Fokker Triplane Flight Characteristics
Originally printed in the EAA's Sport Aviation, March 1983 issue. The author is Ed Lansing, who along with Gary Shepherd built the plane.
This article will discuss the flight and primary aerobatic characteristics of a replica Fokker DR-1 Triplane.
The Triplane was constructed by Gary Shepherd and myself over a 2 ½ year period taking 3000 manhours to complete.
The project has been flying since August 1980 in the Houston, TX area and now has 130 hours of flying time on it.
The construction of the project has been described in Jack Coxs excellent article in SPORT AVIATION (December 1980), so there is no need to repeat those details here.
The Triplane has some interesting flying characteristics and this article will attempt to describe them.
Data Sheets and Information
Refer to Tables 1, 2 and 3 for the so-called "numbers" on a Fokker Triplane. Bear in mind that the project is an exact replica of the WWI original with respect to size, dimensions, rigging, control movements, etc. We tried to build a replica without any compromises in the original configuration. The only modern components we have are a tailwheel instead of the original skid, a Warner 165 instead of the original Oberusel rotary engine, and brakes for the wheels. Otherwise, our replica is the same as flown in the 1917 WWI period.
Taxi and Take-Off
The combination of round cowl, nose-high ground attitude and mid-wing position of one of the wings make the Triplane virtually "blind" on the ground. Lots of S-turns and maneuvering are required. To avoid noseovers we have the brakes set very lightly. They only hold up to about 1200 rpm so braking is marginal and for steering only. Turns have to be planned well in advance.
Gary and I now know why the mid-wing has the half-moon cutouts near the fuselage (so you can see). If you go slow and easy there is really no problem in taxiing and ground handling.
The take-off run is sometimes exciting because you cant see straight ahead at the start. I give it full power, stick all the way back and peek along the fuselage or sight on a cloud for initial direction. The Triplane accelerates very quickly and the tail can be lifted at about 30 mph. Take-off is at around 40 mph after 250 to 300 feet ground run. Visibility is excellent once the tail is up and you can see over the nose. We have a nice rate of climb of 1500 ft./minute at 70 mph so you can fly some tight patterns. In any kind of a crosswind its best to keep the tail on the ground until speed is reached and then "pop" the tail up into the flying attitude. Take-offs present no special problem other than visibility at the start of the ground run.
The first thing you notice about a Triplane is that it is definitely a "rudder" airplane and you have to keep on the pedals constantly to keep the ball in the center of the slip-skid indicator. The elevator is excellent with smooth, powerful responses at all speeds. The ailerons are somewhat heavy but I consider the roll control good. There is a large amount of adverse yaw since there is little aileron differential. If the ailerons are quickly displaced, the Triplane will yaw 20 to 300 if rudder connection is not applied.
The rudder control tends to be marginal. All turns must be coordinated and the ball really rolls around the cage during a turn. Turns are not very stable and I have to constantly work the rudder to keep them coordinated. Only a slight amount of back pressure is needed during turns, even steep ones.
At cruise speed of 90-95 mph, the Triplane is very nimble, quick and responsive. You can really move it around if you keep on the rudder.
The Triplane tends to fly tail heavy and increasing speed requires lots of forward stick to hold the nose down~ It appears as if the effective center of drag is above the thrust line so the faster you go the more elevator correction is needed to hold the nose down. This is somewhat of an explanation for the familiar "tuck" position of the Triplane at high speed. The horizontal stabilizer is rigged at a plus 90 (leading edge high) but even with this forward stick pressure is needed as speed increases. The high lift wing airfoil shape really wants to pull the plane up. All the wings are rigged at 11/20 angle of incidence.
Stalls occur at about 40-45 mph and the Triplane "mushes" down without a definite nose break. It is a little like a parachute in descending.
General visibility is excellent except for the mid-wing placement. In the traffic pattern you have to "bob" the nose up and down to keep visibility. Otherwise, the cockpit is comfortable, roomy and lots of fun to fly.
