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The Fastest Thorp T-18s
This article and its photographs was published in the Winter, 1977 edition of Homebuilt Aircraft. You may click on thumbnail above to see the cover. Saved by Bob Klipp, in May of 2011 the magazine was loaned to Grant MacLaren, who made this website. Grant flew many hours with Howard Henderson in his T-18, N600HH, and wrote the Sport Aviation article about Howard's Pietenpol Air Camper, 444MH. (Read the Piet article here.)
In 1962 light aircraft enthusiast John Thorp of Sun Valley, Calif., brought out his first drawings of his soon-to-be-popular T-18. He was building the all-metal low-wing design as he was drafting. He knew his ship would be a success. And it has been.
Brought into the world as a very simple design, the ship had no canopy so many owners made their own changes in the design. Thorp has also come out with improvements that include the use of power up to 180 horses up front. The top speed will often top the 200 mph mark with a crew of two in the small metal design, that has a wingspan of 20 feet 10 inches. Many ships have been built from Thorp plans but not all ships end up with similar performance.
One such ship is one built and flown by 57-year-old aircraft engineer Howard W. Henderson of Kirkwood, Missouri. First lift off was in October, 1974 after a building program requiring a total of five years of spare time.
The Missouri homebuilder has ideas of his own on the performance of the all metal T-18 and how to get more miles per hour at cruise speed. According to Howard, flight and test engineer in the aircraft industry, the best results will be realized by testing the airplane while building. "In the case of the T-18 I was in a hurry to get in the air and as a result I didn't clean it up as much as I wanted. Didn't make it as low in the drag department as it should have been." But Howard Henderson has done much gathering of drag data and has compiled a set of drag curves for a family of T-18s. He has also plotted what the drag is on his T-18, and where it is on the curve chart. "Now I know what my ship will do," Howard informed me, "at any given altitude, temperature or under any given conditions." The Howard Henderson-built T-18 is flush riveted and relatively clean when compared to the average T-18, although the ship does not have wheel pants.
At the last Oshkosh Fly-in, Henderson asked several engineers who have been gathering data if they could run checks. "you know, one has to be very careful about gathering this knowledge. You have to run a trailing cone test to check your static source. The air speed and manifold pressure gauges have to be calibrated. Then you have to secure the engine performance curves from Lycoming. This allows you to take the data you are getting, go to the curves and correct for temperature. Then you will know the amount of power you are putting into the airplane. The one assumption that Henderson has made on his ship and the other aircraft he has compared it to, is that he is getting 85 percent efficiency on the propeller. This is about average on a metal prop. During these tests, results were obtained on a wood propeller that could be compared with a metal prop used on the Henderson T-18. The wood prop was quieter, and ran smoother with no flutter problems.
 Please excuse the photo quality -- it was printed across the gutter, and the paper is old. That is the reason the Sensenich wood stick was brought out for use with the T-18 aircraft. The Missouri-based T-18 uses a metal propeller (at this writing ... gem). As Howard informed me, "I know where to red line my metal prop. I don't run my ship as hard as I could if I had a wood propeller. I think my metal prop is just a little more efficient than the wood type." He explained that he didn't want to take anything from the wood design and the cost is about the same starting out from scratch, however, a new owner should start out with one of these new wood propellers, to eliminate worry about prop fatigue. The propellers on homebuilts face higher stresses when pilots fly into the higher altitudes. The first mood resonance. Howard explained, occurs at 2600 or 2700 rpm. "To be on the safe side, I never exceed 2500 rpm which limits me to cruise at 55-60 percent power. Up at 9,000 feet where it's nice and cool and where you like to do your cross-country flying, my limit is about 55 percent. But you get better economy. Now, if the small plane pilot is flying a 400-mile trip, this 55 percent is just about where he likes to fly. The Henderson T-18 will cruise at 160 mph with the engine revs at 2,450, using 19 inches. This 55 percent horsepower is nice and easy on your engine. What about cleaning up of an aircraft? Is it worth all the trouble? Is it that difficult? Henderson went on to explain to me that if he had his ship as neat and clean as this fellow up in Minnesota . . . "His name is B. C. Roemer and his son is a very fine engineer. We have been trading data back and forth and he has come up with much useful information. I understand that he has run a very careful trailing cone test on his static source, has double checked his calibrations by running on a ground course." There is a very interesting comparison between the Henderson-built T-18 and the Roemer Thorp, now thought to be the world's fastest T-18. Howard noted that his ship has an equivalent flat plate area of 2.8 sq. ft. while the Minnesota ship is very low with but 2.35 sq. ft. I asked Howard if the difference was all that great? "Well, it represents about 20 percent difference in drag. Of course this 20 percent is a little difficult for the average observer to note just by looking at both aircraft. You have to note all the little details like canopy fit, detail finish and the close attention to close fitting of various structure members. My ship has no wheel pants but according to Roemer they don't really help that much. What about the mounting of the landing gear and higher speeds? Fairings on the gear will add much to performance. The T-18 enthusiasts discovered that added fairings alone will increase the speed about seven miles per hour. This has been proved to be far more important than adding wheel pants. Henderson told me that be has much turbulence at the intersection of the landing gear leg and the fuselage fitting. As a matter of fact this was so great that