Why not a Quad Tiltrotor Pusher-Puller?

[Blair Maynard]

I haven't looked much into the future design of the quad tiltrotor, if
anybody else has, please post any info you can. But a quadtiltrotor
seems to be a promising project. I presume the engines in front and
rear would be standard forward-facing props and I guess they would be
offset so that the rear props would be pulling "fresh" air (as opposed
to air which was already pulled by the front props).

My question is why not configure the rear engines as pusher engines?

Did the German Arrow aircraft not show that this was an effective
combination?

Wouldn't that somewhat alleviate the need to offset the engines/props?
It would certainly space the props further apart.

Disadvantages:

I would guess the main obstacle would be the ground configuration of
the aircraft. Since the rear props would be pushers, they would have
to be pointing down for VTOL. How would the landing gear be
positioned so that the A/C would be balanced properly yet not
interfere with the rear props spinning rather close to the ground? I
can't answer this question off hand, but suspect there might be a
solution.

Another disadvantage would be the rear pusher engines could not tilt
much while on the ground. Since the rear props would be dangerously
close to the ground, tilting them a few degrees would put the front
blade tips very close to the surface. So the rear engines could not
help much in a STO. An STL (I don't know if the current tiltrotor the
V-22 Osprey even does this) would be impossible as the bouncing around
and any yaw or roll caused by any variation in ground surface would be
very dangerous with the rear props so close to the surface.

The aircraft would have to be longer than a quad-forward-facing
tiltrotor. Since there is more distance between the front and rear
props, the tail would have to be set further back to keep it out of
the way of the props.

Yes there are lots of practical disadvantages, no doubt many I haven't
listed. But I bet a quad pusher puller tiltrotor would cook in
airplane mode

It would be interesting to see the effects of transitioning from
airplane to vertiplane mode. I wonder if all four engines could
transition at the same time or whether it would be better to rotate
the front two by themselves before starting the rear to rotate. I
would guess all four engines would have to rotate at the same time as
otherwise there would be an imbalance of the vertical lift.

[Bill Kambic]

"Blair Maynard" wrote in message

> My question is why not configure the rear engines as pusher engines?
>
> Did the German Arrow aircraft not show that this was an effective
> combination?

The total number of "pusher" designs that went past the drawing board can be
counted on about one hand. The FACT that nobody ever produced them in
numbers during the Golden Age of Prop Aircraft (a/k/a WWII) should tell us a
lot about their feasibility.

IIRC, pushers seem more efficient but may not be. A prop in front of the
aircraft operates in "still" air; in a pusher config. it operates in the
"disturbed" air that lies aft of the the wing and fuselage. This
significantly compromises propeller efficiency. A second reason was that
props push air over control surfaces, particularly at takeoff power, making
them more effective at lower speeds (and reducing takeoff run distances).
This effect was also significant in landing configuation, IIRC. From Aero
in Pensacola so long ago I seem to remember that thses were at least some of
the reasons that they were not used

A couple of successful designs from the civilian world were the Republic
Seabee and Trecker Gull. Both were amphibians. I have a dim recollection
that the pusher configuration improved water performance, but I can't quite
remember.

Oh, and I guess I shouldn't forget the Wright Flyer!<g>

> Wouldn't that somewhat alleviate the need to offset the engines/props?
> It would certainly space the props further apart.

I dunno. Maybe.

> Disadvantages:
>
> I would guess the main obstacle would be the ground configuration of
> the aircraft. Since the rear props would be pushers, they would have
> to be pointing down for VTOL. How would the landing gear be
> positioned so that the A/C would be balanced properly yet not
> interfere with the rear props spinning rather close to the ground? I
> can't answer this question off hand, but suspect there might be a
> solution.
>
> Another disadvantage would be the rear pusher engines could not tilt
> much while on the ground. Since the rear props would be dangerously
> close to the ground, tilting them a few degrees would put the front
> blade tips very close to the surface. So the rear engines could not
> help much in a STO. An STL (I don't know if the current tiltrotor the
> V-22 Osprey even does this) would be impossible as the bouncing around
> and any yaw or roll caused by any variation in ground surface would be
> very dangerous with the rear props so close to the surface.
>
> The aircraft would have to be longer than a quad-forward-facing
> tiltrotor. Since there is more distance between the front and rear
> props, the tail would have to be set further back to keep it out of
> the way of the props.
>
> Yes there are lots of practical disadvantages, no doubt many I haven't
> listed. But I bet a quad pusher puller tiltrotor would cook in
> airplane mode
>
> It would be interesting to see the effects of transitioning from
> airplane to vertiplane mode. I wonder if all four engines could
> transition at the same time or whether it would be better to rotate
> the front two by themselves before starting the rear to rotate. I
> would guess all four engines would have to rotate at the same time as
> otherwise there would be an imbalance of the vertical lift.

