Tailplane incidence

[Dave_Trible]

I've always been a 0-0-0 guy (my ex's will verify) and don't usually detect any hunting on my ships-except for one in my current active fleet which hunts a bit only inverted.  I blamed it on a little warpage in the elevators.  Been having this issue too much so I just replaced all three planes elevators with solid 3/8 and carbon.  Hope the hunt goes away.  If not I would think I could experiment with acetate trim strips stuck on the elevators (or a Gurney lip) to trim the issue. Think I'll play with this some and see what I learn. My thought for now is that this is more like a trim issue for an individual model rather than a generic principal to be applied to all.

Dave

[Howard Rush]

I posted some stuff on the Impact stab on the engineering board.  If somebody wants to do the same for a typical airfoiled stab with the appropriate incidence so elevator is centered when the flaps are, we can compare the two methods.

[billbyles]

So you were increasing the drag on the elevator with the trim tab, like a mini speed brake? Is there a dead spot in elevator response around neutral that was causing the "wandering"?

Hi Douglas,

There was no "dead spot" in elevator around neutral in our airplanes; however the pitch response in those airplanes required only a very light stick force around neutral so we trimmed for the approximate 10 pounds of back pressure on the stick.  We found out early on, from flying at the same airshows as the Navy's Blues that they do the same down trim thing & have been since nearly the inception of the team.

[RandySmith]

Hi Douglas,

There was no "dead spot" in elevator around neutral in our airplanes; however the pitch response in those airplanes required only a very light stick force around neutral so we trimmed for the approximate 10 pounds of back pressure on the stick.  We found out early on, from flying at the same airshows as the Navy's Blues that they do the same down trim thing & have been since nearly the inception of the team.

That was what I about to mention Bill, The Blue Angels do exactly the same to help in tracking, From what I read the F-16 had  Zero stick pressure at the very start of it, but then the engineers added stick pressure for the pilots benefit.

Randy

[billbyles]

That was what I about to mention Bill, The Blue Angels do exactly the same to help in tracking, From what I read the F-16 had  Zero stick pressure at the very start of it, but then the engineers added stick pressure for the pilots benefit.

Randy

Hi Randy,

We learned a lot from the Blues & from the Air Force Thunderbirds.  For instance, when flying away from the crowd in "Vee" formation and turning say left, at the start of the turn, I would (being on the right side at #3 position) move from my normal position stepped down & back slightly from lead to a position slightly stepped up with my left tip tank about five or six feet from lead's vertical fin & in line with it not quite overlapping lead's horizontal stabilizer.  At about the 200 degree point in the left turn I would smoothly move back to the normal position.  We would then make a pass in front of the crowd & do two formation "Vee" loops & on into the rest of the show.

All of this made it appear to the crowd that the formation had remained exactly the same.  After the first time watching the Blues do this we talked with them at the airshow performers party that evening & got quite a lot of good info.   

[Randy Cuberly]

Since this thread is about trim stability...or something like that...I would like to relate a recent experience with trimming a Whitely Shoestring and some rather strange (at least to me and a few others here) results.
To begin, the airplane is 650 sq in..about 59oz..has a PA65 on a pipe for power, and is on 67Ft (center to center) .018 lines, is 0-0-0 in alignment, and generally flies a very nice pattern.  Initially in trimming it displayed a slight tendency to hunt (very slow oscillations of low amplitude) when upright but not inverted, but tracked very well in maneuvers.  We added a slight down trim to the elevator (probably less than 1 degree) and the hunt disappered.  However two other things surfaced.   It now had a tendency to turn tighter inside and a little sluggish on outsides.  Handle bias was increased to the down side (about 3/16 inch) and the turn returned to normal (equal inside and outside.  Now however when flying level upright there is significantly more pressure (pull) on the down line than the up line (handle).
Sounds simply like being necessary to hold down elevator to maintain level flight...well this seems different.  The airplane ehxibits very neutral trim when the engine quits, but seems to pull more on the down line than the up line even when flying at a fairly high altitude where it obviously repuires some up bias to maintain that flight path.  
This seems counter intuitive to adding the down elevator trim...and physically seems to be different than a simple handle adjustment which doesn't seem to alter the condition.
Any of you "aerodynamicist have an explanation.  Uhhhhh...be nice now "you're nuts" is not considered an acceptable reply!  

Randy Cuberly

[Steve Helmick]

Since this thread is about trim stability...or something like that...I would like to relate a recent experience with trimming a Whitely Shoestring and some rather strange (at least to me and a few others here) results.
To begin, the airplane is 650 sq in..about 59oz..has a PA65 on a pipe for power, and is on 67Ft (center to center) .018 lines, is 0-0-0 in alignment, and generally flies a very nice pattern.  Initially in trimming it displayed a slight tendency to hunt (very slow oscillations of low amplitude) when upright but not inverted, but tracked very well in maneuvers.  We added a slight down trim to the elevator (probably less than 1 degree) and the hunt disappered.  However two other things surfaced.   It now had a tendency to turn tighter inside and a little sluggish on outsides.  Handle bias was increased to the down side (about 3/16 inch) and the turn returned to normal (equal inside and outside.  Now however when flying level upright there is significantly more pressure (pull) on the down line than the up line (handle).
Sounds simply like being necessary to hold down elevator to maintain level flight...well this seems different.  The airplane ehxibits very neutral trim when the engine quits, but seems to pull more on the down line than the up line even when flying at a fairly high altitude where it obviously repuires some up bias to maintain that flight path.  
This seems counter intuitive to adding the down elevator trim...and physically seems to be different than a simple handle adjustment which doesn't seem to alter the condition.
Any of you "aerodynamicist have an explanation.  Uhhhhh...be nice now "you're nuts" is not considered an acceptable reply!  

