We answer your frequently asked questions about our electric motors here: 

Electric motor

To do that, the following connections must be known: The HPD motors are constructed as alternating current synchronous motors with a constant  torque line from standing still over the whole range of revolution speeds, independently of environmental influences. A combustion engine can only develop its maximum torque at a specific revolution speed point in optimal environmental conditions (air pressure, temperature, fuel quality etc.).  This means in practice that  “Electric  HP“ should not be compared with "combustion engine HP". In our experience and with our measurements with many aircraft  re-equipped from combustion engines to the HPD motor range, the following ration can be assumed in the power design of an aircraft already fitted with a combustion engine : 1.25 x "Electric HP" = "combustion engine HP"

Our power systems  (battery, charging system, motor management, motor and propeller) have a degree of efficiency in energy conversion between  45% and 80% depending on air speed of the aircraft. That means that for each 1kWh of energy charged in the battery, 0.45kWh to 0,8kWh  are converted into the drive.

A good combustion engine power system achieves about 5%, that is, for each litre of petrol with an energy content of about 10 kWh, only about 0.5kWh can be converted into thrust.

In practice, measurement flights show the relationship as follows: A highly dynamic aircraft with our  HPD12 motor and a battery pack containing 3kWh of energy can fly for about 45 minutes under power. The same aircraft with a combustion engine would need about 3 litres of petrol with an energy content of 30 kWh for that.

That corresponds to an energy conversion ratio of about 10:1 (electric motor to combustion engine)

Yes, but only under certain conditions. The noise emitted by a combustion engine  consists  of the engine noise, the running noise of belts or gear transmissions, resonance noises and the slipstream of the propeller. Very quiet combustion engines achieve noise emission figures of approx. 60dbA.

With an electric motor there is only the slipstream of the propeller if it is a direct drive system, such as our HPD xx range of motors.

The running noise of the airscrew rises greatly with the increase in propeller speed. It may well be that an electric power system with a fast-turning small propeller are perceived as "aggressively" loud. Even with electric power systems that are supported by gears or belts there is a non-negligible friction noise.

Our gear-less electrical drive system HPD xx is designed with a nominal rotation speed of  2000 /min. The airscrew specially developed for this speed range,  H25K, H25F and H30V, develop noise emissions of 46 – 50 dbA with an efficiency of about 50% ! That means that you don't even notice this motor at a distance of about 100m. It should also be considered that the noise emission limit in residential areas is 50dbA  and so this motor could also be used in residential areas around the clock; it can thus be said: The electric motor is quieter than a combustion engine.

The purchase costs of an electric power system are about 30%  higher than those of a combustion engine of comparable power and comfort (e-starter, monitoring etc), largely due to the still relatively high battery costs. However, several facts need to be considered in a comparison of the two systems:

  • The operating costs of an electric motor are limited to the electricity costs when charging. e.g.: if a  24Ah Li-Ion battery pack with 14 cells contains about 1.2kwh of energy. With this, about 20 minutes can be flown with a GS. This energy from the socket costs an average of 30 cents. The comparable energy cost for fuel for a combustion engine is about 1.50 €.
  • The electric engine is completely service-free. There are no servicing costs!  The E-motor is highly available in all weather and delivers constant power at all altitudes.
  • The E-motor with our propellers develops a noise level of 47-50dbA. That is below the sustained emission limit for residential areas! This means that flying is possible in noise-sensitive areas.
  • The compact construction of the electric motor allows slim and aerodynamic designs.
  • The system works completely emission-free.

The index after the type designation  High PowerDirect gives the sustained power in Kilowatts. Thus the HPD12 can give 12kW sustained power and the HPD32 32kW. The motors with the additional designation D (for Duplex) are redundant motors which guarantee failsafe operation. The peak power of the motors is each about 25% to 50% higher than the nominal power. This can be given for short durations, e.g., during take-off for approx. 2-3 minutes and increases the thrust. With the corresponding airscrews from our range, the motors develop between 500N (1.30m Propeller on the HPD12) and approx. 1500N static thrust (1.75m Propeller on the HPD40D).

Conventional propellers are designed for the high vibration characteristics of a combustion engine. This requires relatively thick profiles that are not aerodynamically optimal. Our propellers have considerably thinner and more effective profiles, specially designed for electric flight with HPD - motors and need significantly less drive power per thrust.


The battery types in our range come from the automotive sector. They are used there for electric and hybrid cars. This means that these batteries must comply with particular  regulations and  approvals. The batteries are also subjected to special tests, such as altitude simulation, thermal effects, vibration, shock, short circuit, impact, deep discharge, over-charging etc.  The batteries may not, for example, exhibit in practice any exothermic reaction (fire) in a crash following which the battery is mechanically and electrically  (short circuit) destroyed. Moreover, in our power system all relevant parameters of the battery (cell voltage, temperature, current, power etc.) are monitored. Our battery packs are UN38.3 certified.

There are two specific limit values for the lifetime of a battery. Depending on the battery type, there is a cycle number at which the battery ca still give 70% of its capacity and there is a chronological lifetime. The first value (70%)  is at about 800 to 1500 charge/discharge cycles. Charge cycles are complete charges/discharges. The chronological lifetime is 10-12 years. Extensive details of this can be found in the instructions for the E-Drive System.

Our Li-ion batteries have a measured self-discharge rate of about 0.3Ah per month, depending on the charge status and age. Even if a battery is standing for 4 weeks after charging in the flying season, the loss of capacity is negligible.

Our batteries can be charged quickly. Depending on the type, the charging current can be between 1C and 2C (1-2*nominal capacity (C)). The charging time to 90% of the required charge is calculated from the capacitance of the battery divided by the charging current: t = C/I Example: A 60Ah battery pack with a 30A charging device thus needs t+60Ah/30A = 2 hours, and the same battery needs t= 60Ah/8A = 7.5 hours with an 8A charging device.

There are many developments and a lot of research work aiming to bring even more powerful batteries into the market. If the automobile sector eventually engages fully with E-drive technology, these energy stores will also come in the future. In the medium term, the development potential in the next 2 years is about 20%.

Flight or climb time depends exclusively on the size of the battery, or the energy source.

The flight time in powered flight is calculated simply by dividing the energy content of the battery by the required journey power : t = E/P (t in hours; E in kWh; P in kW).

As a calculated example, we will consider the power requirement of a trike when climbing (St) at around 2m/s at 13kW for 180 seconds to an altitude of 360m. In the subsequent cruise at about 60km/h, this kind of aircraft needs about 5kW. we will assume our 3.1kWh battery pack.

  • Climb energy requirement: E = P*t = 13kW * 180sec. = 0.65kWh
  • Remaining energy for the cruise E_rest = 3.1kWh - 0.65 kWh
  • Level flight :   t = E_rest / P;  t= 2.45kWh / 5kW =  = 0.49h = 29 minutes

The flight time and/or the climb altitude can be scaled up with more battery packs, up to 8.


There are currently several ultralight aircraft powered by the HPD-motor range which are approved as UL by the DULV or DMSV.

In principle, the drive can also be operated as a generator. Our system stores the rotational energy with every braking action (braking procedure of the airscrew back into the battery.  Energy reclamation is very inefficient without great expense (automatic propeller adjustment etc.) and so is not very practical.

Yes, absolutely.

The electric drive is even an essential part od an integrated ecological process. The drive system works completely emissions-free in application.

The production process of all components of the E-Drive System are currently procured with 30% eco-electric mix, and the trend is rising.

The batteries can already be charged with 100% renewable energy.

What else might be possible?

Our plans and ideas inspire first us, then our clients.