BEFORE FLYING YOUR HOMEBUILT . . . (Coninued from page 30)

Suddenly on December 11, 1966, I realized that not only had I finished my T-40 but that I also had not flown any kind of airplane in about three years. Right about here is where all of the bystanders begin urging . . . "come on, let's see it fly!" Of course, you can be real

brave and go right ahead and fly it. Some pilots have had enough experience over the years to be able to do this with ease. However, many of us do not fall into this classification, and I admit to the latter. If yours is a two-place ship . . . no problem. You can do as I did, and get a qualified pilot to test-fly the ship and then check you out. However, the single-place ship takes another kind of thinking. The first few test flights of my T-40 indicated several corrections needed; these were done one at a time, each one followed by a test flight, but none of them were of a serious nature. The changes all took time, and it required over 90 days of work and testing to take care of all these little problems. All test flying was done by James Dixon, who is not only a very able pilot but also one who has

IMPROVE PERFORMANCE THROUGH TUFT-TESTING By Luther D. Sunderland, EAA 5477

(Editor, T-18 Newsletter) 5 Griffin Dr., Apalachin, N.Y.

flown continuously.

While Dixon was flying my T-40, I was able to get in a bit of time in several aircraft such as the Piper "Cherokee" and "Colt", Cessna 172, etc., having problems in finding an aircraft available on weekends when I had the free time. The highlight of this entire period was

flying these airplanes, and watching mine go by like I was standing still. It certainly did look beautiful . . . at least, it did to me. Finally, Dixon said that there were no more changes

that he could suggest, and there I was with no more excuses. About this time I was beginning to be just a bit fed up with the usual question, "Why haven't you flown

it yet?", and feeling that perhaps I was "chicken" I started out in the T-40 with the slow taxi routine, then a bit faster. It turned out that this was well done, as I found that the nose gear steering mechanism was far too sensitive. I can't put the blame for this on Gene Turner as the entire nose strut rig was mine. I cut the steering ratio in half and tried it again. This time it felt better. I got Jim Dixon to try it and

he said okay. Still with no idea of when I would feel ready to fly, I again got Dixon to go over the whole deal with me from take-off to landing, and saddled up to try more fast taxiing. On the third pass down the runway I saw 75 mph indicated and suddenly decided that now was the time, and before I had a chance to chicken out I pulled her off and away we went. All I can say is ... boy, there's just no comparing a job like this with the store bought kind. After my knees quit knocking together, I settled down to get acquainted

with it, and after 30 minutes or so went back to the airport. The f i n a l approach was too high, as I was expecting

HE AVERAGE homebuilder is probably not aware of T a very simple and inexpensive means by which he can improve the aerodynamic characteristics of his airplane. Anyone can make use of the biggest "wind tunnel" in the world for free, but most of us pass up this bargain simply because of a lack of understanding of the simple techniques involved in tuft-testing an airplane. It is possible to determine some valuable information about the flow patterns of air over the various parts of your aircraft through the observation of pieces of ordinary yarn taped to its exterior surface. The orientation and movements of these tufts during flight reveal the nature of important flow characteristics such as flow direction and premature flow separation. Separation can be caused by many factors, such as wavy skin surfaces, bad interferences at joints of airframe components, air leakage at control surfaces and joints, wrong orientation of certain parts like air scoops, poor streamlining, poor cooling system air flow, and wrong wing tip shape. These problems cause loss in speed and climb performance due to high drag, poor engine cooling, sluggish handling qualities, or dangerous stall/ spin characteristics. The encouraging part is that all of them can be corrected, or at least improved, once the causes are understood. A case in point is a recent tuft-test which I performed on my T-18. During initial flight tests, I found that the right wing would consistently drop off first in a stall. Although there was plenty of advance buffeting as a warning, and control could be maintained with ailerons throughout the stall, I thought it would be nice to investigate and try to correct the condition.

a faster rate of sink; I overcontrolled on touch-down, but

The T-18 wing has a constant chord with no built-in

all in all I was not too displeased at my "solo checkout" in

twist. It is built using simplified matched hole tooling

the T-40. This is really a dream of an airplane, but as in the case of saying "hello" to the girl at the next table, you will do much better if you get properly introduced. The whole point is this . . . unless you are one of the fortunate pilots who can depend on many past years of experience, you should certainly take the time and trouble to get introduced properly to your airplane. This is hard to do, from your own desire to fly your machine after the many months of work, and also in having to answer that old question with "no, not yet." Go ahead . . . be chicken! You may avoid banging up the whole deal.

techniques without the use of jigs or fixtures. All hole patterns are laid out in the flat and transferred from one part to another with transfer templates which are simply 2 in. wide strips of aluminum. In building my T-18 with this simplified assembly technique, once I had the parts made I was able to lay out, drill, and dimple the skins and completely rivet up the four wing panels in just one evening for each one. Monel pop rivets were used except for the exposed rivets which were easy to buck. The wings were checked carefully during assembly with a carpenter's level to prevent an unwanted twist from being

built in. This matched hole tooling technique, developed 32

MARCH 1969

A thing of beauty is the Thorp T-18, N 4782G, built by Luther Sunderland, and the subject of the tuft-testing described in this article.

by John Thorp, is quite accurate and allows the novice to build an airplane with almost "paint-by-number" ease.

edge first. As soon as one tuft in the front row on the right wing showed a flow reversal, the right wing would

asymmetry, which causes one wing to stall before the other.

