum 1500 kg.cm² / maximum 6000 kg.cm²
- Rotax 915iS : minimum 1500 kg.cm² / maximum 7500 kg.cm²
- Jabiru 2200 : 3000 kg.cm²
Be careful : in case of problems linked to the use of an unsuitable propeller, with a too high moment of inertia, engines manufacturers may refuse any guarantee.
The E-Props moments of inertia are calculated when the propellers are designed. Then the data are verified and measured for each propeller.
Some examples :
- 3-blade tractor dia 160 cm : 2.400 kg.cm²
- 3-blade tractor dia 180 cm : 3.800 kg.cm²
- 3-blade pusher dia 170 cm : 2.700 kg.cm²
- 4-blade pusher dia 172 cm : 3.600 kg.cm²
- 6-blade pusher dia 170 cm : 4.000 kg.cm²
It is important to know the moment of inertia of the propeller, and verify that this MOI respects the limitations of the engine manufacturer.
Symposium organized by association INTER ACTION (Association de Sauvetage Créatif du Savoir Aérotechnique), IUT de CACHAN (French School) and association AERODYNE (Association d’Etudes & Réalisations en Optimisation Dynamique & Energétique)
every 5 years near Paris. Subjets : leisure aviation and technique.
In 2016, this symposium took place on 10 and 11th of June.
Jérémie Buiatti, designer of the E-PROPS propeller, has presented a publication concerning what happens between engine and propeller.