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sink-me
Leading Lifters
Compiled by Frank Zaic
All the scoop from the leading model designers on what airfoils are most widely
used in meets
If you were to ask a model builder what he would like most, he would answer,
"Fast climb and slow glide." No mention of a million dollars. Anyway, the majority
of the builders who cooperated with Air Trails in the making of a survey of the
most significant model airfoils now in practical use mentioned "fast climb and slow
glide" as their main objective in the search for the "ideal" airfoil. Seems like
this end, according to the answers, can be achieved with airfoils having flat bottom
or undercamber.
On radio control models, NACA 6412 seems to be the main choice. Both Walter A.
Good and R.F. (Radio Frequency?) Gelvin like it for its high lift at low speed as
well as its gentle stalling characteristics. Gelvin talks about 5 lbs. on 6 sq.
ft. and 35-40 mph flying speed. He also uses Clark Y. Both men agree on 37% C.G.
and 2 deg. angular difference between wing and symmetrical stab. Walter is tempted
to try a flat bottom next time because of simpler covering. Capt. H. M. Bourgeois,
U.S.M.C., used NACA 4612 on eight R/C models and is pleased with its weight lifting
and low stalling speed.
Response from the control line boys was surprisingly good. Walton Hughes and
Harold Reinhardt favor NACA 0012 and 0015 for stunt ships. Claim tighter maneuvers
and smoother level flights, and small Center of Lift shift. Hughes places C.G. at
5% to 10% and Reinhardt at 20%. Capt. Bourgeois is trying his own which looks like
laminar flow. High point at 50% with C.G. at 15% to 25%. Supposed to give high speed
with little power, good maneuverability, low stalling speed and good gliding ratio.
He plans gradually to move the high point back to 70% and C.G. to 20-30% to obtain
semi-scale stunt models that would perform with pure stunt models. Idea being to
get more lift and lower G-stall ratio with narrower wing:
While still in experimental stage, the section developed by Jim Saftig showed
promise by performing very tight and violent stunt maneuvers while capable of large
smooth patterns and stability. It is of deep symmetrical type with high point at
25%. He also notes that actual performance depends a great deal on overall design
of the model.
H. A. Thomas sent a very good report on the development of the Little Rock high-speed
airfoi. It goes from 1946 streamline to 1950 streamline in front of 40% and Clark-Y-ish
towards T.E., He mentioned that streamlined or similar sections tended to do a "high
speed mush" at times - while present section indicated sharp improvement in lift,
lower drag and stability at high speed. He also believes that it makes trimming
easier and provides good lift at landing speed. Used in combination with streamlined
stabilizer with 1 1/2 deg. difference, and rectangular planform.
Most unusual speed and jet section was developed by William D. Viets. Nearest
relative would be the M-6. He divides the thickness so that it is two-thirds above
center line and one-third below; while high point on top is at 33% and bottom at
50%. He claims more speed, less drag and better handling at top speed. At or near
top speed it flies light but steady, while at low speeds flies evenly without abrupt
stall so that it flies right to touchdown. Did 154 mph. We've heard of all types
of curves but this is the first time that a catenary curve was used for lower portion.
Free flight boys returned the largest percentage of the questionnaires. The surprising
fact was the popularity of the NACA 6409 in almost all divisions. The new "light
wing loading" rules for gas models did bring about a "new" airfoil which can be
best described as a "thinned" Clark Y.
Carl Goldberg is still using his proven section with slight modification (G-610B)
to allow L.E. and T.E. to lie flat on working surface. He believes that its lines
are more nearly correct for airflow at model speeds. As some of you may recall,
Carl made a series of tests to determine best airfoil for models, with his design
as the result. His normal setting between wing and flat bottom stab is 1 1/4 deg.
with C.G. at 57% when using 29% wing area stab.
Another user of Carl's G-610 is Claude McCullough, who likes the glide and the
unusual good climb it gives. He also uses it on rubber and tow-line gliders. Believes
performance equal to if not better than 6409, without the construction difficulties
due to thinness. Everything considered, he prefers deep airfoils in contrast to
thin sections now currently popular. Claims thin section can't get high enough to
justify glide loss, except on small sizes. It has been his observation that as the
size of the model increases, the greater is the necessity for a deep, high-lift
section. On the other hand, he has a suspicion that on AA anything with a bend will
do the job.
William H. Parmeter varies his choice. Thin G-610 good for windy weather, he
states, and especially for large C's. For calm weather and high climb he prefers
flat bottom type. Believes undercamber easier to adjust, while flat is ticklish
and sensitive under high power. And Ocie Randall found Gibson-Goldberg "Arrow" airfoil
the best all-around and uses it for all models.