in the aircraft and no flaps. With 40 deg. full flaps, the stall occurred at 61 mph. To correct the condition, I bent up a '2 by \2 in. angle from a 4 in. long piece of aluminum and taped it to the leading edge of the left wing. By trial and error, I adjusted its position on the leading edge until the stall

However, most T-18 wings have some small degree of

To correct this, John recommends the use of a 4 in.

long wedge-shaped spoiler placed on the leading edge of each inner wing to insure that the inner wing stalls first. But the best location of this spoiler depends somewhat upon the characteristics of the particular airplane. My first blind attemp at installing the spoilers did not help, so I decided to tuft the entire wing. Since my T-18 wing is white, I chose a ball of dark yarn from my daughter's knitting bag to insure a good contrast. First, I wrapped and tied a strand of yarn completely around the wing every eight inches of span, then 1 in. long pieces of masking tape were placed over the yarn on the top side of the wing with a 6 in. tape (see Fig. 1). This seemed like a mighty hairy wing and I had doubts that it would even fly, but I was assured that there would be little effect. After hearing stories about airplanes that could not get off the ground with frost on the wings, you can imagine my surprise when I could not see the slightest change in performance. In normal flight the tufts were all neatly hugging the wing, pointed straight back with the exception of the tuft at the inboard edge of each aileron. This slight wiggle was caused by leakage at the flap/aileron junction. Observation from another airplane showed that the flow was well streamlined at the wing root with no reversals of flow as is customarily found on low-wing airplanes. This verifies that a 90 deg. wing/fuselage juncture is the best choice. It is one reason why the T-18 has a straight center section and a flat-sided fuselage. Tuft tests on another T-18 showed some reversed flow along the fuselage just above the flap. This was cured with a good seal at this point.

drop. This occurred at an IAS of 67 mph with one person

pattern on the left wing matched that of the right wing.

A very precise adjustment of the stall point could be made by placing the spoiler higher or lower on the leading edge contour. With only one spoiler located on the

left wing at the joint between the inner and outer panels, the stall performance was not objectionable. Buffeting

would begin at 75 mph and, as speed was further decreased, the tufts would indicate that the entire inner wing would stall but the outer portion of the outer panel

would not (Fig. 3). The outer portion of each aileron was

in the unstalled region so the ailerons were completely effective in maintaining the wings level. However, John

Thorp said he would prefer seeing the inner wing stall a little earlier, so I added a spoiler to the right wing (Continued on next page)

As I reduced power and speed, the first noticeable

change in the wing flow pattern was a slight wiggle of the last row of tufts all along the trailing edge. As airspeed was further reduced, the turbulence migrated in a V-shaped pattern forward along the joint between inner

and outer wing panels (see Fig. 2). This caused buffeting to begin at about 75 mph. The same thing occurred on both wings except that the pattern on the right wing

got slightly ahead of that on the left wing, and the apex of the separation region reached the right wing leading

The Sunderland T-18 boasts a jump seat which could, if

need be, hold two children. Otherwise, it serves as the baggage area. Note the exceptional interior in the craft. SPORT AVIATION

3J

Pig. 5

IMPROVED PERFORMANCE . . . (Continued from preceding page)

(Figs. 4 and 5). Both spoilers were placed about one foot inboard of the joint between the inner and outer wing, and adjusted to cause the inner wing to begin buffeting several miles per hour earlier than before. With the stick held all the way back and power off, roll control can be maintained with the ailerons even though the stalled inboard portion of the wings is causing severe buffeting. An ex-Navy test pilot, who flew my T-18, said that a person would have to be either asleep or dead drunk to get in trouble in a stall with that airplane. So, the straight inner wing with the outboard dihedral, which may have looked a little unusual at first glance, really pays dividends. According to John Thorp, the V-shaped stall pattern at the wing break is normal on all T-18s. Tuft-tests on other aircraft types with similar outboard dihedral has revealed nearly identical stall patterns. Apparently the slight interference caused by the change in wing direction causes this early flow separation. Finding a solution to the asymmetrical stall problem was only the beginning of a program for cleaning up my T-18. Adding an airtight seal to the ailerons significantly increased their effectiveness and increased speed by several miles per hour. I am in the process of making new airtight seals for the gap covers between the inner and outer wing panels. Next, I plan to experiment with small fillets at the wing/fuselage juncture and also improve the seal around the bottom of the canopy. Air leaks cause drag so, for best performance, they should be eliminated. As nearly as I can determine, I have already matched the original design specifications for the clean configuration of the T-18 as published by the designer, but hope

to improve them even further.

SUNDERLAND T 18 SPECIFICATIONS

Engine . . . . . . . . . . . . . . . . . . . . Lycoming 0-290-G at 125 hp Empty weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826 Ibs.

Passengers . . . . . . . . . . . . . . . . . . . . . . . . . . . 2, plus jump seat Baggage . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Ibs. in jump seat

Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 gals. Gross weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,500 Ibs. Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 ft. 10 in. Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 ft. 6 in. Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 f t. 1 in.

Wing area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 sq. ft. Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600 m i les Maximum speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 mph

Cruise speed at 75 percent . . . . . . . . . . . . . . . . . . . . . 152 mph

Fig. 4 34

MARCH 1*69

Rate of climb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,200 fpm Minimum speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 mph Minimum speed (flaps) . . . . . . . . . . . . . . . . . . . . . . . . . 61 mph Limit load factor . . . . . . . . . . . . . . . . +6 —3 G at 1,250 Ibs. Ultimate positive load . . . . . . . . . . . . . . . . . +9 G (aerobatic) Quick wing removal for towing. (S)