I understand that the propeller speed should not be too much otherwise it will stall at the tip and be useless so higher rpm engine would be useless when we first think about it. However, we can put a single speed gearbox between the crankshaft and the propeller to reduce the speed of the propeller so that it will not stall at the tips. We can have an engine running at 6000 rpm making lots of power and through a 6:1 reduction gearbox the propeller will rotate at 1000 rpm and im guessing at that rpm the propeller tips wont stall. Why is this not done? Why are the aircraft engines so lazy low rpm engines with low power output? For example the 5 Liter GO-300 engine makes a peak power of 175hp at 3200 rpm , what would be the downsides of putting a 4 Liter Porsche 911 GT3 flat 6 engine that makes 500hp at 8500 rpm and then through a reduction gearbox decrease the speed of the propeller so that the propeller tip wont stall? Also im giving the porsche engine as an example because that engine is also used in 24 hour races constantly and they are extremely reliable.
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1$\begingroup$ Running an engine for 24 hours isn't "extremely reliable" at all. An ordinary car engine goes for 5,000-7,500 hours without any internal maintenance. And then, when people's car engine stop, you normally don't have a crash or fatality. $\endgroup$– user71659Apr 14 at 18:57
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$\begingroup$ @user71659 These engines generally require full rebuild after around 100 hours not 24. The regular car engine you are talking about that lasts 7500 hours pretty much never sees the redline in its entire life. It makes no sense to compare a regular car engine that realistically never goes above 3000 rpm to this engine to say "its not reliable". This engine is constantly abused to extreme levels in that 100 hour interval. But yes now looking at O-300 it says interval is at 1200hr , so i guess reason why a high rpm high hp engine is not used is due to much more frequent rebuild interval and cost. $\endgroup$– Timothy SchererApr 14 at 19:21
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1$\begingroup$ Low RPM is good b/c it means less friction generated heat. Engine life is largely a function of distance traveled by the piston rings. Low RPM means more flying time for a given distance of piston ring travel. You want direct drive for the reasons Jan gives in his answer, and that means you generally relay on displacement, and get 1/2 hp per cubic inch in order to achieve a torque peak under 3000 rpm. An aircraft engine's environment is closer to an industrial engine. $\endgroup$– John KApr 14 at 20:50
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$\begingroup$ To add on to @JohnK the idea of small-displacement high-revving engines comes in a large part from obsolete European and Asian car taxes based on pure displacement. There have been examples of car engines brought to the US are modified to be slower and larger, and save fuel. Viscous drag from engine oil and moving parts scales to the square of velocity. As mentioned, it also affects wear, which is also compounded by the use of lighter oils to compensate for high speeds, but is hidden by European vehicles driving significantly less per year than the US. $\endgroup$– user71659Apr 14 at 22:49
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1$\begingroup$ @Dave Those links are nice thanks for sharing. $\endgroup$– Timothy SchererApr 15 at 8:53
2 Answers
A reduction gearbox is a non-trivial piece of fine machinery that needs to be maintained and can fail. And since reliability and low maintenance cost are important design goals for aircraft engines, especially engines for small aircraft, it makes sense to design the engine without need for a gearbox and just accept the need for a bigger cylinder volume for the same power.
But the newer deigns, like the Rotax or Austro Engines ones, all do have reduction gearbox. These designs are derived from automotive designs and those generally run at slightly higher speeds.
Note that the difference isn't that huge. Typical small aircraft propellers have maximum RPM 2,700, with maybe 2,400 RPM being usual cruise setting, and the automotive engines don't normally go over 4,500 RPM, with 3,000–4,000 being typical cruise speeds.
The main obstacle to newer engine designs for aircraft is the certification. While the engines themselves are pretty reliable these days, they are all electronically controlled these days and that adds failure points that all need to be carefully validated, and backup sources of electric power need to be provided and such, which is a huge effort. So instead a lot of light aircraft still use the older tried and tested simpler ungeared engines.
Side note: the tips of a propeller won't stall by spinning faster. Rather they will generate more drag, wasting power, and once the tips go supersonic (close enough to supersonic to start causing shock waves to form), vastly more drag, wasting even more power.
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1$\begingroup$ There's also torsional resonance issues, the bigg bugaboo for geared engines with few cylinders. Molt Talyor, for his flying car that he certified in the early 50s using a Lyc O-320, used Flexidyne dry fluid couplings used in washing machines etc, a housing with a wavy plate filled with ball bearings, to provide the compliance in the drive line to absorb resonant pulses. Rotax relies on rubber couplings and/or slip clutches, and careful tuning of the system. $\endgroup$– John KApr 14 at 20:44
Why are aircraft piston engines so low rpm?
The fuel consumption of a typical aeronautical piston engine depends, all the rest being equal, only on the rotating speed rpm with a typical bell-shaped curve (source):
As visible in the previous plot, there's a range of rpm (more or less from 3'000 to 5'000) where the fuel consumption is minimised. The design goal of a gearbox is to supply the propeller with the needed rotating speed so that the engine's rpm remain inside that optimal range. This minimise not only the fuel consumption but also the engine's wear. Making the engine run at a higher rpm simply consumes more fuel plus requires a bigger and heavier gearbox.
I understand that the propeller speed should not be too much otherwise it will stall at the tip.
The limiting factor is the onset of transonic speeds on the tip which greatly increases aerodynamic drag and therefore the needed torque.