reversed flow caused problems in the fuel vent system. As the fuel vent is manmade about two inches from the intersection, the pilot discovered, during the first five hours on the ship, that when the fuel got down to within 12 inches of the bottom of the tank the engine would starve. Henderson cured this by mounting a set of fairings and moving the fuel vent in front of the gear. What the T-18 builders are trying to learn, and what Henderson wants to explain, is the difference in absolute drag characteristics between his ship and Roemer's. "We will try to set them side by side but it would be a problem to show what was better between the ships. Many of the small points would be impossible to show." The T-18 built and flown by Howard Henderson is a very fine performing aircraft but anything can be improved. What would Howard do to his plane without altering the overall appearance? "I might do several things," he said. "Say I had the ship home for about six or eight months. I would have to refit all the canopy fittings using flush screws. "I would also have to flush mount the wing tips as they are now overlapping the thickness of the fiberglass sheet. Taking up the landing gear fairings, I could make small fairings where the gear meets the underside of the fuselage. Then I could come up with a good pair of high speed wheel pants. My ship has a two-inch venturi tube, I could remove that and use a vacuum pump or figure out a different way to secure vacuum. I use a vacuum scoop under the airplane which is also causing a little drag or turbulence. The Missouri builder told me that you can't measure the amount one can gain with this amount of "clean up" but he thought it would help the difference between his ship and the quick design flown by B. C. Roemer. "I think I could gain about 12 mph and this is much cheaper than adding power. Using a little math, the speed is inversely proportional to the cube root of the equivalent flat plate and to the cube root of the horsepower. In other words both of these things affect speed mathematically the same way. If you wish to go twice as fast in a given airplane, you would have to have four times the horsepower. But you can also clean up your airplane, and it will affect it in the same ratio." By a 20-percent improvement in cleanness, the Roemer ship is gaining about 12 mph at a given horsepower. Now, of course, the Roemer ship is using the 180-hp mill. This brings up another fact. According to Lycoming their 180-hp engine is a more efficient power plant. If both the 150 and 180 hp were to run at 100 hp the larger engine would be getting about 12 percent better mileage. The 180 will use less fuel, has higher compression ratio, and this means better efficiency. When you recall the Roemer ship has 20 percent less drag and the engine puts out more power, the ship is a very, very efficient aircraft. Howard Henderson thinks this T-18 is the most efficient one in the air today. Henderson thinks this T-18 is the most equivalent flat plate and to the cube root I efficient one in the air today. of the horsepower. In other words both of I The homebuilt high speed enthusiast these things affect speed mathematically I must keep in mind the free breather carb the same way. If you wish to go twice as I inlet. The builder must mount a filter fast in a given airplane, you would have to I which will not restrict the flow of air and have four times the horsepower. But you also allow very little pressure drop. If the can also clean up your airplane, and it ship has a nice clean inlet the result will affect it in the same ratio." By a I be almost a supercharger effect. The 20-percent improvement in cleanness, the I engine will pick up an extra inch of Roemer ship is gaining about 12 mph at a I manifold pressure at higher altitudes, given horsepower. which will also allow the pilot to maintain power at higher altitudes than a normal homebuilt. There is also the proper pitch of the propeller. For instance, on the Roemer ship the builder has pitched his prop to 83 inches. According to Henderson this is a very high pitch. No one has used a prop with such a high pitch, but the Minnesota based homebuilt is so clean that the advance ratio of his propeller is very high, thus allowing the efficiency to go up. Howard Henderson explained to me that his aircraft has been made as quiet as possible. "I have sacrificed some weight to make it as noise-free as possible, but have not been completely successful. I have even researched the idea of using lead to shut down the noise. One does not think of using lead in aircraft but it has been used in commercial types. The material is composed of a sheet of lead between two layers of foam. I keep the engine noise down with the mounting of a pair of Pazmany mufflers. This has helped to a great degree I think." Future programs will include the taping of the canopy in an effort to locate noise level. There seems to be no perfect system of sealing a large canopy, which is known to be a chief source of noise. They all have little noises or whistles. Henderson expressed a great interest in gathering more data on improving the T-18, especially regarding the reliability and performance as the ship is intended to be a fine cross-country ship. This means long distance flights at good speed and comfort plus economy. As Howard and I returned from the airport, after photographing his pretty metal design, he told me that of course he could increase the performance a great deal but this would result in disfiguring the airplane. "I don't wish to go that far," he said with a grin. "It's a very attractive design just as John Thorp said it would be back in 1962."
It must have been about 1974 when Howard asked me if I'd like to go flying? We were having coffee on a Sunday morning at Eliot Chapel. "Of course I would," was my reply and we struck up a friendship that lasted 'til Howard died in 2016.
We made many flights in the T-18; some cross-country, some in the neighborhood. We flew to OshKosh where we were mis-identified by the control tower, making for an interesting landing. And we flew to Brodhead after flying into a sucker hole. I'll take credit for finding our way -- I recognised a round barn southwest of the airport on Highway 11. The photo above was taken at Arrrowhead Airport which closed shortly thereafter.
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