Andrew, thanks so much for the detailed response! I learned enough science over the years to follow a lot of that, and I appreciate all of the insight.

Both are putting out the same amount of torque, but the Thielert is putting out less power. The constant speed prop on the Theilert will be at a slightly higher pitch to provide the same torque at the lower speed, but it won't make up for the fact that it is turning 400 rpm slower. The Theilert is the watch here. You're putting less energy into it, even when multiplied by the improved efficiency, so you'll get less out.

If I hooked a block and tackle up to the front of your airplane, I could apply more force than that IO-360, but the best humans out there can only approach 1 horsepower. We can't accelerate your aircraft quickly. Torque and force has to do with being able to move something or not (think torque wrench). Work is the amount of force times distance to move your aircraft from point A to point B. Power is rate (time) that work is being done.

Torque is force times distance
Work is the energy transferred by, a force acting through a distance
Power is the rate at which work is performed or energy is converted

Not entirely true. Every time you use a gearbox, you lose some power in the gear box, plus you carry the extra weight of the gear box around. For a piston engine, the gear box has to be able to handle the significant torque ripple which causes it to be even heavier than just the power would suggest.

Ideal prop RPM is somewhat slower than 2700 RPM. Turboprops typically go for 1500 to 2000 RPM, so that can be taken to indicate the ideal RPMs for a prop. If the engine can directly turn slower, it will be able to swing a more efficient prop.

A naturally slower turning, higher torque, direct drive engine is thus more usable power at the prop than the same input power at higher RPM or through a gearbox.

Mike C.

This is correct assuming you can design a prop to fit the aircraft that will meet the target parameters of the aircraft's mission along with the powerplant limitations. If I put too much pitch on that constant speed prop to slow the engine down, my prop efficiency will go to pot as I start to stall parts of the blade. That slower turning engine must be able to produce the same power at a lower speed typically requiring a larger charge (fuel and air) per cycle putting more mechanical stress on the engine.

This is where a light weight, faster turning engine with a well designed gearbox can put out more power at high RPM, then be geared down to achieve reasonable prop speeds can outperform (lighter weight and same power) a large displacement engine with high combustion stresses.

Getting into details... If I have a small well designed SI IC engine and I want to double the power... I have several major options:
1) Double the displacement (bore and/or stroke) - bigger cylinders, bigger crank, bigger case, bigger valves - nearly double the weight.
2) Double the number of cylinders - twice the weight (nearly).
3) Boost it to 2 bar (15PSI boost). Add weight in the turbocharger (or supercharger), cylinders (running at higher power per cylinder), rods, crank, etc... along with intercooling, etc...
4) Double the engine speed. No more load stresses on the cylinders, more structural stress only on the rotating components. Crank doesn't need to grow much, if at all. Valve train needs to be better designed, better materials, etc... There's a cost impact here, but weight is minimal. Design things for higher number of cycles and higher temperatures (less time for things to cool between cycles).
5) Add an oxidizer to the combustion mix (same load increases as #3), but requires carrying an oxidizer fuel (Nitromethane, methanol, etc...)
6) Can't double power, but increase compression ratio - you can increase power, but this requires a stronger engine again - weight goes up with power. You also have to watch the detonation margin at high power levels. (arguably not valid since the assumption of a "well" designed engine.)

Look at what the automotive space is doing right now - combination of #2 and #3 as they provide both emissions and efficiency benefits and economies of scale are reducing the associated costs of turbocharging. Lighter engines are an important initiative in that market right now. They are also pushing #6 as well because they can actively control timing to limit detonation.

Your example greatly exaggerates the effect.

182 has an 82 inch prop at 2400 RPM, 230 HP. Tip velocity 859 fps.

MU2 has 90 inch props at 2000 RPM, 715 HP. Tip velocity 789 fps, quite a bit less.

It doesn't take much increase in prop diameter to absorb a LOT more power. Adding only 8 inches to the prop diameter yielded a prop that absorbs more than 3 times the power and does it at a slower RPM.

A 182 with a 2000 RPM engine will be more efficient per HP than one at 2400 RPM. That's the nature of props. The prop diameter would grow maybe 2 inches which can be compensated by adjusting the thrust line 2 inches higher for the same ground clearance.

Mike C.

Looks to me like the diesel engines currently available produce slightly less maximum HP than the gasoline engines they are replacing, but maintain that power to higher altitudes (turbo-charged), and weigh slightly more. So I would expect to see slightly longer takeoff distances, slightly lower rate of climb, and possibly slightly lower cruise speeds at lower altitudes, but perhaps slightly faster cruise speeds at higher altitudes.

I believe that propellers can be optimized properly for each engine's HP and RPM, as well as for the desired cruising speed range. If that is done, both propellers should provide similar efficiency factors, so I doubt this is significant difference between the two.

The fact that diesel engines typically weigh a good bit more than gas engines (raising empty weight) is an issue. But diesels can be significantly more efficient (requiring less fuel), which may offset that weight difference somewhat. However, diesel fuel weight a bit more per gallon that AvGas, so the weight goes back up somewhat...

Why am I talking about all this? Well, it's because I think that we're comparing apples to oranges until we see an airframe designed from inception to be powered by a diesel engine, with all the design factors optimized in that direction... And how long has it been since the last entirely new, designed-from-scratch fully certified airframe was introduced? (Not including the LSA here, as their certification process is quite different, as are the goals and limitations of the design.)

Depends on prop. If they turn slower and user a large diameter prop, the extra efficiency can give you back those few lost HP.


No, it can be significant. It is the same reason a glider wing is more efficient, slower speed and less loading. The larger slower turning prop has the same kind of benefit.


This needs to be measured in a system.

Typical takeoff BSFC for an IO-520 is around 0.6 lbs/hp/hr, cruise is about 0.4 lbs/hp/hr, say it average 0.5 over the first hour.

The SR305 diesel is 0.36 lbs/hp/hr for takeoff and roughly the same in cruise.

In 1 hour of flight, 227 HP for the diesel, the diesel will burn 32 lbs less fuel for the same power. Over a 3 hour flight, that is about 100 lbs less. So the diesel advantage is there in fuel weight and it feels like it would cancel the extra engine weight. The SR305 is 455 lbs, the O-470-L is 404 lbs, so only a ~50 lbs penalty from just the engine. Other things (cooling, accessories, etc) might change that, but we are in the range where fuel savings negates the extra engine weight.

Notice the above was not in gallons, but lbs, so the extra density of diesel or Jet-A is already accounted for. For the same range, you can design a smaller tank which saves some weight.

Mike C.