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
usingdrag /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
moveonlyout.  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
roundnumbers 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
restoringmoment of 16 square feet times 26 pounds per square
foot at 100 MPH
or 16 times 26 pounds times 40 inch 16,640 inch poundsor 1386
foot pounds.
The aero drag on each vertical fin cancels out leaving the good
dragfrom 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
8degrees 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.
Commonon 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
straightaheador  kiss
your ass goodbye :)
Paul Lamar
Paul, I'd wanted to check some numbers prior to replying..but
justhaven'thad time.

Do you know what Re# that chart was plotted at? Using your
sizingnumbers I
calculate
a Re of 1.5M at 80mph [1000msl] which is a pretty small number
allthingsconcidered.

I'm thinking it may work, however the CL of 1.0 might be a
bitoptimistic  at
Vmc speed [minimum control]
It does appear that w/ a larger tail than 'standard' this would bea
workable solution.

I would design it so that it is flyable in a non-
crabbedcondition. Reason
being is as the a/c crab's the
wing w/ the good engine is going to get an increased span [due
tothe sweep
of the wing] and your
going to get alot more lift on that side.. this is going to
causemore  issue
than one might expect.
An extra large set of vert surfaces should make it a
controlable unit.

Btw, I wish that FAR23 [I think that's the reg] had stated
thatall twin
 a/c
had to have a Vmc of
85% of the Vso. T'would have stopped alot of deaths due to
poorpilotage..The 414A- Ram IV that
I fly has just about got this.. and I love it... [and yes
I'veflown in
single engine during training]
Best darn twin I've flown yet.. :)
J. Johnson

It is right out of Von Doenhoff page 4. Theory of Wing
Sections.The bible
of
air foils. No Reynolds # cited. You might be able to fly it
atzero crab
angle
but it would take a very effective rudder and you would still
haveinduceddrag.
http://www.amazon.com/Theory-Wing-Sections-Including-
Summary/dp/0486605868Paul Lamar


I'll have to dig out my copy and have a read :)

Thanks for the education!

Jarrett

Paul,
Do you think having the fin with its root operating in the
strong low
 pressure aft/on top of the wing will have any effect on its behavior
 (or the calculations)?

Matt-
I suspect it would reduce its effectiveness.
It is interesting that a lot of airplanes use strakes in front
 of the vertical fin. These were added to the P51 and the B17 to
name a few.
 IMHO these generate a vortex at some finite angle of attackwhich has a
 super low pressure at it's core. This low pressure acts on the
vertical  fin to generate a powerful restoring force. This in
effect stiffens
 the directional stability.
IMHO the FA-17 uses horizontal version
 if front of the wings to increase the lift in a turn.
Paul Lamar

Jean, the comment by J Johnson about roll control of a crabbed
swept wing
is spot on and it worries me that it slipped by my attention, When the
swept wing swings around to it's crabbed position the chord length
of the
wings changes with the shift of relative wind. the forward wing chord
appearing shorter and the aft rotated wing acting like the chord
is longer.
The wing panels are the same thickness so that's like reducing
camber on the
section and with a low or mid mounted wing the fusaloge is blocking
available airflow over some of the aft rotated wing panel
particularly in
the strake area which is a large percent of canard wing area. It
occurs that
treating the desease rather than the symptoms is in order.

There was a twin
engine bomber (british) that had overlapping props with the tips
of on prop
just cleating the prop shaft of the other prop. One prop is
mounted slightly
ahead of  the other. This would close up the amount of offset from the
centerline and reduce single engine asymetrical thrust. I have been
wondering  where the filament wound carbon tube driveshafts  are
routed out
to the gearbokes for the prop shafts. It looks like they are close
to the
thinner trailing edge of the wing.The lack of flaps on a canard
having a
fixed trailing edge helps but it looks tight.The drawing Paul
posted earlier
this week with counterrotating props on vertically/offset stacked
enginessure looks like a winner to me and  looks cool doing it. I
read somewhere
that counterrotating props are efficient due to the redirecting of air
similar  to the alternate opposite rotating turbine wheels of a
jet engine.
Does anyone know if this is true?

george grimes


George, I believe the Russian bears had a couple points higher on the scale
due to their co-axial props.

Fwiw

Jarrett