How To Improve The Performance Of Small Biplanes By Silas H. Wellman, EAA 4335 304 Rancroff Dr., North Tonawanda, N.Y.

IS unfortunately true that small sport biplanes comIever,Tmonly built by homebuilders have a lot of drag. Howthere is only one way in which a biplane has to have more drag than a monoplane. That is in the induced drag or drag due to lift. Table I shows the power required to create lift by five different airplanes. Airplane (a), used as a standard, is the well-known Baby Ace Model "D" with 26 foot span and a 4Vi foot chord. Allowing for small tips, this gives about 115 square feet of wing area. These ships average about 875 pounds gross weight, giving 7.6 pounds per square foot of wing loading. The other four are all biplanes, and all are assumed to weigh 1,000 pounds gross weight. One might think that

This is because the biplane combination has a substantially lower maximum lift than the monoplane. In the table, however, we have credited this airplane with 1,000 pounds gross weight to make it comparable with the others. This biplane has been figured for a wide (3'/i foot) gap, and two equal wings to give it maximum efficiency. Its figures show what could be done to reduce the power required to create lift in a biplane. Airplane (c) is the EAA Biplane with 20 foot upper span, 18 Vfc foot lower span, 3.6 foot gap, and 3 foot chord which, with tips, gives 114 square feet of wing area.

Airplane (d) is the Smith Miniplane with 17 foot upper span, 15% foot lower span, 3 1 " foot gap, and 3.C6 foot chord. With tips and cut-outs this gives 98 square feet of wing area. Airplane (e) is intended to be representative of the ultra-small biplanes that are sometimes built. It has 16 foot upper span, 14 foot lower span, 3',a foot gap, and 3 foot average chord. This gives 90 square feet of wing

area. Table I is based on propeller efficiencies of 80 percent for cruising at a lift coefficient of .3 or .4, 75 percent for normal climbs at a lift coefficient of .5, and 70 percent for steep climbs at a lift coefficient of .8. This represents a propeller which is just about perfectly matched to the airplane. The table tells the story of increasing power required to create lift as the span and equivalent aspect ratio decrease. Thus we come to the first rule of small biplane building. This is, "the smaller the airplane, the bigger the engine must be." References (1) and (3) also point out that to reduce drag, both wings should be equal or, if the lower wing is shorter the lower chord should be reduced proportionally more than the span. Fig. 1. This Laird biplane once won the Thompson Trophy Race. It shows that a biplane can be streamlined.

the biplanes would weigh less than the monoplane, but this 1,000 pound figure is very well established by many examples of the Smith Miniplanc and the EAA Biplane. Airplane (b) is an example of what could be done to save power and improve the rate of climb of a small biplane. It has 24 foot wing span, 27 foot average chord, and 130 square feet of wing area. This extra area is necessary to give it the same landing speed as the Baby

Ace,

if we could keep the weight down to 875 pounds.

The skin friction drag of the biplane will be only a hair greater than an equivalent monoplane, being due only to the slightly greater wing area required to keep the landing speed down.

Parasite drag, that is drag due to sloppy construction and poor streamlining, is another matter. There is no reason why this type of drag has to be any higher in a biplane than in a monoplane. Unfortunately, the practice of building small biplanes as basically scale models of larger ships leads to little less than disaster in the drag department. References (1)

TABLE 1

Plane Number Lift Coefficient Power to create lift (bhp)

Speed (mph)

a

b

c

d

e

.4 .5 .8 .3 .4 .5 .8 .4 .5 .8 .3 .4 .5 .8 .3 .3 .3 .4 .5 .8 5.6 6.4 7.7 10.5 6.7 7.3 9.2 12.5 9.2 10.6 12.6 17.3 12.0 13.9 16.7 22.4 13.3 15.5 18.5 24.4 100 87 78 61 100 87 78 61 107 92 83 66 117 99 88 70 120 104 94 74

REFERENCES

1) Airplane Design by E. P. Warner, McGraw Hill Book Co., Inc., New York, 1936 2) Fluid Dynamic Drag by Sighard F. Hoerner published by the author at 148 Busteed Dr. Midland Park, N.J., 1958 3) Elements of Practical Aerodynamics by Bradley Jones, John Wiley and Sons, Inc., New York, 1942. 28

MARCH 1963

to pull it at 100 mph. This assumes 80 percent propeller efficiency. Installing optimum cowling and baffles will reduce this to around 20.3 bph. A simple canopy will further reduce this to around 13.6 bhp. Interfence drag, drag due to interference between various parts of the plane, is usually less than commonly supposed. There is one place, however, where it can get out of hand. A fitting located at the juncture of the fuselage and the lower wing will have a drag equal to a flat plate of 17 times its area held crosswise in free airflow. (Ref. 2). Plainly these fittings should have a large fairing built over them or be put inside the fuselage as on the Miniplane.

Fig. 2. The fuselage of this Gee Bee racer shows an approach to streamlining that will give minimum drag on tiny airplanes.

and (2) show that the basic requirement for drag reduction is to maintain a smooth airflow and to reduce interference drag. This means that the fuselage itself should be the widest thing on the airplane. In particular it should be wider than either the engine or the shoulders of the pilot. It so happens that every Continental or Lycoming engine in common use requires a cowl approximately 30 inches in width. Making the fuselage narrower than this will not reduce the drag with an uncowled engine and will increase it with a cowled engine. The exception is if you are willing to build a real racing cowl and fuselage for which see "Rivets" or "Cosmic Wind" racers.

Beyond this point drag reduction consists of: 1. Flush construction 2. Elimination of unnecessary parts in the airstream 3. Detailed attention to every juncture and fairing on the ship. Reference (1) is excellent for information on this process. Reference (2) is more detailed but very abstract and difficult for those not trained in math. In addition to increasing the span and streamlining the ship there are two other ways of "improving" the performance of biplanes. One of these is very satisfactory. It is to simply install an engine big enough to overcome any drag present and still give good performance. The other method is only an optical illusion but is very commonly used. The pilot-static tube is installed under the front of the lower wing and has a collar mounted just ahead of the static opening. This will make it read high at high speed and low at low speed. By this means a 115 mph top speed can be boosted to around 140! Also, a 50-60 mph stall can be dropped to around 30-40 mph!

A medium-sized pilot sitting on a parachute and a Similarly, the rate of climb can be made wonderful thin seat will be 3V2 feet high. Thus our minimum fuseby the following method. Install a World War II surplus 1 lage cross section will be 2 '2 feet by 3Vi feet, but we can R. of C. instrument. These were built to have a nineround off the top, giving 8.05 square feet of cross secsecond lag in their readings. The trick is to get a running tion area. start at full throttle in level flight, then pull the plane immediately into a climbing attitude. Take your reading This area cannot be reduced by shrinking the fusenine seconds later when the climb has "settled down!" lage as trying to do so leads to leaving the engine and part of the pilot hanging out in the breeze. This really You should be able to get 1,200 feet per minute out of anything by this method! runs up the drag. Such a fuselage if smoothly built but with open cockpit and uncowled engine will require about 25.1 bhp

Happy flying!

The Challenged By Peter M. Fergusson From mother earth their spirits rose and they gazed on the sands of Mars. The cloud wreathed Venus their footsteps knew as they groped for the distant stars.

Now they stand in the light of the double suns on the rim of the galaxie; They stand and consider the awesome void as their ancestors pondered the sea.

Frail hulls of metal enclosed them round for they were frailer still, But they wrote on a parchment of beacon suns and comets served as quills.

Till the secrets of God and the universe are theirs to know and see They'll forge an empire of conquered suns to fulfill man's destiny. SPORT AVIATION

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