I got to thinking the other day ‘What if car owners had their engines completely rebuilt on a fixed schedule like airplane owners are required to do by the FAA? You know, where the mechanic takes the engine completely apart, measures the wear of the parts, replaces worn parts as necessary, installs new seals, recalibrates, etc., reinstalls the engine. Would that be cost effective or a waste of time and money for car owners?’.
Then – before I could decide one way or the other – I got to thinking "Do these small 4 passenger airplane engines, like Cessena 172’s and the like, do they have transmissions? There’s a lot of problems reported here on transmissions, so it seems like a good idea that airplanes wouldn’t have them. And I’ve ridden in a plane like that, and I don’t recall any operator control for the transmission.
Just curious, anybody know? Do these small airplane drivetrains have a transmission? Is there even any fixed gearing between the crankshaft and the propeller? Or does the crankshaft simply drive the propeller directly?
For the most part, they are much like an outboard motor. The props can be changed by the mechanic to change the flight characteristics. The closest thing would be those that change the pitch of the prop while flying which is essentially like a transmission…
A few of the small aircraft piston engines use gear reduction and the high power models have variable pitch propellers as dagosa mentioned. But most that I see are direct drive with a fixed prop. There is a significant thrust bearing incorporated in the power train though.
The maintenance schedule for engines once specified periodically removing the cylinder head and pan to inspect and repair or replace as necessary to bring within specification. European cars continued to spell out such procedures well into the 1950s.
Things change when you get to larger propeller planes or prop planes that have special requirements, like short takeoff runs, etc - they’re often powered by turboprops, which is essentially a jet engine connected to a propeller. They do have a reduction gearbox, but not the kind of transmission you’re probably thinking of, where you change gears to go faster or slower.
With perkier engines like the Subaru which is commonly used in homebuilt light aircraft the gear reduction unit like the one below. That link shows how it works although some use spur gears and so on.
This lets the engine run at higher RPMs to make use of the horsepower and in turn it keeps the tip speed of the propeller blades down. If the tip speed starts approaching the speed of sound there are noise issues along with the possibility of blade failure.
The same basic principle is applied to wind turbines as they use reduction gearboxes also.
Yeah, most GA engines are horizontally-opposed, pushrod-valved, aircooled engine. Each cylinder typically displaces around 90 cu. in., and the engine outputs around 1HP per every 2 cu in…an O-360 is rated at 180 to 200 hp at 2700 RPM.
In short, circa 1950s technology.
On paper, a modern, high hp-per-displacement engine looks a lot better…but by the time you add the cooling system, fluid, and pump; a gear reduction, etc…the all-up weight doesn’t wind up being much better than the tried-and-true.
Good comments, thanks everyone. I wasn’t even aware that smaller planes like the Cessna 172 and the Piper Cherokee used variable prop pitch, but, yes, varying the angle of attack of the prop (the pitch) is sort of the same as changing gears in a car. A more aggressive prop pitch will move the plane through the air faster, for a given rpm.
I can see the weight advantage of using an air cooled engine for airplanes. Sounds like these small airplane engines are configured similar to the 60’s VW Beetle engine, only w/more HP. Air cooled engines seem to have lost favor for automobiles. Are there even any new cars at all made with air cooled engines I wonder? I think I heard a comment here that the reason was the difficulty of meeting emissions reqt’s with air cooling. I guess the federal emissions requirements are less stringent for airplanes.
@ GeorgeSanJose
The Tucker Torpedo used an air cooled helicopter engine in the rear of the car. Tucker buildt 57 before the big automakers forced him off the stage.
Varying the angle of attack is exactly the way a hydrostatic transmission works with it’s swash plate, the way a torque converter works and the way a prop on an out board motor works in a fluid medium like air, water or hydraulic fluid. I think of the prop pitch as an external transmission. A simple, cheap fool proof way of changing the pitch on props in water could make you a millionaire. Sure, the reduction gear may be part of it, but transmissions come in all shapes and sizes and they don’t have to be gear driven…they could have fluid drive…like props in air.
A stock cessna 172 usually does not have a variable pich prop …but mine did. It had the engine and prop from a 182 to give the variable prop and the nice boost in power we always need here at 6500 ft altitude home airstrip.
And you could truly feel the difference as you changed pitch adjusting for low speed get-up-and-go or high speed cruising.
There is an automobile museum here that has a Tucker, @252525, and I have looked at it from every angle and read extensively on the car and Mr Tucker and the “conspiracy” that supposedly led to his bankruptcy. Often captions under photographs of a Tucker describe the engine as a flat 6 helicopter engine, ignoring the modification. It is an impressive machine.
the all-up weight doesn’t wind up being much better than the tried-and-true.
Plus you have to pay a lot of money to get a new engine design certified, so if it’s not gonna show an advantage over the existing, there’s no point.
Though there are a few newer airplanes using diesel engines (Diamond’s DA-22 for one) that are pretty interesting. Along with the new engine comes new computer management for it - so no more mixture knob.
Understand that when you upshift in a car, you disconnect the engine from the drivewheels in order to change the ratio and slow the engine down to begin it’s uphill climb through its power curve again. Your car’s inertia keeps it rolling while you shift.
Envision, if you will, disconnecting the propellar from the engine in mid air in order to shift. Not a great idea.