Sounds to me like a pretty complicated system. Complexity is spelled
m-o-n-e-y. Costs more to build. Costs more to maintain. Lots more to go
wrong (and give the Safety Officer even more gray hairs).

Personally, I would go with a tri-motor configuration, using three Pegasus
engines (two pod mounted on the wings a la the A3/B66 and one in the
fuselage). The cost savings in gear boxes and engineering and maintenance
would probably more than offset the higher initial cost of the the engines
with their higher fuel consumption.

Bill Kambic

If, by any act, error, or omission, I have, intentionally or
unintentionally, displayed any breedist, disciplinist, sexist, racist,
culturalist, nationalist, regionalist, localist, ageist, lookist, ableist,
sizeist, speciesist, intellectualist, socioeconomicist, ethnocentrist,
phallocentrist, heteropatriarchalist, or other violation of the rules of
political correctness, known or unknown, I am not sorry and I encourage you
to get over it.

[Keith Willshaw]

"Bill Kambic" > wrote in message
...
> "Blair Maynard" wrote in message
>
> > My question is why not configure the rear engines as pusher engines?
> >
> > Did the German Arrow aircraft not show that this was an effective
> > combination?
>
> The total number of "pusher" designs that went past the drawing board can
be
> counted on about one hand. The FACT that nobody ever produced them in
> numbers during the Golden Age of Prop Aircraft (a/k/a WWII) should tell us
a
> lot about their feasibility.
>

There are a few more than that

In no particular order the Vickers FB5, DeHavilland DH-2 , Convair B-36 ,
Beechcraft Model 2400, Nomad II & III and Supermarine Walrus come to mind

There are problems with airflow stability for a pusher design but I suspect
the biggest problem was pilot survivability, especially in single engine
designs.

Bailing out with a prop behind you is hardly a nice idea, as I recall they
tried various fixes on the DO-335 including explosively blowing off the
prop and fitting ejector seats.

Keith

[The Raven]

"Blair Maynard" > wrote in message
...
> I haven't looked much into the future design of the quad tiltrotor, if
> anybody else has, please post any info you can.

A variant of the Osprey has been proposed with two sets of rotors. Consider
it a CH-46 Osprey.......

> But a quadtiltrotor
> seems to be a promising project. I presume the engines in front and
> rear would be standard forward-facing props and I guess they would be
> offset so that the rear props would be pulling "fresh" air (as opposed
> to air which was already pulled by the front props).
>
> My question is why not configure the rear engines as pusher engines?

Because you'd potentially be swinging the prop arc towards the loading ramp.
Yes, a good design would minimise the possibility of resultant danger but it
would be easier to avoid it in the first place by sticking with the same
method as the Osprey. Of course, you then simplify design and support by
utilising interchangeable assemblies etc rather than having "pusher" and
"puller" variants of the engine/pivot/prop assemblies.

> Did the German Arrow aircraft not show that this was an effective
> combination?

Not really, it wasn't a tilt rotor and didn't have to contend with a loading
ramp at the rear.

>
> Wouldn't that somewhat alleviate the need to offset the engines/props?
> It would certainly space the props further apart.
>
> Disadvantages:
>
> I would guess the main obstacle would be the ground configuration of
> the aircraft. Since the rear props would be pushers, they would have
> to be pointing down for VTOL.

Why? You could have them pivot upwards.

> How would the landing gear be
> positioned so that the A/C would be balanced properly yet not
> interfere with the rear props spinning rather close to the ground?

Have the rear rotors pivot upwards.

> I
> can't answer this question off hand, but suspect there might be a
> solution.

Have the rear rotors pivot upwards.

> Another disadvantage would be the rear pusher engines could not tilt
> much while on the ground. Since the rear props would be dangerously
> close to the ground, tilting them a few degrees would put the front
> blade tips very close to the surface. So the rear engines could not
> help much in a STO. An STL (I don't know if the current tiltrotor the
> V-22 Osprey even does this) would be impossible as the bouncing around
> and any yaw or roll caused by any variation in ground surface would be
> very dangerous with the rear props so close to the surface.

Have the rear rotors pivot upwards.

