What determines the type of propellers and the number of propeller blades used in aircraft engines?
When would contra-rotating propellers be used for an aircraft?
I think that more blades would yield more torque at low speeds (for shorter takeoffs), but would be less efficient at cruise.
Counter-rotating props would cancel out the torque from the engines, and the “p-factor” while the plane is slightly nose-up. Both of these factors tend to make a plane (without counter-rotating props) want to veer off to the right while rolling down a runway to take off. So counter-rotation would solve this minor problem, but introduce a different problem - if the engines rotate different directions, then the parts won’t be just alike and it will be more of a maintenance hassle.
Three bladed or greater props are also often used to lower tip speed - generating the same thrust with a two-bladed prop requires longer blades, which generates more tip noise. A lot of high-powered singles like Beech Bonanzas have aftermarket STC’d three bladed props, and they usually advertise quieter operation and better takeoff performance.
Contra-rotating props eliminate torque issues, and also eliminate a ‘critical engine’ issue - the propeller on an aircraft engine develops more thrust on the downgoing blade during slow flight, because the propeller disk is at an angle to the relative wind. This effect is much more severe in helicopters, but it happens in airplanes, too. The key reason is because the blade on one side of the angled disk is moving with the relative wind, and the other is moving against is. So one is flying at a higher airspeed than the other, and creates more lift. So, the asymmetric disc load on the airplane causes it to want to yaw - that’s why you have to add in a bit of right rudder in slow flight to keep the airplane flying straight.
If both propellers on a twin rotate in the same direction, one engine will have the downgoing blade between the engine and the fuselage, and the other will have the downgoing blade between the engine and the tip of the wing. In the latter case, the extra thrust is farther away from the fuselage centerline, which creates a larger arm for the extra thrust, and more of a force wanting to pull the aircraft off line. In a twin this is especially bad because if you lose an engine, then during landing you often have to be flying slow, with the good engine providing significant amounts of power. A single engine landing or takeoff with the critical engine is harder than the same landing with the non-critical engine. By using contra-rotating props, you can have the downgoing blades on both engines nearer the fuselage, making one-engine landings easier.
This is a big issue, because one of the main causes of fatalities in light twins is a stall-spin while attempting a single-engine landing, or a stall-spin when an engine is lost on takeoff.
And of course the P-38 has two critical engines…
Anybody else know why?
I’ve thought about this a little (not especially seriously as I’ve never flown a real airplane), and think that if I had an engine problem in a twin that left the one good engine trying to spin the plane around, I’d cut that engine on final approach and dead-stick it. Is that a good choice? Since you can’t really climb with one engine on many twins, it’s not as if you have another shot anyway, and any such control problem would be worse at full throttle.
Yep. One engine turned one prop, the other engine turned the other. It would be nigh-on uncontrollable if you only had one engine due to the torque reaction.
WWII fighters had some rather innovative stuff in them, as with any mature technology that was facing obsolescence. Bell’s P39 put the engine behind the cockpit. Of course, anyone who’s ever driven a Toyota MR2 or other mid-engined car will tell you that it’s pretty easy to spin in them - and “spinning out” in an airplane isn’t really a good thing. Especially if your stall recovery is poor.
An honest question:
The lift from the blade. How much of a percentage of the overall thrust of the propeller does it provide versus the thrust from the angle of attack from the same blade?
Maybe I’m overengineering it in my mind, but I’ve always thought of the majority of the thrust coming from pushing the air back from the angle of attack versus the pulling forward from the lift of the propeller “wings”.
Tripler
Please, give me a little detail here. . .
Sure, you’re going to cut the power on short final. But a lot of twins are pretty marginal on one engine, and you have to hold a fair bit of power just to maintain your altitude.
Here’s the scenario for a fatal stall-spin: Pilot is coming home, and maybe 50 miles from the airport starts reducing power. Engine failures that aren’t caused by fuel starvation usually happen when you change something, like reducing or adding power. So maybe carb icing sets in, and kills an engine. Or a throttle linkage breaks, or whatever. So now the guy is on one engine, limping home. He gets near the airport, but finds that he’s misjudged his approach and he’s a little low. So he cranks up the power. Now he’s ready to turn onto final, the power is up on one engine, and he begins a turn and raises the nose a bit, and wham. Airplane starts to yaw over, he pushes the rudder hard, and that slows the airplane more. So he adds power, and the torque pulls him over and gone.
Or, he’s on final and a little low, so he pulls the nose up. Airplane slows down, rudder loses effectiveness, and it begins to yaw. He puts in more rudder, which slows the airplane down, which causes the rudder to lose more effectiveness, and now the airplane is descending and turning, so he applies power, and now the yaw moment overcomes the rudder and pull the airplane right over…
It takes a lot of training to handle engine-out work in a light twin, and having a critical engine makes it a lot worse.
The op asked about contra, not counter, rotating props. Contra rotating is when there are two propellors on the same axis, rotating in opposite directions. They aren’t common because of extra weight of drive gears required and the engineering is not nearly as simple as stacking a left hand prop in front of a right hand one. The Avro Shackelton bomber used then and a few P-51 racers have had that plane’s Griffon engine, gear box and props installed but none have proven as successful as Merlin engines with conventional propellors. The Russian Tu-95 Bear patrol plane has turboprop engines with contra rotating propellors. Aside from that I can’t think of any other examples that saw widespread use.
As for number of blades it tends to be higher with bigger prop diameters. The larger you go the slower RMP has to be to keep tips from reaching transonic speed. There is a larger time interval between blades so each is in air relatively undisturbed by the previous blade. Less common in smaller props for the same reason. At the extreme small end, models which have prop speeds upwards of 28,000 rpm it isn’t unheard of to use single bladed props. It looks funny as hell but with a proper counterweight it works.
Tripler: 100% of the thrust comes from ‘pushing the air back’. Just like the lift from a wing comes from pushing the air down below it. It’s a misconception that suction pulls the airplane up, but it’s such a widespread misconception that it even gets taught in schools.
Airplanes fly because of Newton’s laws. A wing or propeller does work, and the only way work can be done is to move the air mass. In the case of the wing, the airfoil keeps the air attached to the top of the wing as it descends off the trailing edge, and the wing therefore imparts a net downwards vector to the air mass. Plus, the flat-plate lift from the underside of the wing deflecting air down creates lift. That’s why airplanes have vortices that are pushed down behind them. A big jet has vortices so strong they can toss a light aircraft that gets too close around like paper.
Aaaaaaah, and here I thought that the Bernoulli Principle was “strong enough” to provide enough “suction” because of the airflow over the airfoil, that the lift came more from the camber of the wing versus the angle of attack.
Tripler
By all means, please correct me if I’m wrong in my new interpretation.
Camber and angle of attack both play a part. But the function of the camber isn’t to provide a ‘suction’ that pulls the airplane up. The camber is designed to keep the airflow attached to the top surface of the wing long enough for it to be deflected downwards efficiently. A flat plate can create lift both through air striking the bottom surface and forcing the wing up, but also some amount of air is pulled over the top and deflected downwards. But the flow over the top surface goes turbulent at low angles of attack, limiting the amount of lift that can be created. So you streamline the top surface of the wing and design the curvature to create maximum lift for the regime of flight of the aircraft in question.
In my opinion, the ‘suction’ on the top of the wing is more effect than cause. It’s created by the inertia of the air being pushed backwards and being separated from the normal free stream. But it’s the acceleration of an air mass downwards that does the work required to keep the airplane flying.
Right engine clockwise, left engine counterclockwise? Just guessing.
Seems like that’d put yer P-factor outboard of the spinner on either powerplant.
???
Pullin