First post / Prop sizing questions

[Abcole]

Hello all, first post here. I made an account because I have a couple of questions regarding experimental aircraft and modern automotive engines. To frame the question, I feel I should walk you through how I arrived at the question. I have recently become interested in aviation as a hobby and with all new hobbies I have been attempting to absorb as much information as possible. I discovered experimental aircraft after looking at what is required to maintain a certified aircraft. Since I am a mechanical engineer and a hobby mechanic/fabricator and don’t trust even my automotive maintenance and repair to anyone other than myself the fact that there are no restrictions to owners performing maintenance functions on the airplanes in this category seems like a huge plus. While doing more research I discovered that having an aircraft engine overhauled can cost upwards of 30 thousand dollars. I thought that sounds ridiculous considering how old and simplistic the technology seems. This is especially true when you consider that a brand-new gm long block is only $5000 dollars. This got me to the point I was thinking about the possibility of putting LS power into an experimental aircraft. After some research I found the Murphy Moose that has the naturally aspirated LS engine. Due to the higher revs of the automotive engine they used a 2:1 step down gear case to run the prop at the correct RPM.

This finally brings me to my question. From my understanding prop size is determined or rather limited by the speed of the tip. From my research the prop tip must stay under 0.85 of Mach 1. For example, if the given engine develops peak power at 2700 RPM then the diameter of the prop would be at the largest 80” which would result in a prop tip speed of 287 m/s. If all the above is true then why mess around with a 2:1 step down gear case? Just go direct 1:1 and properly size the prop. For an RPM of 4500 the prop diameter would be 48” to have the same 287 m/s speed as the previous example. Is there any issue with reducing the prop size by this much? I would assume that you would need at least 3 blades potentially more to utilize the full power of the engine at WOT, but I cant see that presenting any issues. Is there something that I am missing? The idea of replacing an engine with a brand-new crate engine for $5k as opposed to $30k would lower the cost of flying per hour quite tremendously at least 13 dollars an hour and not needing to buy and maintain the reduction drive would be a huge win as well.

[Bill Berson]

A homebuilder can overhaul a used but repairable aircraft engine for perhaps $5000.
The aircraft engine is engineered for the stress of turning a prop, and at an efficient diameter and rpm.

[Dana]

For low to moderate speed aircraft, larger propeller turning slower is more efficient than a smaller propeller turning fast.

[Jeffrey Meyer]

Propeller vs Impeller, incompressible vs compressible flow, extremely low rotor solidity vs extremely high rotor solidity. Completely different engineering design criteria.
Propellers are rotating wings. the free-air wing tip contributes zero lift to the wing - not some lift, not a little lift, zero lift. On the other hand, the tip of an impeller blade contributes a major portion of the lift. This is because the shroud (or duct, or whatever you want to call it) around the impeller prevents radial flow of the air that then tries to flow tangentially around the blade. To prevent that tangential flow we put another blade very close to our blade - so close that the air has no choice but to flow in the axial direction of the rotor. This "no choice" is called "pressure". We are now working in the compressible regime rather than the incompressible regime of a free-air propeller, and the closeness of the blades is called the "high solidity" of the impeller.
And last but not least, on a bypass turbofan, the diameter of the first stage bypass rotor is bigger than the inside diameter of the intake duct. This helps to smooth the transition from incompressible flow to compressible flow.

[Jeffrey Meyer]

Prop efficiency is inversely proportional to prop diameter. As the saying goes "keep it as long as possible as long as possible". So theoretically the most efficient prop for a given airspeed and thrust (read "power") is a single bladed prop. However, that being somewhat difficult to implement probably the best compromise from the efficiency point of view would be a two-bladed prop.

[Frank Giger]

Okay, lemme have a stab at short vs. long on props (y'all feel free to correct me!):

First off, the inner two or so feet right behind the prop is airplane, what with that pesky fuselage and everything. Not a lot of efficiency there.

Second, we're looking for thrust. A smaller prop just doesn't have the same surface area as a large prop, so even if it's moving half as fast, the longer prop is going to make more.

Third, we want "clean" air for that blade. A small prop going fast is going to catch disrupted air from the other blade.

[Airmutt]

Don’t want to sound like the curmudgeon of the group but you really need to ask yourself why don’t you see a lot of automotive big block engine installations in the EAB world. It can and has been done but has it’s challenges.

[Sam Buchanan]

Some reasons why there are few auto engine based aircraft:

Scarcity of reliable gearboxes

Cooling

Cowling

Weight

Lack of insurance

These are significant hurdles that have to be jumped and aren’t fully appreciated by those with limited aviation experience.

[Frank Giger]

Excepting the VW. Loads of VW engines (and variants based on VW blocks) flying around.

[mmarien]

Interesting question. I read somewhere that prop efficiency was best at about 1800 RPM and as mentioned here the one blade prop is the most efficient. On the other hand there are modern turbine engines that are mostly pushing air that bypasses the jet engine. The turbines have multiple vanes turning at ~30,000 RPM. The vanes could be considered multiple blade props. So there is probably room somewhere between slow moving single blade props and very fast multi blade props that would be efficient at the peak hp range of a piston engine.

There is the heat problem of running any engine at higher RPM's. My O320 run cooler at 2300 RPM as opposed to 2700 RPM. I needed a nice cool day to open it up and keep the CHT within reason. I'm pretty sure that if you put your automobile in a lower gear and got the RPM up to 4000 or 5000 RPM at highway speeds you'll probably have a heat problem.

And there is that cowling problem behind the prop on most planes, except maybe the VariEze and derivatives. Maybe a smaller prop would be more efficient beside or behind the fuselage rather than in front of it. Just thinking out loud here.

Having said all that, if they put the amount of engineering that goes into an automobile engines into airplane engines, we'd probably have better solutions. Rotax comes to mind. Nearly 90% of LSA's use the Rotax engine. Maybe there is a reason for that. Same hp at twice the RPM and half the weight.

One final thought. The PT6 turboprop uses a unique solution of running the exhaust gases of the engine through a separate turbine that runs the prop at a much slower speed. Maybe what is needed is a torque converter to separate the prop from the engine.

Any solution needs to be done quickly before fossil fuel powered engines become obsolete.