A520
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Why So Few Propulsive Propeller Plane?

Thu Mar 01, 2007 7:23 pm

For what I understand, propulsive propeller aircraft have two advantages:

1. the propeller wash doesn't interfere with other parts (wings, tail, ...) so the lift surface have a better, more laminar flow.

2. the ride is smoother for the passangers (same reason as above) and quiter (noise generated further away and at the back), + better visibility (case of single engine aircraft).

Why then so few designs of this type are succesful? The Beechcraft 2000 Starship (twin) was a complete failulre, as was the Cirrus VK-30 (single). Why don't we see propulsive private aircraft, or Bombardier or ATR commuters?

Thanks for your thoughts.
 
BAE146QT
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RE: Why So Few Propulsive Propeller Plane?

Thu Mar 01, 2007 9:07 pm

The "pusher" configuration isn't very common, no. There's a few reasons for it;

Having the propwash go over the wings is (I believe) more efficient because it generates additional lift. It also helps in situations where a stall might occur, because again, it increases airflow over the wings. Not forgetting that it also helps give rudder authority at low speed on the ground.

Additionally, the big fan at the front helps with engine cooling.

And to add to all that, you have to be very careful with component placement. The B36 had terrible trouble with carb icing because the engines were mounted "backwards" - the carb got no warmth because it was in front of the cylinders. So if you were to design a pusher plane, you would need to design (or heavily modify) engines to go with it. Economically, it's probably not worth it when you consider the other drawbacks.
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Starlionblue
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RE: Why So Few Propulsive Propeller Plane?

Thu Mar 01, 2007 9:56 pm

The Piaggio Avanti was somewhat succesful.

So was the Pfeil, but that one had both pushed and puller.


Apart from the reasons outlined by BAe146QT, it is simply tougher to place pushers. On singles you have to find a place in the ass of the plane. With wing mounteds and singles you have to start worrying about prop strikes, meaning a possible increase in landing gear height.
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pygmalion
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RE: Why So Few Propulsive Propeller Plane?

Thu Mar 01, 2007 11:38 pm

the other big reason is that you are now putting the propeller in the wake of the airplane parts. Propellers are complex and do much better in clean smooth air.
 
bond007
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RE: Why So Few Propulsive Propeller Plane?

Thu Mar 01, 2007 11:46 pm

Quoting Starlionblue (Reply 2):
The Piaggio Avanti was somewhat succesful.

Actually, it continues to be quite successful. They are ramping up production, and are backlogged for the next couple of years.

It also is the fastest turboprop aircraft in production I believe (for certified pax ops anyway).

Last year they sold around 20% the number of King Airs sold (C90/B200/B350). Not bad at all.

Jimbo
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KELPkid
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 1:16 am

In piston engine installations, the primary challenge to pusher installations tends to be adequate cooling air...ask any Cessna 337 owner which engine makes it's TBO time  Wink

Also, prop planes with a pusher tend to not "feel" right...much of a prop plane's feel that us pilots are accustomed to comes from the effects of propwash on the tail surfaces.
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EridanMan
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 6:40 am

The Beechcraft starship was a failure for many reasons, the pusher prop configuration was only one of its downfalls... (Dealing with an untested design on such a large scale, with new constructions materials and techniques to boot... The engineers simply bit off more than they could chew.. They were able to make the innovations work, but not work particularly efficiently.

The biggest nightmare for pusher prop configurations is keeping the fan spinning in relatively wake-free air (the fuselage wake causes massive variations in blade AOA, increasing noise and decreasing efficiency tremendously). This can either be done through painstaking streamlining of the aircraft body, or in the case of the Avanti, simply moving the prop discs out from behind the fuselage to where they only have to deal with the wake of a thin laminar wing (and extraordinarily thin engine nacelles, something not possible with piston engines). Without proper control of the propeller intake flow, however, any and all efficiency gains from freeing the aircraft fuselage from the prop slipstream are quickly erased.

I read somewhere that during initial testing, at best climb configuration, the original starship had issues where, literally, portions of the prop blades would be completely stalled while others would be essentially feathered, simply because of the relative airflow around the cabin and wing... not a good situation.

For tractor configurations, knowledge that the fuselage will sit in the prop-blast can be utilized by the aircraft designers as well... drag-inducing variables such as control and stabilization surface sizing and location all must be optimized for the lowest-speed regions of the flight envelope (where control effects are marginalized by low flow rate, and thus large areas are needed). The prop-blast artificially speeds up this low-speed flow, allowing for smaller control/stabilizer surfaces within its wake, and leading to lower net drag if properly designed... (To the obvious limit that control must still be retained during a power failure, but having the surfaces directly behind a powered stream certainly enhances the 'control feel' of the aircraft down near its lower performance limits in a correctly designed aircraft, something a pusher is not capable of).
 
N231YE
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 8:18 am

I wonder why the Embrear CBA-123 failed...


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Same reasons as above (like the BE-2000)?
 
EridanMan
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 9:11 am

According to Wikipedia (so take it for what its worth), it was done in by political issues...

I have never heard of the type before today though, so I don't know Wink

Its fun to compare that ship to the Avanti though... 90 knots slower with 800 more shp (granted, a little less than twice the useful load...)...

it really lets you see what the Avanti designers were thinking when they laid the ship out... (I have a bit of an obsession with the Avanti, I think its a genius design).
 
L-188
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 2:14 pm

As has been mentioned already propellers are most efficent when presented with clean air. So when it has to take a bite out of turbulent air from the wake of the aircraft it's won't be as efficent. I don't recal the exact number but supposedly the rear engine installation of the Cessna 337 produced 2/3's of the power that the front engine did on the same HP.

And as been mentioned here haveing the prop in front also provides a convient airflow for cooling the engine.
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KELPkid
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RE: Why So Few Propulsive Propeller Plane?

Fri Mar 02, 2007 3:06 pm

Quoting L-188 (Reply 9):
I don't recal the exact number but supposedly the rear engine installation of the Cessna 337 produced 2/3's of the power that the front engine did on the same HP.

IIRC, the single engine climb rate on the front is around 200 FPM, while it is 500 FPM on the rear...I've heard more than once that the front is the engine you want to loose in the Mixmaster, however it rarely happens that way (due to the ground cooling problems on the rear...).
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