Great concept, I would not have a T tail, I can see from the drawings
that you are trying to let the rudder and elevator run in as clean air
as possible.that has obvious advantages, but a couple of
disadvantages,
one of them is structural integrity, and just by eyeballing the big
elevator and rudder surfaces in this design, and then looking at the
size of the body between the two propellers....thats an awfully thin
stucture, that will hold that T tail.

One of the disadvantages with letting the rudder surfaces run in clean
air is less rudder authority, thus the need for increasing the
surface areas.

Having that little structure on the boly at the location of the
propeller
arc....., in this case I would suggest to go the other way. make an
ordinary elevator and rudder, and let it ride in the propeller wash.

You will get higher drag, as the airspeed will be higher in the
propwash
, but instead you can go down in size of the surfaces, so the drag
will
end up being the same.

Second advantage of this is rudder authority,  You can get it down to
standstill, and that is something that can come in really handy come
crosswinds.

Some designs did a point of getting as much rudder authority as
possible, and especially propeller twins or 4 engine aircrafts, had
their rudders at the end of the elevator, , so they looked like an H ,
that is actually not a bad idea.

One engine stops, and you are coming in on landing with one engine
running, are getting on the slow side, and have strong crosswinds,
those
big huge surfces  sitting in the air ( that you need to get rudder
autority in free air), are now losing authority more and more, but are
instead catching more and more of the crosswinds.

If you are trying to minimize adverse jaw at a one engine loss, look
at
ME110, look at B-24.... Big rudder autority on small surfaces, right
in
the midst of the propellerblast.

With those long slender wings, and slick design with a good
powerpackage, you can do some serious altitude with this thing,
however
your propeller diameter, and high altitude capability are in conflict,
with that wing arrangement and with that power, you can probably go as
high as when you see the sky blacken, and see star constellations in
daytime.

That will make for some serious speed and distances.

There is a lot to propeller science, but as a very broad generality,
big
slow blades are more effective than small speedy things.

This may... and I say may be a concept to pursue.

Of all engines, none fit the bill better than a rotatry engine to do
this. ( that I know of , up to date) . To connect two engines, through
the crankshaft is considered very hard.  First  because of a balance
aspect, most other type of the few engines that can do it,  because of
that, needs to line up in a certain way, or be an industrial heavy
type
with a lot of cylinders to make this possible,second, there are not
many
type engines that because of a long line of technical reasons are
suited
to be connected in series. In some very big earhmoving equipment and
big
huge industrial Catepillar applications this is done.

Having not done it, but seing what I see, I would say that the
prospect
of using two rotatys, and connect them, where one will run through the
crankshaft  (or center shaft) is very high. Because of it's perfect
balance, there would not be any need for synchronisation gears, to
make
it connect at a certain position....most probably the only thing
needed
would be a simple clutch.

The power transfer in a rotary does not go through a crankshaft that
looks like a snake, it's a straight shot.

You would use the rear engine (of you use a tractor type
configuration)
in takeoff and climb and fast cruise, stop and  disconnect the rear
engine at economy long distance cruise. In case of an engine failure,
you would use the other engine. If the rear fails, you just disengage
the clutch, and run on the front. If the front fails, you keep the
clutch engage, and run both engines on the rear engine, you will run
the
prop via a dead engine in that case, so lubrication to the front
engine
needs to be maintained.

Connect and disconnect procedures would need to be worked out, but
as a
dry clutch is pretty forgiving in speed differential, just two dials
with RPM for each engine, match the RPM, and engage..will probably be
enough.

If that could be done, you could get rid of the two motor gondolas on
the wing, getting one straight push/pull vector and don't worry about
any adverse yaw at engine loss. you will get power from the other
engine
still, and make a tractor or pusher, two engines, one big huge
propeller, one gearbox.

Because of the huge power diffeneces you will get, you can not go
with a
fixed prop in this application, the prop has to cover a much broader
powerband

You start and climb on two engines, then get a looooong range fast
high
flying aircraft sipping fuel on one engine.

You most probably will get an intercontinental aircraft in this way,
perhaps even the Pacific. You may even be able to take on so much fuel
that you have to wait for a while to be able to maintain altitude on
one
engine.....oh man..... you will get long legs.

If the engines can be done this way, you will save a lot of
aerodynamic
worries.

The worries you will get with retracting long gearlegs...in return for
what you get....I'll take it.

Not all aircraft styles will lend itself to this engine concept, your
style, almost a glider type aircraft,  certainly fit the bill
perfectly.

I am pretty sure that there are very few things that have not been
done, so I am sure someone somewhere have tried to connect two
rotaries
in tandem, but I don't know if it has been done for aircraft purposes.
IF and that is IF it can be done, it probably require the least
possible
arrangement of all the existing engines, in order for it to be
accomplished.

Structurally I give it a very high chance. I don't know if it can do
it,
but I really don't see anything that says that it is impossible. On
the
contrary, I see very good chance of success on that venue.

The answer that you need to find out is...can a rotarys center shaft
accept twice the load reliably?

Roger


Roger,
Thanks for your thoughts.  I want to avoid tandem
props because I want all props in the rear for the
noise, the view from the cockpit, and for the look.

