REFLECTOR: elevator range - increased drag

[Keith Hallsten]

John,

 

Yes, this was jus discussed within the last few days on the Canard Aviators
list with respect to the Long-EZ.  Excess down elevator will act as a
spoiler.  The elevator should have a stop so that maximum down travel gives
the canard-elevator system the maximum lift coefficient.   Unfortunately, I
don't know exactly what elevator deflection gives CL max with the Velocity
canard airfoil.

 

Here's an except from the "Canard Pusher" newsletter for Rutan Aircraft
Factory planes:

 

CAUTION  CP 48
ELEVATOR CONTROL STOP POSITION.  This applies to VariEzes as well as
Long-EZs using the original GU canard (Roncz 1145MS not affected).  The
design philosophy of the EZ canard type airplane calls for the canard
airfoil to develop maximum lift coefficient (CLmax) at full aft stick.
Thus the elevator trailing edge down (nose up command) stop must be set
correctly.  On an accurately built GU canard/elevator, this will
usually be at approximately 22 degrees (trailing edge down).
 
Recently, we have heard from a few builders, both VariEze and Long-EZ,
who have noticed stall characteristics that were not "per the
handbook".  In all cases, the cause was the elevator nose up stop set
to allow too much elevator travel.  If you have noticed any of the
following symptoms, check that you have no more than 22 degrees to 22-
1/2 degrees trailing edge down travel on your elevator.
 
1)  Perform a 1-'g', wings level, straight ahead stall with sufficient
power to maintain level flight.  Slowly pull the control stick back to
full aft stick.  This should result in a nose high attitude with a
"pitch bucking" that can vary from hardly noticeable to quite vigorous,
perhaps "one buck" per second, with a deck angle change of several
degrees per "buck".  This is normal and will vary depending on the cg.
If, however, you notice a strong stall break (canard stalls) and the
nose comes down through the horizon until you are in a stable shallow
dive, even though you are still holding full aft stick, the speed may
build up to over 100 KIAS before the EZ begins to climb again.  This
very long period pitch "bucking" can be as long as 30 seconds per cycle
and is indicative of too much elevator trailing edge down travel.  You
can verify this by releasing back pressure on the stick during the nose
down phase of the cycle and gently raising the elevator trailing edge
perhaps 1/8" at a time.
 
This should allow the canard to develop more lift and pitch the nose
up.  Try to determine by experimenting with elevator position, where
CLmax is, then set your elevator stop at that position.
 
2)  Another classic symptom may be noticed during a take off.  At full
aft stick, it may take a longer take-off roll to lift off that it does
at, say, slightly forward with the stick.  If you have ever noticed
this, it should be corrected.  Under certain circumstances, this could
become a serious problem.  A Long-EZ builder/flyer in Alaska,
attempting to take off on a rather short runway, discovered that he was
rapidly approaching the end of the runway and, even though he was
holding the stick all the way back, was not rotating.  Realizing he was
not going to make it, he backed off from the full aft stick stop and,
to his surprise, the airplane literally jumped into the air!  Again,
his trailing edge down elevator stop was set for too much travel.  This
same scenario has also been reported to us by a San Diego VariEze
pilot.
 
What causes this?  If the elevator stop is set so that at full aft
stick your canard can develop its maximum possible lift, this will
result in the lowest possible rotation speed for take-off and a good,
clean canard stall (limiting the main wing angle of attack) or classic
"per the book" stall at full aft stick in flight.  If, however, you
have set your elevator stop for too much travel (perhaps you thought
you could lower your rotation speed?!!) what happens is that you are
now on the "back side" of the lift curve, lift is less than maximum,
and the elevator is creating lots of drag.  The result may be running
off the end of the runway.  Keep in mind that this condition could be
aggravated even further if it were raining.

 

Keith Hallsten

 

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of John Tvedte
Sent: Thursday, March 16, 2006 8:37 AM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: elevator range - increased drag

 

All,

 

Factory Spec (minimum) is for 26 degrees down and 23 degrees up or 49
degrees total. 

 

Our Elevator has 30 degrees down and 28 degrees up or 58 degrees total. 

 

I am wondering if anyone knows if having too much elevator range can result
in getting the canard in the "back side" of the lift curve, lift is less
than maximum and the elevator is creating lots of drag?  

 

Wondering if I can get a survey of what people are getting for their down
and up elevator travel - flying airplanes greatly appreciated?

 

Thanks,

 

John

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