> The aircraft would have to be longer than a quad-forward-facing
> tiltrotor. Since there is more distance between the front and rear
> props, the tail would have to be set further back to keep it out of
> the way of the props.

No, because the rotors would be further apart if you had a front pull, rear
push configuration. You put the rear rotors rearmost on the engine nacelles.

>
> Yes there are lots of practical disadvantages, no doubt many I haven't
> listed. But I bet a quad pusher puller tiltrotor would cook in
> airplane mode

No idea. I haven't seen anything beyond some promotional advertising of the
4 tilt rotor concept.

> It would be interesting to see the effects of transitioning from
> airplane to vertiplane mode. I wonder if all four engines could
> transition at the same time or whether it would be better to rotate
> the front two by themselves before starting the rear to rotate.

I suspect you'd need to transition all simultaneously, or close to it. You
would have one end at the extreme opposite of the other...

> I
> would guess all four engines would have to rotate at the same time as
> otherwise there would be an imbalance of the vertical lift.

Yup, unless that "imbalance" was used for forward motion or tilting the
airframe.

The Raven

[The Raven]

"Blair Maynard" > wrote in message
...
> On Wed, 2 Jul 2003 09:16:51 +1000, "The Raven" >
> wrote:
>
> >"Blair Maynard" > wrote in message
> ...
> >> I haven't looked much into the future design of the quad tiltrotor, if
> >> anybody else has, please post any info you can.
> >
> >A variant of the Osprey has been proposed with two sets of rotors.
Consider
> >it a CH-46 Osprey.......
> >
> >> But a quadtiltrotor
> >> seems to be a promising project. I presume the engines in front and
> >> rear would be standard forward-facing props and I guess they would be
> >> offset so that the rear props would be pulling "fresh" air (as opposed
> >> to air which was already pulled by the front props).
> >>
> >> My question is why not configure the rear engines as pusher engines?
> >
> >Because you'd potentially be swinging the prop arc towards the loading
ramp.
> >Yes, a good design would minimise the possibility of resultant danger but
it
> >would be easier to avoid it in the first place by sticking with the same
> >method as the Osprey. Of course, you then simplify design and support by
> >utilising interchangeable assemblies etc rather than having "pusher" and
> >"puller" variants of the engine/pivot/prop assemblies.
> >
> >> Did the German Arrow aircraft not show that this was an effective
> >> combination?
> >
> >Not really, it wasn't a tilt rotor and didn't have to contend with a
loading
> >ramp at the rear.
> >
> >>
> >> Wouldn't that somewhat alleviate the need to offset the engines/props?
> >> It would certainly space the props further apart.
> >>
> >> Disadvantages:
> >>
> >> I would guess the main obstacle would be the ground configuration of
> >> the aircraft. Since the rear props would be pushers, they would have
> >> to be pointing down for VTOL.
> >
> >Why? You could have them pivot upwards.
>
> I am assuming the rear pusher engines are mounted on the tips of a
> second wing positioned behind the first, probably slightly heigher
> than the first wing. Basically a biplane, but with the top wing set a
> significant distance behind the bottom wing.
>
> The rear engines are pushers, so if they rotated up they would be
> pushing up. This might help the Quad Tiltrotor to arrive at the
> ground, but not in one piece.

Oops, I realised my gaff regarding pusher props and rotating up after
posting it. I see no real benefit in using pushers, the engine is there so
why not stick on the front as a traditional "puller".

>
> Picture an Osprey with pusher props on it (I am not saying that is the
> way it should be, it is just to help visualize it), instead of the
> current puller ones. The engine would be pointing backwards and
> pushing air behind it. Try imagining what would happen if you rotated
> those engines upward. They would push the plane downwards.

Yup, agreed.

> Anyway, I will work on a drawing in a month. Thanks for everybody's
> input.

You should see if you can hunt up pics of the Osprey four rotor concept.
Perhaps that will help.

>
> And yes the rear props would come close to the cargo ramp. That is one
> of the disadvantages.

One wonders if jets could be used, thus eliminating the prop...........Of
course, you'd need a lot of thrust to lift it.

> But I am sure there are a few disadvantages in the current design
> which, i believe, would put rear puller engines partially in the wake
> of the front puller engines. They must lose a lot of power if much of
> the air that is running thru them has just been pulled by the front
> engines.

Stick a normal rotor on the back of an tilt rotor? Give you a hybrid setup.
Hmmmm, sounds more like a Rotodyne every minute.

The Raven