REFLECTOR: elevator range - increased drag
Terry Miles
terrence_miles at hotmail.com
Tue Mar 21 07:33:05 CST 2006
Oh boy,
It is only w/ great trepidation that I wade into this one. I'd be
better at explaining with old notes from pilot school, but they are
tucked away deep somewhere in Chicago, and I'm in Greenville.
Take a sheet of paper and make yourself an X and Y axis chart. Label
the vertical line Trust Available/Trust Required. Label the horizontal
line speed. Now draw and smiley face on your chart and a second
straight line right thru the smile.
The straight line is trust available and doesn't change. It is your
engine/prop max output. The speed line goes from zero to Vne. Notice
that a some speed less the zero you do not have enuf HP to maintain
flight. Notice the lowest point on your smile. That speed is equal to
C sub L max (Cl-max) (sorry I don't know how to type subscripts.
Now, the other factors that go into determining C sub L max are air
density and the shape of the wing. Example lower flaps and the smile
line will shift left. Go to Denver on the 100 degree day and it shifts
right.
Now, only as best as I recall all this stuff, deflecting the elevator
(much like lowering the flaps) changes the shape of the wing and so here
is our debate point about too much elevator deflection. Using the Cozy
story about the guy who took off with the stick in his gut trying to
minimize takeoff ground run and in reality lengthened it applies to all
airplanes not just canards.
To offer my thoughts on Keith's question below, there is no set
deflection associated with C lmax because V sub stall is a function of
angle of attack and (I think anyway) that a of a is a function of lift
and lift is a function of multiple factors including wing shape which is
changed by elevator deflection.
I think another way of stating our common concern could be: "Is the
possible to improve the lift coef of the canard wing with greater
downward elevator deflection such that the main wing would stall first?"
Think of it as two separate C lmax speeds. One for the canard wing, one
for the main wing. How close are the separate stall speeds now in
theory?? Also, in theory, were to this happen it would be at airspeeds
below current stall speeds, and you'd likely be hitting your prop if in
the landing mode.
Terry
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Keith Hallsten
Sent: Monday, March 20, 2006 9:14 PM
To: 'Velocity Aircraft Owners and Builders list'
Subject: RE: REFLECTOR: elevator range - increased drag
Ron,
The lift coefficient is not a function of the loading, only of the
airfoil. Whether a stall will occur IS a function of the loading, among
other things. What we really need to know is what elevator deflection is
associated with CL max. We don't want to have less, because we need it
to rotate. However, we DO want a hard stop to prevent deflecting the
elevator any further, so you don't end up on the "back side" of the lift
curve, where we find LESS lift and MORE drag - an unhappy combination.
The post from the Canard Pusher includes a procedure for determining
experimentally whether your stop permits excess elevator deflection. I
don't have a flying plane yet, so I can't perform the experiment. Have
any of you with a flying plane done this?
Keith Hallsten
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Ron Brown
Sent: Monday, March 20, 2006 11:27 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: elevator range - increased drag
John,
I saw the discussion on the Canard list. I couldn't tell you what the
CL
max is for Velocities. I'd guess it is dependent on the individual
airframe, nose loading, etc.
I haven't run that test. I was just trying to say, you'll need the down
travel - but the listed up travel isn't as important.
Ron
----- Original Message -----
From: "John Tvedte" <JohnT at comp-sol.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Monday, March 20, 2006 1:07 PM
Subject: RE: REFLECTOR: elevator range - increased drag
> Ron,
>
> In our case with have 30 deg down - what we are wondering though is
> what
> down elevator results in max. lift - CL max -
>
> In our case - mechanically we are limited to 30 deg down - so - will
> being
> beyond 28 degs result in drag
>
> From Chapter 3 (XL) NOTE: wording - minimum acceptable
> The minimum acceptable deflection is as follows:
>
> . Up Elevator: 24 degrees min (preferably up to 26 degrees)
>
> . Down Elevator: 28 degrees (approximately 2-3/4" deflection from
> neutral)
>
> ------------------------
>
> From Chapter 10 (XL) NOTE: wording - least
>
> Full elevator deflection results in at least 26 degree trailing edge
> down
> elevator travel
>
> and 23 degree trailing edge up elevator travel.
>
>
> John
>
> -----Original Message-----
> From: reflector-bounces at tvbf.org on behalf of Ron Brown
> Sent: Mon 3/20/2006 11:56 AM
> To: Velocity Aircraft Owners and Builders list
> Cc:
> Subject: Re: REFLECTOR: elevator range - increased drag
>
>
>
> You'll need the recommended down travel. Up is less important unless
> you are planning to fly upside down - not recommended. Bottom line,
> make sure you have the down travel.
>
> Ronnie
>
> ----- Original Message -----
> From: "John Tvedte" <JohnT at comp-sol.com>
> To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
> Sent: Monday, March 20, 2006 12:28 PM
> Subject: RE: REFLECTOR: elevator range - increased drag
>
>
> > Keith,
> >
> > Exactly -
> >
> > Here are some inconsistant #'s in the manuals (for XL).
> >
> > Chapter 3 - states 24-26 deg. up, and 28 deg. down. total - 52-54
> > deg. http://www.velocityaircraft.com/manuals/03_XLG.PDF
> > <http://www.velocityaircraft.com/manuals/03_XLG.PDF>
> >
> > Chapter 10 - states 23 deg. up and at least 26 deg. down - so total
> > of
> > at
> > least 49 deg.
> > http://www.velocityaircraft.com/manuals/10_XLR.PDF
> > <http://www.velocityaircraft.com/manuals/10_XLR.PDF>
> >
> > John
> >
> > -----Original Message-----
> > From: reflector-bounces at tvbf.org on behalf of Keith Hallsten
> > Sent: Thu 3/16/2006 11:51 AM
> > To: 'Velocity Aircraft Owners and Builders list'
> > Cc:
> > Subject: RE: REFLECTOR: elevator range - increased drag
> >
> >
> >
> > 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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