I like the T tail, to keep the elevator
out of wing wash if wing is stalled and engines are
out, to increase the lever arm between the aerodynamic
center of lift and the elevator, and for the look.

I think I can design
a stiff enough structure to properly support its forces.
A bunch of robust sandwich composite ribs surrounding a
central sandwich composite spar system, transmitting pitching
control forces ought to be feasible.  The composite skin
of the tail structure ought to have considerable ability
to resist torque forces just as composite wing skins do.

I think a flutter simulation/test will be the real test.  Yes,
this large-ish tail structure will
weigh more than if a thicker fuselage extended farther
to the rear, but fuselages have weight too, and I wanted
the props as close together as possible, to minimize
adverse yaw.  I entertained ideas about connecting
the engines to both props when I was considering
the classic wing-mounted tractor configuration, but

I think by minimizing adverse yaw, and maintaining
good rudder authority, there is less need to worry about
such connections.  I appreciated your thoughts about
possible cross-wind effects on the tail, but wouldn't
the same apply to any tail...if the tails are smaller but
farther away from the cg and ac, it would be the same
effect as a larger tail closer in?  Thanks again.

Gene Kahn


Gene,

No it would be a single prop, not a tandem prop.

It's just an untried proposal, two rotary engines, in line, a clutch
inbetween, ...one redrive ....one prop.

Two engines for take off  and fast cruise....stop one engine , disengage
the clutch, and run on one, for long economy cruise.

Get exactly the power that you are planning on, but in this way you can
get rid of the two gondolas on the wing, you will get a cleaner wing,
less frontal area, one bigger and thus more effective prop, ( push or
pull, your choice, I would go with tractor...but any will do)one less
motor mount, one less reduction gear, less heavy structure inside the
wing towards the gondolas,

I am always weary when a propeller arc is sitting in such a way that in
it's extension are cruicial airplane structures. Heck I dont even like
to sit in the propeller arc's extension in comute flight aircrafts. Was
never a fan of overhead pusher engines in amphibians, but realize that
there are not too many places to put the power package on thise...so I
made it simple...I just don't like amphibians.
On tail surfaces, well, yes, but the differenc is the effect of prop
blast over the tailsurfaces....it's a compromise either way,...basically
keep it in clean air, but you have to have bigger surfaces, or keep it
in the propwash, and you can go down in surface area in order for it to
do the same, the numbers have to be crunched of course to get it right,
but that is the general difference, when it comes to drag and tail
surface autority.

An additional difference is ...of course..that a tail surface in free
air, will have less and less autority the slower it goes, while
a tailsurface  in the propblast will have autority down to, and
including stand still.

On final, the throttle is throttled back of course,...and depending on
landing technique, you can be in a glide, with no thrust ( and thus no
propwash) or you can have a slight positive thrust, ( and some
propwash). ...but , if the surfaces are mounted in the propwash zone,
all you have to do is to throttle up, and you will get instant
authority, something you can not get if the tailsurfaces are riding in
free air, that was my point. If the surfaces are riding in free air, you
have to wait after the throttle up, until you have build up some speed,
before you will regain autority.

In most small aircraft classic configurations, there is a prop up front
blasting the fuselage and the tail surfaces, , but larger jet airliners,
of course don't have that, so if you step back, and look at the
tailsurfaces on those, ( by propotion) you will see that they are
somewhat larger.

Usually both configurations are not too much of a problem in practical
terms, but I thought to mention this, as you had a concern about adverse
yaw in case of one engine failure.

Your aircraft proposal approaches a litle bit a hybrid between a U2 and
an airliner, some airliner have T tail some don't, and as this aircraft
is still a paperproduct , the need for a T tail is as of now unknown.
Those require a lot of structure, and if you are able to get away from
it, and you find that you actually will not blanket the tail when there
is a stall, and can do a classic tail instead, you have saved yourself
some engineering headache.

I looove your aircraft  idea, this one will go places, if you get it up
in the air,....why don't you build a big R/C model of it first.

You will not get the right Reynolds number, but you can at lest work out
obvious aerodynamic problems. ...( and there will be)

It's not a full substitute for wind tunnel testing, but as most of those
resources are out of reach for most people, you could learn a lot from a
big R/C model of your concept, and it is easier and cheaper to make
changes in one of those, than finding out something once the whole
aircraft is built.

You will have endless combinations that you can test, T tail, H tail,
straight tail, motor gondolas, prop up front, in the back, or whatever.
Do a lot of slow landings, and find out problems , basic balance
windows,  benefits and disadvantages, with any of these configurations
on the basic airframe.

You might want to take contact with the R/C community in your neck of
the woods, and explain to them what you want to do. By using foam
cutting technique, you can experiment with any airfoil, maybe even make
a batch of different wing configurations, and a set of T, H and straight
tail configuration, and start flying it until you find your best
combination.

Absent of a big airplane companys resources, that would probably be one
of the better develpment ways to go. Burt Rutan did a rig on a car,
and fixed a model of the VariEze on it,, and ran it back and forth on a
flat spot in the desert somewhere, so he could be able to study it, and
be there at the same time. You don't have to reinvent the wheel, you can
do the same, it worked for him, and out of that a long line of
successful canards came out.

good luck

Roger