Negative Stagger

An analysis of the Beechcraft design
by Winfield Foster

An airplane with a wing span even smaller than our light planes of today, with a wing loading only slightly higher, resulting in a landing speed of not more than 50 miles per hour and with the ability to make an emergency landing with a run of less than 100 feet — yet able to carry five passengers at a normal cruising speed of 190 miles per hour, and to climb 1,500 feet per minute — those specifications pretty well sum up what the advanced private flier is going to want in the way of an airplane after the war, in the opinion of many aircraft company executives.

Maybe 190 miles per hour is more than will be required, the designers say — but no one is going to object to high speed if it can be combined with a reasonable landing speed. And for real utility, for ability to cover ground in a hurry regardless of head winds, a speed above 150 miles per hour is considered minimum to provide a real margin between air travel speeds and ground transportation speeds.

It won't surprise many aeronautical engineers, but it may be news to the nation's hundreds of thousands of fledgling fliers and prospective airplane owners, that specifications like the above have been achieved and proved in performance for nearly a decade in the biplane manufactured by the Beech Aircraft Corporation of Wichita, Kansas. The model specifically referred to is the Beechcraft D-17-R.

Furthermore, these specifications were achieved with prewar aviation fuels, with octane ratings as low as 80, and with a Wright Whirlwind engine, model R-975-E-3, designed and built nearly 10 years ago, and developing 420 hp. The advantages inherent in newer fuels and more advanced engines should result in higher cruising speeds and lower landing speeds, due to reduced weight, with the same developed horsepower, or, alternatively, in a substantial lowering of weight and consequent increased economy, while holding to the same performance figures.

The Beech D17R carries an ATC numbered 638, which places its design and approval by the Department of Civil Aeronautics of the Department of Commerce as far back as 1937. Right now, with no changes in design, this airplane is being produced in quantity for the Army and Navy, for the speedy transport of personnel and supplies. The ship's useful load of 1,685 lbs, and cruising range of over 800 miles, add up to a real utility airplane.

The distinguishing feature of the Beechcraft biplane, which enables it to be readily identified as far as it can be clearly seen, is the unique arrangement of the two wings, as shown in the photograph. The lower wing is placed substantially forward of the upper wing — the reverse of traditional design.

This design is known as "negative" stagger — and in many ways it has proved a positive virtue.

Take, for instance, the most important element of airplane performance — behavior in a stall. An airplane with bad stall characteristics, such as a tendency to stall without warning, and to develop a spin without extreme provocation, just isn't made for the average pilot.

In this respect, Beechcraft's negative stagger has proved its worth. In actual performance, the ship will approach a stall, recover automatically, approach it again, recover, and continue until the condition is corrected by releasing backward pressure on the control stick, and gaining flying speed.

Even if the ship is completely stalled, it will recover as above, unless the stall is made intentionally violent, when it will recover in the same manner as any conventional airplane.

The reasons for this performance are bound up with another element of design known as negative decalage — which means simply that the angle of attack of the lower wing is greater than the angle of attack of the upper wing.

This means that when a stall is approached, the lower wing, having a higher angle of attack, will stall before the upper wing. And since the lower wing is forward of the upper wing, the airplane will be nosed down, pivoting around the center of pressure on the upper wing. As soon as the ship noses down, the lower wing regains its lift, the stalled condition is ended, and the ship is again under full control unless another stall is developed, when the above process is repeated.

To be specific, it is possible to stall a Beechcraft in a steep bank with power on, trim the stabilizer to maintain this condition, and then sit there with hands and feet off the controls while the ship bobs and dips and wallows its way around a turn, with never a tendency to spin, even under what are ideal spin conditions for the ordinary airplane. This unique flight characteristic, and vital safety factor, is, of course, materially helped by the slight dihedral of the wings, and the washout of the lower surfaces near the elliptical wing tips.

Other advantages of negative stagger, some of them pertaining to flight characteristics and others to servicing and handling on the ground, are as follows: The forward position of the lower wing enables landing gear to be retracted flush with the bottom of the fuselage. This does two things — it makes possible higher speeds because of decreased drag, and it also permits belly landings to be made in emergencies, with a landing run of less than 100 feet, and with positive protection against nosing over regardless of the landing surface. A normal belly landing, according to experience, results in damage to the aircraft aggregating less than 4% of the total cost of the plane — which adds up to cheap insurance for plane and passengers.

This ability to make belly landings means that the airplane can be put down safely and with minimum damage on virtually any kind of fairly level terrain — marshland, sand, or water. Test pilots who have intentionally made belly landings report that they are easier to make than regular landings, as the cushion of air at ground level tends to level off the wing for a perfect landing position.

Just as the forward position of the lower wing permits the landing gear to be perfectly streamlined when retracted, so does the rearward position of the upper wing permit a perfect streamline shape for the upper fuselage. In addition, the rearward position of the upper wing enables the pilot to get a good view above and behind — an important factor when landing at a crowded airport.

A time-saving feature under present airport conditions is the fact that the attendant can stand on the lower wing while filling the upper wing tanks, thus eliminating the annoying and apparently inevitable delay while a stepladder is hunted up and adjusted.

The lower wing can also be used as a convenient step in getting into the cabin, and makes unnecessary the provision of any extra steps, while permitting the location of the door in the most convenient position — between front and rear seats. Although the monoplane has been accepted as standard for pursuit ships and heavy cargo ships — Beech is building large quantities of low-wing twin-engine monoplanes which are serving the armed forces as bomber trainers, navigational trainers, and personnel transports — many aeronautical engineers believe the biplane may prove ideal for personal use after the war.

Requiring a smaller hangar space, being easier to maneuver around a busy airport, permitting extra structural strength because of the possibility of cross bracing, and finally, making possible the use of negative stagger to achieve a positive benefit in the prevention of stalls, the biplane has plenty of what it takes to make a really usable, safe, and economical airplane for post-war use.

This article was originally published in the July, 1943, issue of Air News magazine, vol 4, no 1, pp 30-31, 50.
The original article includes 1 uncaptioned photo and a page of captioned diagrams.
Photo and diagrams credited to Beech Aircraft Corporation.

Diagram captions: [ diagrams ]