1988 SP Air Race – Improving your chances – 2

1988 State President’s Trophy Air Race


A series of three articles written by Vic Kaiser prior to the 1988 Air Race

Part 2

In last month’s article we looked at Navigation, Terrain, Wind and Draughts associated with terrain. As promised we will look at a few more factors starting with DRAG.

We all know what drag is and how it affects our aircraft in general, but lets take a closer look at the various components of drag. We can divide TOTAL DRAG into two major components viz ZERO LIFT DRAG and LIFT DEPENDANT DRAG. Firstly lets analyse zero lift drag.

Zero lift drag can be further divided into 3 parts viz:

  • 1. Surface friction drag
  • 2. Form drag
  • 3. Interference drag.

Surface friction drag, as the name implies, is dependant on the smoothness of your aircraft’s skin. If you polish your aircraft well and remove any unnecessary “dents” in the leading edges of your aircraft, you will minimise surface friction drag as the whole surface area of the aircraft has a boundary layer, and thus causes surface friction drag. Form drag is a factor that really only the manufacturer can minimise in the design of the aircraft. Likewise the interference drag which, for instance is found where the wings meet the fuselage etc. As a competitor in this event, you are not allowed to “fill” the rivets of the fairings in order to reduce interference drag, as that renders your aircraft as non-standard.

Next we look at LIFT DEPENDANT DRAG which we categorise into:

  • 1. Induced drag or vortex drag.
  • 2. Increments of;
    • (a) Form drag
    • (b)surface friction drag
    • (c) Interference drag.

As you remember from basic principles there exists a positive pressure below the wing and a negative pressure above the wing. Now the difference in pressure between upper and lower surfaces causes air to spill around the wing tips and to form vortices. The effect of these is to produce a downflow past the wing additional to that resulting from the production of lift. This additional downflow causes additional drag and is known as induced drag.

The factors affecting induced drag are:

  • 1. Planform
  • 2. Aspect ratio – as a pilot you have no influence on either 1 or 2.
  • 3. Lift and weight.
  • 4. Speed.

Consider weight. The higher your weight the more lift required, at a given speed to support this weight. If you require more, lift, you require a greater pressure differential between upper and lower surfaces and hence a stronger vortex resulting in higher induced drag. Keep your aircraft weight to a minimum whilst making sure it remains standard.

Lastly we look at SPEED. At a given weight. lift remains constant during level flight. Should you now increase the speed in our lift formula
L=Cl .5 P V2 S
we can see that in order to keep LIFT constant we must reduce our Cl (angle of attack). Now from the previous discussion, when we do this, we decrease the pressure differential between upper and lower surfaces thereby decreasing the vortex as well as the induced drag. After all the purpose of this race is to fly as fast as possible, and if you do this with your minimum permissible weight. you will minimise your induced drag.

Next we will look at C of G position. If we have an aft C of G we need a “lifting” force in level flight. In order to get an upward moment from the tailplane our elevator will be deflected downwards slightly, the mean camber of the whole tail section now being the same as that of the wings. The tail section now produces additional lift and if we look back at our lift formula, we can see that with all other factors contact, the V2, being TAS, must increase.

However, small and insignificant these factors may seem, remember that if you gain 1 knot at a handicap speed of 150 knots, you should gain 52,45 seconds over an average course distance. Now if you can improve your performance by say 3 knots. you gain over the whole race is already 5 minutes over your fellow competitor, in a similar aircraft who has not prepared his aircraft to fly only 3 KNOTS faster.

And finally consider your engine and your propeller. Your propeller is like a rotating wing and exhibits the same properties as an aerofoil. So if you file out all the “nicks” and polish both sides of your propeller, without removing the paint as such, you can reduce your propeller drag and at least gain 1 of the 3 KNOTS you are looking for. Set your mixture at the correct setting for your power setting and remember that provided you keep the engine parameters below the red line, you should not damage your engine in the race. The final do’s and don’ts will be continued in the next issue.