The Optimum C/G of Sailplanes

A Caution from Wilhelm Dirks

In the “classic” aerodynamic theory of airplanes the wings generate lift and the tail plane generates stability. Because curved wing profiles are used, the aerodynamic moment generated by the wings, which tries to push the nose down and has to be countered by the tail plane.
For this the tail plane has to produce downward force dependent on the airspeed and C/G. The higher the airspeed and the more forward the C/G, a higher downward force is produced. In a rearward C/G the tail plane can even produce a lifting force.
Most contest pilots trim their sailplanes to the most rearward permissible position. In theory this improves performance, especially in circling flight one does not have to “pull” on the stick as much.  At the “German Soaring Symposium” in Stuttgart a paper was presented and discussed which showed these well known facts in graphic detail for several sailplane types.
In the first phase it was researched just how much the performance was affected by forward or rearward C/G positions. The permissible C/G values are determined by the designer. A forward C/G determines the size of the tail plane and elevator, i.e. that sufficient force is available in the elevator to make circling flight possible. The aft C/G is determined so that the airplane has satisfactory pitch stability and is able recover from a spin. Generally the calculations for all types showed a performance difference of 1.5 to 2 points between the foremost and rear most C/G positions. That is a significant difference! Is it therefore correct to fly with the rearmost C/G? Let’s think about it.

Performance Factors

The L/D of a sailplane is calculated by the ratio of total lift to total drag. If the tail plane produces downward force the wings have to generate more lift, and that causes increased induced and profile drag, reducing the L/D. In spite of this the rearmost C/G does not necessarily produce better results. The tail plane is not designed to produce much lift.  It normally has an almost symmetrical profile.
The wing profile is designed to produce lift and is much better at this task.  In addition the tail plane produces a disproportionate amount of induced drag because of it’s low aspect ratio. The optimum condition would be one in which the tail plane in slow flight produces just enough lift to compensate for the loss of lift of the wings in the fuselage area. That would minimize the induced drag of the sailplane.
This condition is obtained, more or less, depending on the design, by the distance of the tail plane from the most rearward C/G position.

Flight Characteristics and Safety

At aft C/G stability is minimal, especially in the roll axis, and the sailplane must be “flown” at all times to avoid air speed variations usually encountered when thermalling. Depending on experience and skill, the pilot may tire faster and his concentration can diminish, so that the theoretical advantage is greatly reduced. At the rearmost C/G the sailplane will enter a spin much easier at less than the minimum airspeed then at forward C/G, where spinning in many instances is not even possible. This can have deadly results, especially close to the ground. Different sailplanes react differently, but the tendency is clear. Even an experienced contest pilot should be very wary of choosing the extreme rear most C/G position.

The C/G definitely influences the flight performance and efficiency.
A position in the forward half of the C/G envelope produces negative results and should be avoided.
A good compromise is a C/G position about 30 to 40 % forward of the aftmost position.
Flying with the C/G at the rearmost position is endangering your life.
Check your C/G and do a weight/balance of your sailplane, and weigh yourself.

Wilhelm Dirks
Translated by Albin Schreiter – CDN


Add on after the soaring symposium Stuttgart 2000 and Braunschweig 2001:

Members of the Akaflieg Braunschweig have once again focused on achieving an optimal center of gravity. They have therefore completed a number of test flights with two ASH-25 which had been carefully inspected, set up and checked. Both planes were equipped with highly complex measuring equipment due to the expected marginal rate of differences between the two planes. One was declared to be the reference plane, the other was designed to allow altering the center of gravity before every flight.

The test flights have been completed with various speeds and flap settings. The results were later evaluated by computer. The specific results are of no real interest for us. A common pilot flies at various speeds with a variety of flap settings during a normal long duration flight. What was of interest to us was the average result of the test flights. And even without the complete set of results one thing can be noted:

The aft position of the center of gravity is not desirable in order to achieve a really good result. The best results were achieved about 30 – 35 % in front of the aft limit of the specifications for the center of gravity. That is caused by the elevator since it is not in a neutral (streamlined) position with a centered C/G. Instead it has to produce lift (an up elevator deflection) which it’s not actually designed for resulting in increased drag.

