DOWNWASH AT THE TAIL

All considerations of tailplane design and rigging must allow for the downwash effect of the wing on the airflow at the tail. The lower the a. r. of the wing, and the shorter the fuselage, the more downwash effect there will be. As a result, the tailplane’s true angle of attack is often significantly less than the geometry of the design suggests. At the trailing edge of the wing, if the planform is roughly elliptical (see Chapter 6), the downwash angle is approximately given by the formula:

„ 18.25 x Cr

Downwash = e = —A—

where A is the aspect ratio and Cl the wing lift coefficient (i. e. not including tail areas in the calculation).

At the tail for an orthodox model layout, the tip vortices will have almost completed their ‘roll up’, so the downwash angle there will be almost doubled. For most design purposes, it is found from the approximate equation:

„ . „ 35 x Ci.

Downwash = e (t)——– A—

With a canard, the forewing is in the influence of the vortices of the mainplane, since, as has been said, the upwash effect is felt ahead of the lifting surface as well as behind. The effect is roughly half as great, i. e. the upwash at a foreplane would be about half the downwash if a tailplane were at the same distance aft of die wing aerodynamic centre as the foreplane is ahead of it

It may be seen from these rough equations that a model trimmed for a high angle of attack, such as a free-flight gliding model, may easily experience a downwash angle of 3 or 4 degrees at the tail. In setting up rigging angle on the drawing board, account must be taken of this. A tailplane set at zero degrees geometrically would, in the downwash, be operating at a distinctly negative angle of attack. In a dive, of course, Cl is much reduced and the downwash approaches zero, so a tailplane set at zero geometrically would be closer to its aerodynamic zero. Many models built with ‘lifting’ tailplanes in fact when trimmed have their tails set, relative to the true local airflow, at a negative angle of attack. The camber then being the wrong way, they create more drag than they should.