Weight Where? (The Fuel Versus Baggage Dilemma)

By Bob Stephens

A constant dilemma for any pilot must be how much weight is there and where is it? I think that Alan Cozen’s article covers this aspect very well, and its certainly something that I’ve been giving a considerable amount of thought to recently as I start to construct my wings.

In general the standard location for fuel is in the main fuel tank, forward of the instrument panel, and if greater range is required then additional tanks are positioned in the wing lockers. This however means that you are limited to putting baggage behind you with a small amount being put in the wing lockers. As mentioned by Alan this can severely restrict the amount of fuel/baggage you can carry. The problem lies in the fact that lever arm for the rear parcel shelf is nearly 3 times longer than that of the wing lockers. Therefore for every pound of baggage put on the baggage shelf you lose nearly 3 pounds of fuel or baggage in the wings.

 Now if you put the constant weight (i.e. the baggage) nearer the C of G you can start to carry more fuel - especially if the fuel in the wings is over or forward of the C of G (as it is in the leading edge). To illustrate this I have used the table Alan produced for his aircraft’s weight and balance, and adapted it for use in a spreadsheet. It is now very easy to see what affect a change in location and quantity will have on the C of G of an aircraft. I hope that the following few tables will illustrate the advantages of leading edge tanks in preference to wing locker tanks.

Table 1 shows how an aircraft with baggage on the parcel shelf, and fuel in the wing lockers will just remain within the C of G limits (10.5” to 16.1” measured from leading edge) when its main tank and wing tanks are full (typical weight for take off for a long X-country trip).

Table 1 - Full fuel with fuel in the wing lockers and baggage on the parcel shelf

Table 2 however shows what happens to the C of G half an hour or so later when only 5 litres of fuel has been used from the main tank. The aircraft immediately has a C of G greater than the maximum permissible. The problem can easily be resolved by removing baggage from the parcel shelf, however with the wing tanks full of fuel where do you put it?

Table 2 - C of G after 5 litres of fuel used up.

Here is where leading edge tanks and baggage in the wing lockers come into their own. Table 3 uses the same figures as Table 1, but we are now using leading edge tanks and baggage in the wing lockers (we’ve even included 10 lbs of baggage on the parcel shelf). The aircraft is well within the C of G range.

Table 3 - Leading Edge tanks and Wing Baggage at take off.

Table 4 - C of G after half of main tank used up.

Table 4 shows what happens to the C of G when half of the main tank is used up (quite a different picture from Table 2). In fact for this loading the C of G will remain within limits until there is no fuel in the leading edge tanks and 23 litres in the main tank. If you remove the 10 lbs from the baggage shelf then the C of G remains within limits until only 11 litres are left in the main tank.

Hopefully this article will provoke some useful debate, so if you have any comments please send them to me so that I can publish them in future issues. The examples presented are for illustration and are obviously open to interpretation, but I think they make the point quite well. I am currently producing a design for leading edge tanks (with useful advice and comments from Alan Cozens and the possibility of retro-fitting to completed aircraft) which I hope to get approved by the PFA, and once this is done I would be happy for anyone to use it. I hope to be able to have more news by the next issue.