Randy Cuberly

I should wait for Howard, but won't. Are the controls free moving while under tension? If so, I'd add a degree of downthrust first. If it doesn't turn outsides as easily as insides, I'd dial in more down elevator.  Can't abide handle bias of any sort, tho I'd like to read what exactly you did to the handle. In my view, it's an airplane problem, so fix the airplane! What handle was being used? There are plenty on the market that are not very good, in my view. I would also look at it to see if the handle fits the pilot's hand snugly, as it should, or if it's too wide in the grip area.    Steve

[Randy Cuberly]

Hi Steve,
Actually I have tried more down elevator and it just made the condition worse.
The handle is a small Fancher style and fits my hand perfectly.
Yeah, I usually don't like handle bias, at least not to any great extent, but tried this as a stop gap since it was suggested by Mr RJ hisself!  It isn't extreme only about 1/8 inch or so.

The controls are very free both with tension and without.  Remember I said this was a little strange.  I've trimmed a lot of airplanes in the past but this one is different.  Your suggestions are certainly good ones and I'll probably try the engine downthrust next.
Unfortunately I been under the weather most of this week and unable to get out to the field to try anything further( dammed old man stuff) .  Also working on several other airplanes at home.

I'll let you know if the downthrust helps.  The airplane is quite flyable as it is but...well you know the pursuit of perfection and all that stuff.

Randy C.

[Brett Buck]

I should wait for Howard, but won't. Are the controls free moving while under tension?

    Randy got an example of testing for that one a few weekends ago! 

    Brett

[Al Rabe]

OK, short answer, or opinion, if you prefer.  All control
line propeller powered airplanes experience a bit of
gyroscopic precession from the very fact that the airplane is
tethered and in a constant turn as it travels in a circular
flightpath.  This is an indisputable physical result of flying
a tethered airplane with a gyroscope mounted on the engine.
This precession, typically nose up on airplanes flying
counter-clockwise, is there all the time while the engine is
running and must be compensated for.  You can either attempt
to trim it out using incidence or down thrust, or you will
carry a bit of down in the handle, whether you are aware of it
or not, to make the airplane seem to fly neutrally correct.

The effect is slight, but it is always there as long as a prop
is turning.  I am not comfortable with the thought of using
down thrust.  Incidence works and will reliably add a bit of
compensating nose down pitching moment (nose up inverted) to
compensate for precession.  Dave and Brett have done some test
with variable adjustable stabs and , if I remember correctly,
think that about a half degree of incidence not only counters
precession but adds a bit of longitudinal stability.  I forget
the reasoning, but probably has to do with "longitudinal
dihedral".

And then there are asymmetries in airplane configuration.
Most airplanes have the stab located higher than the wing.
When maneuvering, up flaps on outside maneuvers direct wing
wake toward the tailplane reducing their effectiveness.  Flaps
down on inside maneuvers direct the wake of the wing away from
the tailplane.  This is noticeable in asymmetrically
configured airplanes showing a tendency to turn better inside
(up).  There is no perfect trim for this asymmetry but,
practically, we can balance the effect by trimming the
elevator down until the turn rate seems balanced.  Or, by
anticipating the need for trim by building in a bit of
incidence to correct not just for precession but also for
configuration asymmetry. Again, I think that just 1/2 to 1
degree of incidence is enough for typical airplanes.  My
semi-scale airplanes have a much greater asymmetry of
configuration in that my stabs are usually located much higher
relative to the wings.  For this, I need more incidence.  

Years ago I found that while trimming a series of competition
airplanes to turn equally that I was typically winding up with
about 3/16" droop of the elevators on most of these
airplanes.  The 1972 and 1973 NATs winning sea Furys both
trimmed out with 3/16" of down elevators.  With the
Molded Mustangs, I anticipated the need for asymmetry and
gyroscopic trim by splitting the 3/16" down elevator trim
between the elevators and stab to "fair" (align the
stab and elevators) the surfaces.  This resulted in 3/32"
incidence in the stabs and 3/32" of droop in the
elevators which "faired" the assembly with what
turns out to be about 2 degrees of incidence.  All of my
airplanes since have 2 degrees of incidence,
"faired" elevators, and track straight in pitch with
loss of line tension.

You pays your money and do what works for you.  This is how
and why I use incidence.  

Also, I use 2 degrees of engine offset on all of my airplanes,
except those powered by .91s, where line tension is excessive
anyway.  As far as I can tell engine offset has no noticeable
effect on pitch trim in maneuvers.  Undoubtedly, there are
some negative effects on maneuvers of this configuration, but
I have no idea of what or how much.  I think this is something
which, on balance, probably adds a bit more positive effect to
the handling characteristics of our acrobatic models than
negative.  Whatever.  I build it in, trim the airplanes
carefully, and learn, with practice, how my individual
airplanes "like" to handle in each maneuver.

Al

[Chris Wilson]

..... "longitudinal
dihedral"........

First time that I have heard of that term, but I get what it means.

Thanks.