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 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 poundsor 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 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't
 had 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 allthings
 concidered.

 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 haveinduced
 drag.

 http://www.amazon.com/Theory-Wing-Sections-Including-Summary/dp/0486605868
 Paul 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 attack  which 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 engines
sure 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



Don't forget about Beech Starship.  Despite it being FAA-mandated
overweight and hence not able to meet the original performance specs,
it apparently is a nice flying airplane - even with a fan shut down.
It does have it's engines mounted quite close together in terms of the
scale of the rest of the airplane.


Matt-