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Low RPM Engines - Nearly Flat Torque 'Must Have for Helicopters'

10 years ago, we wanted to build a helicopter. As pilots, we had our wish list at our R&D wall… All went fine, until we had to choose an engine for our helicopter.
All existing aircraft engine builders started their product to power up a fixed wing or a 4 wheel drive vehicle.
We needed an engine that would allow us to have 95% of the torque when the RPM dropped 300 … a normal operation in helicopter flying.
We did not find such engine.
We decided to build that engine ourselves …

D-Motor Belgium, was born

Now 10 years later we are proud to announce that we have over 150 4 and 6 cylinder LF engines in the air as a small engine builder, and we finally have accomplished what we had in mind back in 2011 : A light weigth helicopter engine, delivering more the 350 Nm torque, delivering 155 HP from 2.700 RPM to 2.850 RPM continiously.

We have build an engine for a typical drone and helicopter regime, running on MoGas, weighing less then 100 kgs, producing 155 HP continiously, 175 HP peak.

Our flathead boxer engine line runs at 20% reserve – meaning that we limit the power to 80% of the peak performance, as we knwo as pilots, that there are times when we need that extra HP to avoid dangerous situations.

Our R&D department is a ‘open house’ – we always seek for ideas and optimalisations from fellow pilots or engineers.

Interested ? contact [email protected] – our facts and figures can be found at : D-Motor

Last Edited by at 27 Dec 11:20
Belgian Lightweigth Engine manufacturer
EBKT - EBMO, Belgium

Interesting stuff, despite being an infomercial

I think it’s worth noting that a flat power curve is also beneficial for a fixed wing plane, if equipped with a fixed pitch propeller, when climbing at reduced airspeed and loading the engine to lower rpm at full throttle. Based on the data I’ve seen aero-engines like e.g. Lycomings have constant (flat) torque around the normal operating speed, and it is the higher rpm, lighter smaller displacement engines (e.g, VW-based and to some degree Rotax) which don’t have the same characteristic. I assume the intent with the D-Motor was to replicate the flat torque curve of a Lycoming while reducing weight, and it’s interesting that the side valve (flat head) design was chosen to do this. A good example of how engine design choices appropriate to an aero-engine can be exactly the opposite of those appropriate for a car or motorcycle engine.

Last Edited by Silvaire at 27 Dec 16:15

Welcome to EuroGA, Kurtghekiere

despite being an infomercial

Indeed. Please remember this from the Guidelines

We want to encourage businesses to participate – because a lot of the time they are the only ones who might have answers. The only conditions are that it is done under the company’s full name and that the poster does genuinely participate in the forum generally…

Administrator
Shoreham EGKA, United Kingdom

How’s the uptake of the 4 cylinder engine been in fixed wing aircraft? And at what rpm does it turn a fixed wing prop?

I thought flathead was something for very old American cars and lawn mowers. Simple and cheap to make but not very efficient. But why is the rpm range so important? I assume a helicopter will need some reduction gear anyway so why not rev the engine higher to get more power?

www.ing-golze.de
EDAZ

2,800 rpm is about the maximum for a ~150 HP fixed wing aircraft propeller and I’m sure that was how the engine operating speed was set. The helicopter application with gearbox would not necessitate that speed limit. From that max rpm requirement and the interest in low weight, one can imagine looking at the compromise between efficiency/power and reduced weight associated with a flat head engine.

The post was not intended to be infomercial.
It was merely an invitation to get some feedback.
We are not Rotax or Lycomming, we are a small company, alltough apparently we compete with them….

Low RPM Silvaire : As the drone industry (not utility but militairy) needs a silent power source, the low RPM is usefull.
But our intentions were not those specs, we wanted to build an engine that last long, is solid and is above all safe.

Yes the 2.850 RPM is ok for fixed wing aircraft (direct driven) which is done by the customers that fly our engines in a ULM Class 6 or PPL lic fixed wing. (BOT, Piper, …)
The flot torque curve is designed for helicopters – and indeed, the weight of a 320 Lycomming exceeds the specs for ULM Class 6 helicopters – that is why the engine is used in that kind of aircrafts.

A flathead that runs at half the RPM of a Rotax and double the Torque is rather efficient :-), Sebastian.

bests,

Kurt

Belgian Lightweigth Engine manufacturer
EBKT - EBMO, Belgium

My experience with helicopters is you hear the rotor noise clearly, sometimes from several km. The engine is not heard unless very close.

The elephant is the circulation
ENVA ENOP ENMO, Norway

LeSving wrote:

My experience with helicopters is you hear the rotor noise clearly, sometimes from several km. The engine is not heard unless very close.

Yes indeed. IIRC it is the rotor blades going supersonic at their tips which creates the distinctive noise.

Kurt wrote about a low RPM for silent drones though, not for silent helicopters. I guess he means fixed-wing drones, not heli drones where the advantage of a low RPM engine would indeed be lost.

Low-hours pilot
EDVM Hildesheim, Germany

Noise is a complex phenomenon, and especially when the human ear and brain is taken into account. Noise itself is merely a side effect of engine power, and rather proportional. The RPM has nothing to do with it. People tend to react more to high pitch noise than low pitch noise. The flapping of a Bell UH-1 is severely loud, but the low pitch makes it pleasant (for normal people ) The scream of a jet engine is not very pleasant. Also, low pitch noise travels much farther than high pitch. The higher the pitch, the more attenuated it becomes at a distance. I really don’t think the RPM of a Rotax (4-5000) vs direct drive (2-3000) is very much difference. Muffling the engines, and it is the propeller noise that remains. Getting the propeller RPM down is important for noise. It’s not the RPM itself, but the speed of the blades.

The elephant is the circulation
ENVA ENOP ENMO, Norway
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