The CofG of a sailplane can have a considerable influence on the flight characteristics and performance. As long as it is flown within the tested weight and CofG limits a sailplane is always safe to fly. But the CofG limits for optimal performance of a sailplane are quite small. Because the CofG of the sailplane changes with addition of weight (pilot/passenger) basically two things are necessary to change the situation by adding trim weight when adding water ballast or varying the cockpit load. The latter is especially important for two-seaters because a broad range of pilot/passenger weights has to be accommodated.
1. Water Ballast
In nearly all sailplanes the ballast tanks are located in front of the main spar, that is forward of the CofG. This is not ideal, but cannot be avoided. It cannot be put into the spar, and behind it are the aileron and flap rods. So one puts the water bags in front, or creates waterproof compartments in a part of the wing. The latter has the disadvantage that after a few years one often can see the location of the tanks on the wing surface.
If the ballast tanks are located in front of the CofG then it changes when the tanks are filled. To off-set this it is usual to add an additional tank in the vertical tail fin. However, it is essential to ensure that the wing tanks cannot be drained without also draining the tail tank, because that could lead to a dangerously rearward CofG. That is why the valve handles are constructed in such a way that both wing and tail tanks are drained at the same time. This is also an airworthiness requirement.
So far this is nothing new. The DG-1001 also has a normal tail tank to off-set the wing tank water ballast.
2. Cockpit Load
The consideration of varying cockpit loads is of equal importance. Normally a sailplane is delivered so that the minimum cockpit front seat load is 70 kg. A variation in the front-seat weight is more noticeable in a two-seater because of the longer leverage.
The second (rear seat) load can have different results. In an old K-7 the passenger sits almost atop the CofG, resulting in poor lateral visibility. In newer types both pilot and passenger sit in front of the CofG. In the DG-1001 one needs 1/10th of the passenger’s weight as off-set in the tail to maintain the trim. One can put a suitably large tank into the tail fin and add or subtract water every time one changes pilot or passenger. But who would do that?
The ASK-21 has no adjustment.
The DG-505 was more advanced and offered two tail tanks – one in conjunction with the water ballast system, and a second one to optimize the CofG for varying cockpit loads.
But the handling of water canisters is not much fun, and, although it was progress, we were never altogether happy with this solution.
That is why we thought of the more costly solution of a trim box.
A compartment in the tail fin with 6 slots can take 6 exactly fitting brass weights. 4 pcs. have 2,4 kg each and 2 pcs. have 1,2 kg each – together it is 12 kg. We chose brass because it is heavier than iron and harder than lead. Lead weights would cause safety problems. Drop one on it’s corner and it would never fit properly again. But the fit must be exact to make sure the weights don’t rattle around.
The box has a transparent cover, so that the number of weights is immediately visible. The cover must also be quite strong. The rear of a sailplane is often subject to hard landings. All this causes considerable extra construction costs, which we feel are worthwhile to guarantee proper trim at all times.
The DG-1001 well get an electronic indicator in the cockpit showing the amount of ballast in the tail. Even though you can clearly see from the outside whether you have weight in the trim box, it sometimes happens that the pilot is strapped into the cockpit and asks himself, Have I taken the weight out of the tail or not? He then simply presses a button and an LED next to it tells him how much ballast he has in the tail box.
There is nothing better or safer!
The brass bars can also be used as forward trim weights if a very light pilot flies solo, perhaps even his first solo, and wants to have the same CofG as with two persons. A often not very comfortable lead bag under the seat becomes superfluous.
An ASK-21 with two persons on board has of necessity a far forward CofG – that is why it does not spin!
In a Duo-Discus with water ballast you normally also will fly with a far forward CofG.
In any event, the constant changing of the fill amount is not very desirable in club operations, and therefore is often ignored. But that has an undesirable effect on performance.
And if the rear ballast is carefully brought to the rearward limit specified in the flight manual the DG-1001 will surely spin. And that is what many instructors are waiting for. How else can students be familiarized with a spin?
But be sure to double check your weight and balance for this type of flight.
The trim box itself comes as standard – after all it is not practicable to produce two different types of stabilizers. The specially designed trim weights and the display unit however have to be purchased as optional extras. Please don’t try to build your own homemade weights. They probably won’t fit properly, and using the trim box without the display unit is not permitted anyway.
Additionally to the trim weights we deliver a beautiful plywood box, so your weights always have a proper place among other accessories.
translated by Albin Schreiter, CDN