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Why is it better to have propellers on the front of airplanes but on the rear of submarines? I presume it has something to do with airplanes being heavier than air while submarines are lighter than water since zeppelins also had propellers more towards the rear.
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What determines how many blades to have on an airplane propeller? One would think that there would be a certain optimum number. Most planes have only two blades – which is intuitively too few I would say – but on occasion you see some with three or four blades.
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It’s mostly to do with fluid density. If the prop of a sub was in the front, there would be too much eddy induced drag. There has been planes designed with rear mounted props, but they’re quite rare. [Aside: Hasn’t this question already been answered?]
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Yeap, there’s a trade off. To get the least amount of drag you would use a single bladed propeller - I’ve seen a picture of a model aircraft with a counterweight on the other side of the shaft to balance it. However, not only is it lopsided, you’re not getting much air being pushed per revolution. You’re weighting up drag (and hence power consumption) verses volume of air flow per revolution. Other considerations include vibration and noise. More details on marine propellers here.
Cool. Thanks.
Good guess, but nope.
I originally started in airplanes with “pusher props”. They actually have the advantage of improving airflow over the tail control surfaces. They also have disadvantages as well.
Propellors can also appear on the sides of airplanes - multiengine planes most frequently have the engines/props on the wings, roughly midway between the front and rear of the fuselage.
“Pullers” had some marked advantages in single engine airplanes that lead to their widespread adoption fairly early in aviation, but “pushers” are still designed and built.
If it works - that is, gets you off the ground - I’d say it’s not “too few”.
Supposedly, more-than-two bladed props are quieter - some European countries, I’m told, mandated them in microlights for that reason. But it complicates the problem of properly balancing the blades. It is very important that props be balanced - unbalanced props cause vibration. In the most severe cases, it can shake and engine lose from an engine, which is a Bad Thing.
There is at least one aircraft in current production that has a single-bladed prop: the Silent-IN motorglider. To see it, follow this link, scroll partway down the page to the section marked “Silent-IN”, and click on the upper righthand picture.
A prop on an airplane creates a low pressure in front of the plane that essentially helps to PULL the plane through the air. This is why propellors have the same shape as an airplanes wing. This is also why a normal front mounted “puller” prop tends to be more efficient than the rear facing “pusher” props. The best (efficient) planes that have rear pusher props often also have a front prop also.
the density of water while not negating the effect requires much higher speeds and water displacement than is necessary to propel a boat or submarine simply because the propulsion is not needed to provide “lift” i.e. bouyancy. If it were necessary to create this type of effect boat props would be thinner, longer and spin faster just as they do in an aircraft. and would also benefit by being mounted in the front.
This appears to be advancing the discredited “Bernoulli” explanation for what a propellor is doing. FMI, see the current thread on inverted flight.
Ground clearance is an important factor too; in general, a large propeller is better than a short one, just like the long, slender wings of a sailplane are more efficient than the stubby wings of jet-fighters; however the plane has to take off and a propeller bitting the runway is quite bad indeed; so the propeller is as large as the clearance allows, and if it still can´t absorb the engine horsepower, more blades are added.
One important thing about propellers is that the tips should not break the speed of sound, a supersonic propeller makes a lot of noise, in some experimental airplanes with supersonic propellers, ground crews phisically suffered the effects of such intense noise, not just in the ears, in their whole bodies. So instead of making a small propeller spin faster the solution is to add more blades.
For the reasons I explained above, a multi-blade propeller, compared to a two-blade one produces the same thrust but at lower RPMs; although it´s, all other things equal, less efficient.
A couple of other points:
- Maneuverability. An airplane uses control surfaces to maneuver, as does a sub. However, the speed needed for effective use of the controls is completely different because of the density of the fluid being moved through. The maximum speed for a sub (35-40 knots?) is about the absolute minimum speed in an airplane to get any semblance of control. Different speeds mean different propulsion methods; an airplane needs speed before anything else so the prop is often in front in undisturbed air. A sub needs SOME speed, but nothing horrendous.
Also, with the added density of water and the slower speeds, it makes sense to place the control surfaces aft of the props. Spin the “screws”, send water over your rudder and you are controllable at speeds around 2 knots.
- Stealth. Subs are designed to be quiet. Sticking a big screw in front of a sub would wreak havoc with it’s acoustic signature. A sub with neutral bouyancy can shut off it’s screws and coast until it stops, remaining at the same depth. A screw in front would cause all kinds of turbulence and noise while the thing slowed down. An airplane, OTOH, needs it prop turning ALL THE TIME, otherwise it isn’t flying (well…it will until it either lands or stalls).
On the prop-blade issue, designers are still arguing about this. One of the first airplanes I flew had three-bladed props. This was to increase short-field performance. (Link to a C-27 picture taken by me!) Three blades are good, but the C-130 uses 4 blades. And the new C-130J and C-27J use six-bladed props. (Link to the C-27J with the new-fangled props). As composites are used more in propellers I’d expect to see more exotic designs like this show up.
As has been pointed out, there are planes with the propellors on the rear.