NACA 6409 has a good following. C.O. Wright's report just about summarizes the
opinions expressed. He uses it for all free flight models, from AA to C. Tried many,
including thin Clark Y and some of his own, and finds 6409 good. Climb is good although
not as fast as thin Clark Y, but the glide is much better. He likes thin sections
and 6409 is about as thin as can be had and still allow space for spars. It is used
in combination with thin 40% Clark Y stab set on long moment arm. This sets C. G.
at about 60%. He finds 3 deg, angular setting a good compromise. Although glide
may be improved with two surfaces closer to similar angles, the chances of ship
not pulling out of a dip are not worth taking.
Wright ends by saying that in truth he believes the airfoil section is not as
important as some may think; it is the overall efficiency of the model that counts.
Clayton Newlin, David H. Hunt and C.P. Tiers agree practically 100% with Mr, Wright.
Charles Folk also uses 6409 but close coupled to obtain tight turning ship.
Sal Taibi does a lot of flying over weekends and after 6 p.m., and here is what
he says about his modified 6409 (likes sharpened leading edge): "Use it on A, B,
C, and D Pacer type. Power charactertstics are excellent, cuts down zooming tendencies,
does not stall out easily and is much faster on power. First tried on A size and
found that it glided smoother, flatter and better in respect to flight time. Angular
difference between 1 deg. and 3 deg. Very slight lifting stab with C.G. between
50% and 70%."
Benton Cleveland uses 6409 straight in combination with 9% Clark Y and 45-50%
stab. Finds this set-up allows him to balance at T.E. and set stab until a slight
mush develops. Then by tilting stab the desired glide, circle is obtained, while
rudder is used for determining power turn.
Still in the NACA camp but on the thicker side we have Parnell Schoenky who uses
6412 for rubber and gliders, and 6411 for gas. Reason being that he uses high aspect
ratios of 10 to 13:1 and heavy fast ships to obtain good glide characteristics.
He should know, as he started with Eiffel 400, Goettmgen 497 and RAF 32 in 1938-39.
Made three originals in 1940-42 period. Since 1946, he's been using 6412 in combination
with Clark Y stab with about 4 deg. difference and 2-3 deg. downthrust. Harold Stofer
ranges from 6409, 6412 and 6512 for gas and 6712 for rubber. Believes that "6" camber
factor best for all around gliding qualities.
Delving into the "original" field we find Paul Del Gatto going strong for Davis
type. Claims that it has higher L/D ratio than majority of airfoils used and backs
it up with tests. Henry Cole, who uses Davis :lt5 (A.T. May 1942) on Wakefield and
gas, states that Davis airfoils are noted for their anti-stall characteristics and
low drag at high lift coefficients, also their excellent recovery from stalls and
exceptional pull-outs on gas models. No. 5 gives 1.3 lift coefficient in glide.
"Dutch" Hess lets the record speak for his airfoil, sixteen places between 1st
and 3rd during 1948-49. It's 12% thick with Clark Y top and Grant bottom, set at
5 deg. Developed through experiments. Edward J. Lidgard spent 20 years in cut-and-try
to develop his favorite Wakefield section. Made test in an armory and found that
it has 1.7 ft./sec. sinking speed and 11:1 glide angle and 17 mph. It's thin section
with high point at 25%.
Remember the "Ritz" wing? Curtis D. Janke still uses single surface type on rubber
designs. It is his opinion that drag depends as much on thickness between upper
and lower camber as it does on camber depth. He notices a definite lower sinking
speed.
Dick Everett finally settled on Marquardt S-2 for rubber, McBride B-7with flat
bottom and slightly rounded L.E. for Half-A's and A's, and Goettingen 602 for B's
and C's. According to his report, B-7 climbs with the best and is still up when
others are down. 602 and S-2 compared on a Wakefield. S-2 climbed better than Goettingen
602 but glide not as good, and S-2 also touchy due to C.P. movement. On gas models,
602 glide makes people stare, reports Dick. On all ships, except Wakefield, stab
area 60% with C.G. at 75% to 125% of chord behind L. E.
James Noonan also uses B-7, though for indoor. His outdoor favorite is Eiffel
400. Chester D. Lanzo has not changed his winning Eiffel 400 on 300 sq. in. rubber
models.
Vernon W. Oldershaw ("Slim Jim" and "Glory Bee") is now working with NACA 4612
as a change from his thick original sections developed from Davis formula. His originals
were regular weight lifters, 14 oz. per sq. ft. According to Vernon, 50% change
in C.G. made no difference. Talking about 4612, Hal Roth said that after careful
check-up it proved no better than Clark Y. Hal uses RAF 32 and Clark Y on Wakefield
and 6409 on gas. He believes in flight and glide by wing loading and incidence set-up
rather than by airfoil. With a warp-free surface at correct incidence and wing loading,
most any airfoil provides good results, hence the use of Clark Y.
Frank Parmeter also likes thick sections with lot of undercamber. He uses it
in combination with thin 50% stab on short moment arm with success. By using high-lift
wing a large stab can be employed to carry part of the load without being critical
under power. His C.G. around 90-100% point.