Some of our customers who want participate in competitions ask us to carefully weigh the aircraft and set it up at 98% aft center of gravity. Although we generally fulfill all of our customers wishes this is one we should not comply with. The sailplane will not fly any better by achieving this extreme aft center of gravity. It will simply react more nervously to pitch and roll inputs. In fact, should the pilot loose 2 liters  (1/2 gal) of water during a long flight (de-hydration!), the sailplane has exceeded the specified limits of C/G and control in all axes becomes very sensitive.

Please choose a center of gravity that is 30 to 35% before the rearmost limit of the aircraft’s specification.
This will grant an optimum in safety and performance.


A Comment:

Hello Friedel,

Thank you for your newsletter which is excellent as usual.  I have a comment  about item  “4.  The Optimum C of G of Sailplanes – new results”

You say “Flying with the c/g at the rearmost position is endangering your life.”  I am glad that you make this point because I myself have nearly
crashed after an accidental spin and my wife did crash after an accidental spin — luckily she was only scratched.

We are both BELOW average weight.

So in many sailplanes we are “Flying with the c/g at the rearmost position” and as we both know that “is endangering our lives.”.  This is not through
choice, but because most gliders are designed for the average male pilot and have a MINIMUM cockpit load of 70Kg.

Last November we hired two modern gliders (actually not DG gliders) in South Africa and we had to specially ship 6 extra lead weights to Africa.
Luckily  we anticipated the problem and were able to ship the lead weights in advance  in the container rather than carrying them in our luggage.
If we had arrived in South Africa without these weights would we have canceled our holiday, or perhaps flown with a dangerous c/g position?
Like you we put safety first because we have been gliding long enough to have several good friends killed.
But someone less experienced would be tempted to fly with an aft c/g not for performance reasons, but because the alternative is not to fly at all.

I believe it would save lives if all new sailplanes were manufactured with a much lower minimum cockpit weight so that the majority of pilots can fly
with the c/g forward of the rearmost position.  And for those who are very light there should of course be a simple way to secure ballast in the front
of the glider.

Why do new gliders have a MINIMUM cockpit load of 70Kg?

With best wishes – Phil King

Dear Phil

Thank you very much for your interesting posting.

To answer your question:
There is no problem to realize a minimum cockpit load of 60 or even 50 KG. Especially with our DG-808S we can vary the C/G in a wide range by installing the battery in the luggage compartment or in the fin, by using a light plastic tail wheel or a heavy brass one.

For the DG-1000 we even have a trim box.
A little problem is given with the DG-808C because of the heavy engine compartment, but we also can adjust the C/G in a certain range.

The problem is not the minimum but is the maximum cockpit load.
The difference between these two limits is about 36 KG only and that is not too much. For example my weight is about 92 KG :-(((
Plus clothes, parachute, a breakfast and a water bottle it is about 102 KG, so I am flying near of the maximum weight and of the formost C/G.

It is easily possible to add additional weight by a lead cushion, the normal trim weights and a special trim weight, which we offer for the foremost fuselage position. But we cannot increase to maximum weight for heavy pilots.

The “normal procedure” is a minimum weight of 70 KG. But we ask your customers for their weight and their preferred C/G. So we also can deliver gliders with a C/G exactly according to the customers preferences.

Okay?

Another Comment of Phil King:

It is also easily possible for a lead cushion to slip and jam the controls.
A lead cushion should only be used when it is securely attached to the glider by some means that is properly constructed for the purpose.

Some small people may not agree that it is easy to add additional weight by a lead cushion because they find it difficult to lift the weight.  The weight of a lead cushion on the seat has to be much greater than a lead weight placed further from the C of G of the glider.

 

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