Pusher Vs Tractor configuration

[kmpillai]

Hi friends,
What are all the merits and demerits of pusher over tractor configuration aircrafts?

 

[foamhead]

Very broad question. Are you talking pusher ultralight vs. tractor ultralight or RV vs Velocity or ?

Things that might not be an issue in an ultralight might be a big deal in a 4 place high performance airplane.

Are you looking at buying, building or desiging your own?

What would be major factors to you? These same things might be minor to someone else.

Happy Holidays

Bob Farmer

 

[DoubleGeek]

I'm not prepared to provide ALL merits of the pusher configuration over tractor, but let's start with my favorites and limit the discussion to a single-engine. First of all, the tractor configuration gets demerits for contaminating the fuselage with a noisy, vortex-laden propwash that changes widely with the throttle position. By contrast, in the pusher configuration the propeller is in the fuselage's slipstream, so the air velocity at the leading edge is slightly less uniform than in the tractor configuration due to drag effects from the fuselage. This makes the propeller slightly more effective because it receives relatively slower air and the propwash is unimpeded. And since the propwash is behind the fuselage, The noise experienced by the occupants is also reduced. One last consideration is that additional engineering efforts are usually required to prevent dragging a tail-mounted (pusher) prop during takeoff and landing, since it is mounted on the lowest part of the fuselage during those portions of a flight.

 

[chasbo]

Generally, the tractor prop configuration loses less efficiency with its noted deficiencies than the pusher prop configuration with its noted deficiencies. The adverse impact of the fuselage on the quality of the air entering the pusher blade disk is perhaps best evaluated in the Cessna 337, and is probably the reason we don't see more pushers.

 

[motorglider]

The slipstream issue with tractor vs pusher configuration is less of an issue with low power flight such as motor gliders and self launching gliders. However in these craft the best of both worlds is obtained by an engine mounted above the fuselage on a pylon. This has obvious advantages for water based aircraft too. Something we may all have to consider as airfields disapear in our increasing urban population.

 

[eewings]

A useful general comparison can be found at

http://www.capellakitplanes.com/capella/tvp.htm

A suprising finding for almost all popular pusher aircraft
is that they are 5-6dB noisier than "equivalent" tractor
aircraft. This is claimed to be due to the turbulent flow
into the prop disk. Extended rear fuselage sections,
requiring long driveshafts, are apparently required to
maintain reasonably smooth airflow. A rule of thumb, which
I would appreciate someone verifying, is that the prop disk
must be at least 3 chord lengths behind the main wing to
avoid downwash flow disturbances. No recent pusher designs ( mostly of the Varieze type ) have this characteristic.
All apparently have a characteristically "raspy and beating" prop signature.

Although the noise referred to here is external, it often
translates into increased cockpit noise.

An interesting new design that may improve on these dismal
historical tendencies is the Aeriks-200

http://www.aeriks.com

which has had careful attention paid to aero efficiency and fuselage streamlining.

 

[GregLocock]

Yes, that is a well known result in acoustics. The reason is that the prop acts as an amplifier for small vortices created by the non uniform airflow - the raspiness is the sound of these high energy vortices 'exploding'.

I'd be interested to know if the overall efficiency comes out ahead or not, offsetting lower stream velocity for the skin friction , against the worse airflow over the fan itself.

Cheers

Greg Locock

 

[DenisNA]

Overall efficiency of a pusher prop is alvays lower than that of a tractor one. The additional drag of slipstream is often overestimated. Instead, the slipstream can (and in many cases do) improve the L/D ratio of the plane!
This results from two physical mechanisms: 1. The high-speed slipstream blows over the central wing section and fuselage of a single-engine plane and corrects the spanwise lift distribution, which is always disturbed at this area. 2. The deflected slipstream adda to thelift and requires lower Cl values at all wing sections. This positive effect is most pronounced at relatively low airspeeds where the induced drag is the most important component of the total drag.
The example No1 of the most effective utilisation of this effect is Antonov 2 biplane. The positive effect of slipstream in this graceful flying machine increases the L/D ratio by 14% during climb, and reduces the stall speed to completely unbeleivable 25 knots (the gross weight is 5500kg). Antonov is in the class of her own.

 

[eewings]

At Aero 2003 in Freidrichshafen and at Le Bourget, another
pusher designed by Broussaud Ameur was shown. The company
SC2A, whose website is

http://www.slicom.fr/sc2a

manufacturers this aircraft, called the Altania.
This configuration gets extremely high performance out of a very small Jabiru 2200 80hp engine ( the design family listing is somewhat confusing - there are two 2-place designs, one meeting VLA regs, the other having a higher
useful load, plus a 4-place design with Jabiru 3300 power,
nearing first flight).

The thing to note here is that the engine is in the middle, with an extremely long drive shaft to a prop at the end of a carefully-tapered and small cross-sectioned tail "boom".
This should help airflow streamlining even more than the Aeriks-200 design I mentioned earlier. It is also imperative since the seating arrangement is side-by-side.

Of further interest is that the design is NOT 3LS, but conventional, with good landing/takeoff distances, and there is no prop strike protection...

 

[DenisNA]

The Performance of Altania plane is quite consistent with its small dimensions and is not actually improved by the pusher design. Rather the long drive shaft from the center-mounted engine is nothing more than an unnecessary complication. The clasiic side-by-side tractor plane will be shorter in length and the payload will be placed much closer to the mean CG position. The takeoff and landing performance will be better too.