By chance Earl L. Cayton, W. Harold Bunting and Don Justice worked out almost
similar airfoils. The surprising part is that Cayton used "intuition" while Justice
used NACA reports. The basic pattern is an arc median line with about 10% thick
stream-line plotted around it. The result is similar to 6409 but with camber's high
point further back. Justice said that T.R. 460 mentioned that highest CLmax
is achieved by having maximum camber at 60-70% from L.E. Such sections, however,
had higher minimum drag coefficient than normal type. Justice also tried a birdlike
section with upturned T.E. and reports terrible stall characteristics. On outdoor
H.L. gliders, Cayton uses thin Clark Y, and thin undercamber for indoor type.
So far we have mentioned Clark Y in combination with others. This does not mean
that it is not being used much. Just think of all the kits. Actually, since the
advent of light wing loading rules, Clark Y is making a serious comeback in the
thinned version. Theoretically speaking, it is no longer Clark Y. but you get the
idea. Clark Y with occasional modification on lower camber to slow down the glide
is being used by C. L. Bristol, Rogers L. Barton and Albert Casano because of ease
in construction and excellent power on and off stability.
In the thinned-out class we have Carl Hermes, Robert Hatschek, Leon Shulman,
Malcolm Smith and Frank Ehling. Thickness varies from 9% to 10% with high point
at 30%-40%. This type is definitely intended for fast climb where thermals are strong.
The boys do not brag about the glide but mention it as being fair and good enough.
Type has good stalling personality, and is for current light wing loading and fast
simple construction. As a group the boys use angular difference ranging from 3 deg.
to 5 deg. in combo with 5 deg. to 7 deg. thin stab. Hatschek section seems to average
those submitted, and since he supplied ordinates we might as well present it as
a standard for this particular group. We are calling it AA-1950. (All American-1950).
This completes the actual tabulation of the survey, but some of the boys had
comments that seem worthwhile to pass on.
William L. Effinger, Jr., advises not to bother using ordinates anymore. Old
fashioned. Just bend a good symmetrical section around the desired median. Generally
use "64" series median and streamline it with laminar flow, or the regular NACA
symmetrical section, depending on structure and aerodynamical requirements. Recommends
that one should quote Millikan (from "Aerodynamics") on airfoils. It is all so simple.
Bill Winter does not hold high opinion of the present crop of thin sections.
Believes they have poor recovery after stall, are difficult to adjust, and that
only light wing loading saves them. Likes Eiffel 400 for rubber; tried Midwest kit
wing of this section on Half-A and what a glide! At least equal to Eiffel 400 and
possibly better due to extreme difficulty in stalling is NACA 4612. It gives more
lift than 6409 at small increase of drag.
De La Mater said that median line ordinates of 400 and 4612 are almost identical,
which may explain their similarity. He uses thick section stabs on pylons, thin
lift on moderate high wing and symmetrical on cabin. On difference between wing
and thrust line he begs to differ with theory. Adding incidence is not automatic
down-thrust, as he has repeatedly noted. Stab angle must also be increased. And
he had lots more to say which will eventually show up in his writings.
C. C. Johnson recalled an incident which occurred in 1938. It seems that Eastman
N. Jacobs, then of the NACA staff at Langley Field, offered to demonstrate flow
patterns about airfoils for members of a local club. He asked all members to bring
any section of airfoils that interested them for demonstration ..
The tests were conducted in a small two-dimensional "smoke" tunnel and were not
set up to indicate the performance of airfoils except as could be visually determined
by observing the pattern of flow through a glass window. About the only significant
characteristic that could be determined was the angle of attack at stall. Nothing
unusual occurred with the exception that one test specimen stalled at an angle of
attack several degrees higher than the rest.
This particular panel was taken from an old model and was poorly constructed.
Several spars on the upper surface were projecting above the fair surface about
1/16 inch. Mr. Jacobs demonstrated that a piece of string or wire located on the
other airfoils in a similar position would increase their stall angle to equal that
section. He also stressed that a high stall angle was not necessarily important
to model planes and that such devices would be expected to re-duce the performance
at more reasonable angles of attack.
In spite of Mr. Jacobs' warning many of the fellows rushed madly off to cement
balsa strips all over their model wings!
On his returned questionnaire Mr. Johnson stated that he used medium camber airfoils
on all types of models. Nothing in particular led him to develop this section, which
has flight and glide characteristics the same as all the rest. He mentions that
he uses stabilizer which has flat lower surface for ease of construction but keeps
quiet about angular difference. Very uncertain about C.G. position but says it varies.
However, he's definite about construction; lots of spars and all on top surface
only. Very frank about the advantages of his airfoil over others by saying it has
none. Also sees no future for it. We can only say: Mr. Johnson, you speak like a
man who is hiding something very special until the next big meet!
And so the summary of the survey is finished. It may have different meaning and
value to different people. No attempt has been made to distort the issue, and original
wording was used whenever possible. We close by quoting Mr. Bristol who mentioned
that the Clark Y "served me well. I am now too old and fat to chase free flight."
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