An article originating in the Oakland Cloud Dusters newsletter and
eventually republished in The Hangar Pilot, Dr. John Martin, editor
Circle Trim
by Aaron Peterson
For several years, I’ve been
exploring the Circle Trim flight adjustment system
for rubber-powered (and now electric) sport and
scale models. To begin with, I wanted a scheme that
would a) give a good, solid, predictable procedure;
b) allow me to fly in confined spaces when
desired—gyms, armories, city parks, school yards,
etc.; and c) produce good contest results indoors
and out. All objectives have been met. I can trim
most new models quickly and effectively.
I can fly in small spaces (when the wind is down).
Using Circle Trim, I have won 10 straight local
outdoor Embryo Endurance events and have attained a
97.8-second contest flight under a 30-foot ceiling
with an Ord-Hume 7 Walnut model that weighs 20 grams
without rubber. Maximum climb or steady low cruise:
it works for both.
The basics of Circle Trim:
1. Wing wash (vs. turn) to control back angle;
2. Rudder offset to establish the cruise/glide turn;
3. Forward CG and matching (usually generous)
decalage; and
4. Thrust offset to control power climb and turn as
necessary.
This system is hardly new. The 1933 Jimmie Allen
Bluebird plan shows considerable wing wash to offset
torque—the keystone of the whole idea. One can find
good references all through modeling literature—Dick
Korda, Henry Struck, Butch Hadland, Bill Warner, Wm.
McCombs, Tonda Alfrey—all of these experts (and many
others) have laid it out at one time or another.
Power fliers use washin to stabilize climb turn. The
famous Square Eagle P-30’s success is partly based
on Circle Trim. Many of you are now using Circle
Trim, even if it’s a matter of pasting a drag flap
onto a wing trailing edge.
So, I have invented nothing, but maybe I can offer a
few little slants on the subject that might be of
use. One thing’s for sure—I’m sold on it!
Many construction articles (and some kit
instructions) admonish you to build everything
straight and to steam out any and all wraps. On that
point they are clear, but when it comes to flight
trimming, things muddy up very quickly.
Often it seems given that if the precious model is
only built “correctly,” it will fly with little or
no trouble at all. We all know that is not the case!
Indeed, a normal sort of model can be trimmed to fly
okay in a large circle or in a large “s” pattern
just by getting it balanced properly, maybe giving
it some downthrust, and tweaking the rudder to the
right to offset torque. Maybe some hot breath on the
stabilizer. But those aren’t the kinds of patterns I
want, indoors or out. They take up way too much
room, and the model either hits a wall or grows
smaller and smaller in the distance as it heads for
the trees. I like to see my outdoor airplanes fly,
especially the scale jobs. I want them close at
hand, overhead, with the sun shining through the
tissue. (Not that an efficient high-powered model
like an Embryo can’t get really high in a hurry with
Circle Trim. It most certainly can.)
Circle Trim starts in the building phase. Several
factors ought to be incorporated right in the
structure. The first thing to decide is which way
you want the model to turn—left or right. Indoors, I
definitely believe that left is better, Right turns
get you to the ceiling too fast. Outdoors, I don’t
think it makes any big difference, at least for
high-wing models. I’ve flown sport models and old
timers that had big climbs, nice glides, and good
thermaling behavior in both directions, right and
left.
Supposedly it’s better to fly a freewheeler to the
right, though I’ve seen no proof of that in my own
models. Because of torque effects, a right-hand
pattern will require less side thrust than a
left-hand pattern. Some left-hand models require
huge amounts of right thrust. Anyhow, once you
decide which way to go, something has to be done to
the wings to oppose that turn direction. You can
washin the inside wing. You can washout the outside
wing. You can make the airfoil of the inside wing
15% thicker than that of the outside wing.
All these devices work, but there’s another way, a
refinement that I prefer above the others:
differential washout. We know, of course, that
washout is desirable for stability, indoors and out.
Washin can shoot you down (via tip-stall) if you hit
a ceiling or a gust. (Never washin both wings!)
Washout each wing, but to differing degrees. On a
Peanut flying to the left, washout the left wing
1/16-inch, and the right wing about 5/32-inch. On a
Bostonian, make it 1/16 inch and ¼ inch. On a
24-inch model, make it 1/8 inch and 7/16 inch. My
Howard GH-1 (32-inch span) has a 1/16-inch and
½-inch setup.
A heavy model might require a bit more differential.
It’s not critical. Just so you have that good solid
influence vs. the spiral, yet not so much that your
rudder has to be offset unduly to overpower the wing
wash in the glide, thus requiring even more side
thrust to offset the rudder in the climb, etc. It
all works together, you see, keeping a nice steady
circle all the way up and all the way down. A hinged
rudder (or tab) is a big help in all this, possibly
even essential. Glue it fast when the final set is
found.
With a new model, use rudder offset, CG, and
decalage to set the desired turn and glide.
(Hand-toss or fly under low power.) Then slowly
build up in power, working thereafter almost solely
with thrustline (noseblock) offsets to control the
climb and/or cruise. Use downthrust to stop zooms or
stay away from the ceiling. Use side thrust to
maintain the turn radius. Remember: a change in turn
always affects climb angle, and vice versa. Take
your time, no more than 100+ turns per flight. Work
up to maximum power desired, and there you have it.
Just don’t rush things and all will be cool.
So how do you build in the wing wash? The best way
is to make a wing-size (oversize actually) 1/8-inch
sheet balsa platform jig that sits atop your regular
building board, with the root-rib end and the
leading edge border pinned down, and with a
wing-length wedge of the proper size glued to the
bottom of the trailing edge border. Build your wing
on top of this platform, and it will have the
correct angle set into it. (Shrink your covering on
this jig also.)
Another way is to build flat as per usual, then when
the glue is dry, wedge up the trailing edge as
desired, leaving root + leading edge tightly pinned
down. Saturate the structure with water spray, let
dry for 24 hours, then re-glue main joints and let
dry.
The least desirable method is to cover a flat wing
structure, spray it, and pin it down, wedging the
trailing edge. This way you depend on the tissue
alone to hold the wash. Under ordinary conditions,
this works okay, but let some sag get into that
tissue, and you are flirting with a death spiral.
Always be cautious with Circle Trim models in humid
conditions. Those with good structural integrity
will be okay; others may not. Lots of dope will help
avoid sag if you do fly in humid conditions
frequently.
Some will tell you that those “warped” wings either
look funny or downright ugly. It’s in the eye of the
beholder. All I know is they sure look good in the
air, up there on the “rails” they’re
flying on. Q
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