I’m going to advance a hypothesis that has nothing to do with aerodynamics. Airplane engines are mostly air-cooled, and if the prop is on front, it serves double duty as a cooling fan.
Oh, for heaven’s sake!
First, it’s propellEr, not propellOr. Actually, there are some out there who claim that, in the case of airplanes, they should be called “airscrews.” Whatever…
Second, and please let’s get this one straight, an airplane’s propeller creates THRUST. The thrust is applied (one would hope) parallel to the line of flight. Whether it “pushes” or “pulls” is relevant only if you’re considering the mechanical effect on a particular part of the airframe. Whether the overall configuration is “pusher” or “puller,” the effect and the physics are precisely the same.
While it’s true that the majority of prop-driven planes are “pullers,” there is seldom an overriding reason for that configuration, other than simple practicality. It’s generally easier to design and build and airplane with the engine and prop in front than otherwise, in terms of balance, etc. On the other hand, there are some quite well-known examples of “pushers” that come to mind… The original Wright Flyer and most of the subsequent Wright designs were “pushers.” The Boeing B-36 was equipped with four “pushers” and was the backbone of the infant Strategic Air Command in the late 1940’s and early 1950’s. Also, wasn’t Rutan’s “around-the-world-on-a-tank-of-gas” plane a “pusher” design?
It was a pushmepullyou design with a propeller in each end of the fuselage.
Merriam-Webster lists both spellings.
In a perfect world it goes something like this:
Single blade propeller.
Two blades.
Three blades. and so on. Even in Water!!!
who wants to deal with a 100 foot diameter propeller under water?
Real world constraints and objectives, clearance, noise, structural complexity, tip speed, et al…
Amphibians have their engines up high for one reason only, water clearance. It is really necessary and causes many problems in design and flying performance and control difficulties. But it is really necessary.*:: rinse - repeat many times:: *
A two bladed skinny propeller that could soak up 1-4 thousand HP engines would be way to long and have way to high a tip speed and etc…
YMMV
The South African Defence Force converted old WWII Shackleton bombers for long range patrol flights around our very long coast line. These aircraft had 4 engines, 2 props per engine (both in front of the engine), with the one prop spinning counter to the other. What would be the advantages/reasoning behind this?
BTW, the aircraft are all retired out of service now, I think there’s 1 being used as a “museum flight”. They have not been able to replace these old workhorses with anything remotely as efficient or with the same long range capabilities.
Could be for several reasons.
Had the engines been changed and were more powerful and needed more prop to harness the power.?
On single engine aircraft, they also use up more HP with a smaller overall diameter but remove the “P” factor on takeoff and allow the engine to be pointed straight ahead. Most single engine aircraft have the engine pointed to the right and down a bit. In this country where clockwise engine rotation ( from the cockpit ) is the norm.
Also allows a slower and therefore quieter aircraft.
And could be for a reason I have not remembered or never knew.
YMMV
Airplane with 56 (!) total prop blades:
http://www.aeronautics.ru/an70.htm?clkd=iwm
4 engines, each driving 14 counter-rotating blades (8 in front, 6 in rear)
Brian
I hate to be nitpicky… no wait, I don´t… 
anyway; the B-36 had six engines not four; and later variants added another 4 jet engines to the equation; I guess it was quite a handful for the maintainance crew!
The main reason to add counter rotating propellers is to cancel torque, the propeller spins in one direction and as a reaction the airplane tends to spin in the opposite. Also, it´s a real pain in the arse when I´m trimming my model planes 
As for the Shackleton, I remember that the engine mounts didn´t cope well with the torsion of the structure caused by the torque. Though I don´t remeber if that was on the IC engine nacelles or the ones that incorporated jet engines too (the outer pair)
In any case, the norm is that counter-rotating props make a whole lot more noise than single propellers, the rear blades slam headlong into the wake of the front propellers. Actually, any object near a propeller will increase the noise, it´s easy to test it with a fan, turn on the fan and put your hand over the mesh cover, you´ll hear a distinctive pulsating, low-pitch noise. Multiply that by the upteen power and you get the picture.
We’ve done a lot of this before:
For airplanes, a front-prop design puts the prop in ambient air. A rear-prop design has to deal with the wake of the fuselage, which may make the prop much less efficient. For subs and other boats/ships, pilot141 mentioned one important reason: the prop drives water over the control surface which makes the controls much more effective.
As for the number of blades, there are several factors in the design. From a construction standpoint, you’d like to make fewer blades since the cost/blade is likely to be the similar regardless of their length, more blades equals higher cost. From a fluid dynamics standpoint, it is very important to keep tip speed below the speed of sound and, in water, to keep the low pressure region on each blade above the cavitation pressure. This argues for more smaller/slower blades rather than a few large/fast blades. You also would like the leading edge of each blade to encounter ambient fluid rather than riding in the wake of the blade ahead of it. This considers both revolution speed and forward speed because if the revolutions are too high and/or forward speed too low, you don’t get good undisturbed flow on the leading edge of each blade. There are a lot of other design considerations like mechanical balance and ground clearance of the vehicle. Design is a compromise dictated by specific situations, not a “best” overall standard.