The pusher prop placed behind the tailplanes is not free from the unwanted influence from the turbulent airflow coming from the wing and other parts of airframe. The propulsive efiiciency will be by 5-10% worse against good tractor configuration. I don't know any plane with similar pusher configuration, which could show any considerable improvement over conventional designs. Moreover, the highly turbulent airflow entering propellor disk in this configuration may result in excessive vibration and subsequent fatigue loads of the prop blades. As far as I know, German aircraft manufacturer Grob even was studying a multiblade prop with elastic blade coupling to the hub to overcome this issue in their business class single-engined plane project of the same configuration. This project seems to be abandoned now.

 

[eewings]

Agreed!

I didn't say that the pusher design of the Altania or the Aeriks-200 is the reason that these designs have good performance - rather that these are two examples of the aircraft designers trying to minimize the adverse affects of the pusher configuration. There are scads of similarly sized and powered tractor configuration designs that have, as you say, equivalent cruise performance and significantly better takeoff and landing performance. The Europa and Fascination D4 come immediately to mind.

In both the Altania and Aeriks-200, the designs
were driven by a desire to improve the passenger's comfort and pilot's visibility by relocating the noisy engine and prop. And, of course, to look sexier and different than a tractor configuration aircraft - otherwise, what would be the point of yet another tractor aircraft with performance similar to all other tractor aircraft?

The sad truth with history's single engine pusher light aircraft is that they haven't delivered on passenger comfort - often, cockpit noise in particular is worse - while adding uneeded complexity, worsening reliability,
and degrading takeoff/landing performance.

So, to date, the only pusher configuration which has delivered on claims of efficiency and passenger comfort is the Piaggio Avanti. And it is a cabin-class twin.

NASA is pursuing a "new thinking" light/personal aircraft design, with similar goals of passenger comfort AND exploitation of high-volume system component cost savings (i.e., derated auto engines). Their contention is that to do this, aircraft have to move from a "performance-optimizing" design flow to a "passenger-optimizing" flow. The tractor configuration is the best "performance-optimizing" design - but the NASA "new thoughts" are auto-engine driven pushers.

Hope springs eternal.

 

[DenisNA]

I have serious doubts if the pusher configuration of Piaggio Avanti can improve performance. The blades of propellors positioned just behind of the trailing edge encounter powerful strikes of the wake trace during each revolution. this definitely does not enhance efiiciency/ Moreove, if the prop shaft is positioned too close to the wing chords plane, these strikes may actually destroy the blades. And the noise produced bu such props resembles machine gun! This effect was noticed, for instance, in B-36 bomber. Needless to say, the positive effects of wing surface blowing are not exploited. The three-surface aerodynamic configuration of this plane is also meaningless from the viewpoint of balancing drag reduction. Those who invented this layout thought that the negative lift of the conventional tailplane necessary to trim the aircraft increases the induced drag. In fact it is possible even to recover the part of energy going into the induced slipstream of the wing by means of the properly placed conventional tailplane. By other words, the interference between the wing and stabiliser can be constructive, and the designer can optimise the layout in such a way to make this constructive interference maximal at the range of cruise angles of attack.
The only advantage of the Avanti configuration is the possibility to place the passenger cabin in front of the wing. Indeed, this advantage is also doubtful, because the fuselage wiil be longer and the payload will be placed too far from the mean C of G position. This drawback can be judged relatively insignificant only if the most part of the useful load is the fuel.
I also agree about NASA "new concept". The development of auto engines went into absolutely different direction as compared with the air-cooled piston aero engines. In fact the stable concept of the latter during last 40 years simply means that they reached such point of perfection that their further improvement becomes more and more difficult. This considers combination of fuel economy, reliability and specific weight. The use of other fuel than leaded 100-octane avgas remains indeed a challenge, but I dont think the use of automotive technologies can help here.

 

[eewings]

We should be careful of veering off into a topic that's part of another thread ( three-surface aircraft in aerodynamic engineering ).

The Avanti is a far more successful pusher design than the ill-fated Starship. However, the Avanti is not efficient because it is a pusher, but because it is one of the most aerodynamically optimized designs to come out in the last 20 years. The pusher configuration resulted from the desire, again, to improve passenger comfort by removing the props from the vicinity of the cabin. This is a major reason why jets have dominated the executive aircraft market for years, even though turboprops are more efficient and a better match to this market's typical mission profile - turboprop cabins are noisier. I won't get into the sexiness and perceived modernity of the jet.

It's interesting to speculate what would happen if someone would optimize a twin tractor to the same level as the Avanti. But who would buy it?

In any case - the original topic was the pros and cons of pushers and tractors. Aerodynamically, we've hashed that out pretty thoroughly. However, an aircraft's configuration is based on its mission. Sometimes that dictates unusual arrangements. If the mission isn't well thought out, the arrangement doesn't last.

 

[DenisNA]

A thoroughly optimized twin tractor can be fantastic. The cruise speed will be directly comparable with that of jet, but the taktoff and landing distances will be second only to helicopter. What could one say about cruise at 400 knots and stall at 50(IF THIS IS STALL)?
At least one plane with such proven capabilities is already flying, Antonov 70. Indeed, this is a 130 tonne military (maybe civil) transport powered by four propfans. One could ever imagine the results of implementing such technology in a far smaller corporative plane. The very same people buy business jets and helicopters. Such advanced turboprop could help them save money and, possibly, lives too.