J  Johnson, A center mounted tail on a canard has virtually no
lever  arm
length and would be huge, if moved back wetted area and CG
problems  are a
concern. The wing tip rudders deflect outward only on  most
designs and
affect drag far from the CG to yaw the  aircraft, and a 2 degree
incidence is
believed to turn drag into a small  thrust vector when not deflected.

george grimes

George, I think we're in agreement! Having the tail right near
the props is not going to have alot of effect [hence the  thought
of extending the tail [empennage] . Control of  asymmetric thrust using
drag /tip rudders seem's to be counter  intuitive in that during single
engine operations one needs  ever ounce of thrust they can get..

I think the better  option here [my opinion only] would be to create
some form of  a dual props in an axial config, best of both worlds
so to  speak.

Fwiw

J.  Johnson

You guys are missing a little something. The Rutan  rudders move only
out. They do more with drag than they do  with side force.
Since the lever arm is so great a little bit  of drag goes a long way.
Pardon the pun :)

Sort of like th split ailerons on the Northrop flying  wing.

Paul Lamar

Paul, I'd noted in my  previous email that I wasn't to excited about drag
rudders
to compensate for asymmetric thrust. I did some napkin math.

Assumptions:

Engine thrust = 400lb's [assume at lower speed near  a 'realistic' Vmc]
Engine is 40" offset from A/c  centerline.

I checked and a cozy wing is 28'1" so 1/2 span is  168"

400lbs x 40" = 16,000 in-lbs [1,333 foot pounds yaw  force.]

16000in-lbs / 168" = 95lb's drag [at the wing  tip]

If we do the math we come out w/ a needed drag of ~95lb's  at the tip to
compensate for the asymmetric thrust. No counting any  extra drag from the
feathered prop, which would exacerbate the  issue.  That means for round
numbers we're looking at losing  1/4 of our thrust just to maintain
directional
control..  I'd count this as un-acceptable if it was my design.. but  then
maybe I'm
missing something?

J.  Johnson

Yes. Very good.  I'll buy your math.

To get 95 pounds of drag at 100 MPH you would need a rudder of
3.6  square feet perpendicular to the air stream so I agree. Rudder
drag  alone is not going to get it all.

Lets add in the lateral  restoring force from the vertical tip fins
in the horizontal plain  about the CG.

Lets say it is 100 MPH ten degrees yaw.
The aspect ratio of the vertical tip fin and rudder is close to
6:1  so C sub L of the vertical tip fin is close to one. Dynamic
pressure  is 26 pounds per square foot at 100 MPH. I would need a
good drawing  to be more precise.

The CG is somewhere up there between the  canard and the main
wing. Give it 40 inches in front of the vertical  tip fin 1/4 cord
point. The size of the vertical tip fin, lets  assume, is 4 feet by 2 feet.

Lets say they are 8 square feet  each (2 fins) so we have a restoring
moment of 16 square feet times  26 pounds per square foot at 100 MPH
or 16 times 26 pounds times 40  inch 16,640 inch pounds  or 1386 foot
pounds.

The  aero drag on each vertical fin cancels out leaving the good drag
from the one rudder deflection.

Total restoring force is now  1366 foot pounds from the tip fins and
1,333 foot pounds max from  rudder drag or about 2700 foot pounds total.

IMHO the stabilized  yaw angle will be somewhere  around 8 degrees plus
or minus 2  or 3 degrees. There will be some lower speed down to take off
speed  where this is not going to work and the airplane will spin. Common
on all conventional twins except huff and puffs.

Minimum engine  out control speed or some such.

You either pull the power on the  good engine and crash straight ahead
or kiss your ass goodbye  :)

Paul Lamar


How much could be gained by altering the thrust line of each shaft to
assist in single engine
performance?

I lost track of the gearing ideas. Could the pieces be recovered from the
plain versions of a Franklin or Speedway engineering quick change rear
ends. The axle housing tubes are ground to size 3" OD mild steel.  The
 stuff
for sprint cars has no internal axle at all. The 3" tube is the axle and
 the
suspension attaches to bearing cages in 3" bearings. So a 3" carbon tube
could work as well.

Lynn E. Hanover

Some for sure. The airplane should just fly in a crab like
fashion. Maybe tending to roll toward the good engine.

There is a lot of good sprint car parts out there.

The aluminum axle would be too heavy and not stiff enough.
Aluminum modulus of elasticity is only 10 million while
carbon is 30 million. That is as good as steel with 25% of
the weight or less.

I have thought a lot about it however.


Paul Lamar

Paul, just a thought/question actually. If the twin canard has a shut down
engine is it necessary to force the nose completely back to centeror
or could he just leave it yawed a little like a cross wind landing.It occurs
to me that the goal is to maintain control to a landing location and adding
a lot of drag when down to 1/2 power seems like a waste of thrust. I do
however have an opinion on angeling the thrust lines of the prop shafts
to counter single engine aw. This would put the props  operating in P-factor
like a high angle of attack in climb which is less efficient and noisy and
causes cyclic loading of the blades. I realize some planes are rigged with a
non centerline thrust up to 2 degrees, but the airframe is not restrained
from weather vaning behind the prop and in flight the thrust line would be
less than the 2 degrees. the offset to counter an engine 40  inches off
aerodynamic center running at near max power would be considerably more.

george grimes