Here is where the real adventures begin. Approaches are made at 70 mph and visibility is excellent over the nose. With its high lift/drag configuration the Triplane really comes down steeply at about 1800 ft./minute in a "glide". I use wheel type landings at 95% of the time since its the only way to see. I only try full stall landings in a dead calm. Once the wheels are on the ground, I hold the nose up, slow it down, and then pull back the stick to get the tail planted and firmly on the ground. When the 3 wings stall out and the tail starts to come down, the rudder is effectively blanked out and directional control disappears. With the marginal rudder control you are essentially a passenger during the "twilight zone" of transition between flying and rolling. So, when the tail comes down, its best to be headed in the right direction. Fortunately, the slow landing speed of 40 mph makes for short landing distances. With any kind of a headwind, the Triplane stops in 250 to 300 feet after touchdown.
Crosswind landings are very difficult to impossible to handle. With any kind of crosswind component (3 to 5 mph) these exercises are a real adventure. Before I flew the Triplane, I got lots of advice from the old pros Walt Redfern and Jim Appleby on how to handle this. The consensus on crosswind landing seems to be as follows:
a) Keep it in the hangar if a crosswind is there.
b) Always land into the wind. Land diagonally, off to the side, on taxiways, aprons, etc., to minimize the crosswind component. Our home field has a 200 ft. wide grass strip and most landings are on a diagonal directly into the wind.
c) As a last resort, land on the downwind side of the runway with a wheel landing, slow it down and let it weathervane up into the wind then the tail comes down. With the large wing area and wide fuselage it will weathervane.
My EAA associates in the Houston area kid me by saying with the short fuselage, small rudder, aft CG and large side area, I have "everything working for me" on landings. I now know why the original Triplane has the wing tip skids on the lower wing. However, once the tailwheel is on the ground at low speed, good control returns. The transition zone from flying to rolling is the dicey area. Sometimes you can use cross-control techniques on the landings by using the "down aileron" as a speed brake to help directional control. In summary, every landing is a real experience especially in any kind of crosswind.
Gary and I have taken the Triplane to numerous Texas Gulf Coast air shows and we have tried some primary aerobatics with it. Bear in mind that the following comments are from the perspective of private pilots with 600 hours flying time. We do not consider ourselves experienced in aerobatics and we stick to the basics.
Loops Loops are easy and fun in a Triplane. From cruise condition drop the nose to pick up 110 to 115 mph entry speed. A smooth back pressure of 3 Gs produces an amazingly tight loop. With the high drag, it is very easy to control it on the down side coming out. Caution: Keep the ball in the center or it can really come out of the loop cockeyed.
Hammerheads This is another good maneuver. With entry speed of 110 and a 3 G pull up you go straight up with good control. Rudder kick at the top produces a kind of pirouette rather than a turn and you come straight down. The high drag of the wings ooming down makes exits easy and quarter rolls can easily be done coming out of the Hammerhead.
Rolls The Triplane can be rolled but this maneuver gets difficult due to the differences in the control sensitivities of elevator, aileron and rudder. The best way seems to be to use high speed entry of 110, nose up 30º, full aileron and hope for the best. Lots of rudder correction is needed to overcome the adverse yaw. Once the roll starts indiscriminate use of the rudder or elevator will quickly stop the roll with some interesting results.
The best rolls seem to happen when you use very little corrections once a roll starts. Big barrel rolls also seem to be better than rolling on a point. Slow rolls are very difficult because of the control sensitivity. The bottom line is, keep it fast and quick with lots of altitude for corrections if you fall out of one.
Split-S This is another easy maneuver. With its quick response, the Triplane comes quickly around. Sometimes I can build up airspeed to 110 mph, haul the nose up 300, half roll at 80 mph and exit in the opposite direction at entry altitude.
Immelmans Tough to do because of low speed at the top of the loop. The high lift/drag airfols really slow down at high angles of attack.
Inverted Flight, Negative Gs, Tall Slides, Snaps, Etc. These maneuvers are totally unsuitable for a Triplane. It is not configured to try any of these.
The best description of flying a Triplane is that it is "fun after the first frightening hour". Once you get used to its characteristics and quirks, it does very well, provided you keep within its limitations. It is desirable to stay well inside the flight envelope. It is easy to see why it has the reputation of being quick and nimble. It is also easy to see why WWI airfields were all round or square with the wind sock in the middle. If you always land into the wind, the Triplane is reasonably easy to live with and a lot of fun.
I guess thats why Gary and I went to all the trouble in the first place.
All material not specifically credited is Copyright © by Randy Wilson.
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