Engine types and effects.

[Tzoli]

Hello

I wish to know how the engine type and placement of it effects an aircraft?
I'm thinking in a civil sports plane and a fighter with these arrangements:

a, 1 engine front
b, 2 engines front (like the P-38 Lightning or any twin engine aircraft)
c, 1 engine aft, pusher configuration
d, 2 engines aft, pusher configuration (were there any such design anyway?)
e, 2 engines, front and aft, pull-push configuration
f, 1 engine front, contra-rotating propellers
g, 1 engine aft, pusher configuration with contra-rotating propellers
h, 2 engines, front and aft, pull-push configuration with contra-rotating propellers

What are the pros and contras of these? Like performance, manoeuvrability, maintenance, everything you could think of.

 

[Nik]

Consider the torque and engine-out behaviour. Also, is the sports plane acrobatic, 'hang on the prop' rated ??

 

[Tzoli]

Not an acrobatic just a simple one which you could fly around in your free time like the cessna.

 

[DWG]

Extra options:

2 separate engines driving contra-rotating propellers at either front or rear, examples Fairey Strike Fighter, XB-42 Mixmaster.

2 engines in a single crankcase with separate driveshafts and a combining gearbox, example Napier Double Naiad and AS Double Mamba in the Gannet and the Blackburn B88.

The different combinations reflect several different needs:

HP: What HP do you need for the desired performance? How many engines do you need to provide that?
Propellers: how big does the propeller need to be to turn that HP into thrust? How physically big can it be? For twins, How sensitive is the aircraft to one engine out performance? Does it need handed props, contraprops, all power on the centreline?

>> d, 2 engines aft, pusher configuration (were there any such design anyway?) <<

Mixmaster, Beech Starship

 

[Tzoli]

But what are the pros or contras? Like one having smaller fuel consumption but higher maintenance or longer range but slower speed etc.
Like why an engine with contra rotating propellers better than a normal one? If so why there aren't more of these designs?
Same with engine placement, front, aft both?
I wish to know these points.

 

[DWG]

Like why an engine with contra rotating propellers better than a normal one? If so why there aren't more of these designs?

There is no single answer, certainly no simple one. Every design decision has to be weighed against every other design decision. What is right in one aircraft may be utterly wrong in the next.

The primary reason you might choose a contra-prop is horsepower. The more horsepower you need to force through the prop, the bigger it needs to be, the more blades you need to consider. If the size of the blades grows to the point you cannot physically accommodate it within the aircraft because of ground clearance or physical clearance with the aircraft, then having more, but smaller blades, in a contraprop may solve that. But there are drawbacks: a more complex hub, extra gearbox stages, new failure modes unique to contraprops.

Or the issue might be having two engines to deliver the power from through a single axis. But similar drawbacks.

Or the issue might be countering the torque from a powerful engine without needing to increase the keel area of the fuselage. Again with the drawbacks.

Or it might be sensitivity to the single engine out thrust-assymmetry of a twin. Drawbacks again.

Every design decision is a compromise.

 

[Tzoli]

Interesting.
So for example an 2000hp engine rotating a 4 blade propeller roughly equals an 1000hp one with 2 pair of propellers contra rotating?

Also how effect an aircraft of engine placement? Like a Pull, Push-Pull or Pusher configuration? (front, front-aft, aft)

 

[sagallacci]

"So for example an 2000hp engine rotating a 4 blade propeller roughly equals an 1000hp one with 2 pair of propellers contra rotating?"
Absolutely not. Contra props don't make more power, rather if you have more power, they can deliver it. So, as you add more power, you either add more or larger propeller blades to the point where there is no room for them, or do contra props. In your example, the 1,000 hp engine with contra props simply doesn't have the power to compare with the 2,000 + four blade arrangement. A clearer example would be having a 1,000 + three blade versus 2,000 + two- three blade contra props. Twice the power with twice the propellers.

Regarding pushers, most pusher designs were done for reasons other than performance, usually to have the nose clear for weapons and/or better pilot view. If anything, pushers can have complications, like no direct blowing of air for engine cooling or on to control or lifting surfaces. On the other hand, pushers can have slightly more efficent thrust, as the Do335 could fly a bit faster on just the rear engine instead of just the front engine. As for push/pull arrangements, the drag and structural weights of a normal twin engine type are reduced with having both engines in the one fuselage. For example, though about the same weights and engines, the Do3335 was nearly 100mph faster that the Me410.

 

[frank]

The other, bigger issue with contra props is the elimination of torque. Torque from very high HP engines can make control at slow airspeeds difficult or maybe even impossible. Torque also affects the trim (aerodynamics) of an aircraft as well.

"So for example an 2000hp engine rotating a 4 blade propeller roughly equals an 1000hp one with 2 pair of propellers contra rotating?"
Absolutely not. Contra props don't make more power, rather if you have more power, they can deliver it. So, as you add more power, you either add more or larger propeller blades to the point where there is no room for them, or do contra props. In your example, the 1,000 hp engine with contra props simply doesn't have the power to compare with the 2,000 + four blade arrangement. A clearer example would be having a 1,000 + three blade versus 2,000 + two- three blade contra props. Twice the power with twice the propellers.

Regarding pushers, most pusher designs were done for reasons other than performance, usually to have the nose clear for weapons and/or better pilot view. If anything, pushers can have complications, like no direct blowing of air for engine cooling or on to control or lifting surfaces. On the other hand, pushers can have slightly more efficent thrust, as the Do335 could fly a bit faster on just the rear engine instead of just the front engine. As for push/pull arrangements, the drag and structural weights of a normal twin engine type are reduced with having both engines in the one fuselage. For example, though about the same weights and engines, the Do3335 was nearly 100mph faster that the Me410.

 

[brutus]

Like why an engine with contra rotating propellers better than a normal one?

In addition to the reasons already stated, as engines became more powerful, designers fitted larger propellers to handle the increased power. The problem that eventually arose was that as the speed of the propeller tips approached the speed of sound, they became less efficient. Designers could then 1) increase propeller size further, making them turn more slowly (with at best no gain or a small loss in efficiency); 2) add more blades (probably increasing mechanical complexity); 3) adopt contraprops.

If so why there aren't more of these designs?

As mentioned previously, the drawbacks include greater weight and complexity. The gearbox of a contra prop could potentially be more difficult to repair in the field. And in the case of US designs that tried to employ contra props there was a tendency to experience problems with serious vibrations (the Curtiss XF14C comes to mind for this problem).