Weight & Balance
From Bondline
Contents |
Introduction
The weight and balance for any aircraft is very important. An overweight aircraft may not perform well, have reduced structural margins, or both. An out-of-balance aircraft will either require more elevator movement than available or have greatly reduced stability. In some cases, an out-of-balance aircraft will tip down onto its tail while loading.
Once you have determined your normal pattern of flying, doing a weight and balance calculation is not usually warranted if you always fly under similar conditions. The key, however, is recognizing when you are not flying according to your usual pattern.
In this section, basic theory, the effect of repairs and alterations, and operational considerations are presented and discussed.
Basic Theory
Archimedes was the first to show that the torque exerted on a lever by various weights is the same as if the weight were grouped at a special location called the center-of-gravity (CG). The CG is an invariant property of an object and does not depend on where measurements are taken.
So in a nutshell we have the basic formula.
- Multiply each component weight by its corresponding arm. This is called a moment (torque).
- Add all of the moments together (Archimedes' torques by various weights)
- Add all of the weights together and multiply by the unknown arm of the CG (Archimedes' weights grouped at the special location)
- Set the two calculations equal to each and solve for the CG (take the total moment and divide by the total weight)
The invariant property of the CG also allows component groupings to be used. For example, rather than cite the weight and moment (lever) arm of each bolt, washer, skin, etc., it is possible and preferable to group these items together into a larger group whose component weight and CG are calculated ahead of time. For example, this is commonly done with the empty weight of an aircraft so that pilots only need be concerned about the effect of fuel, passengers, and other payload.
Effects of Repairs and Alterations
The FAA Weight and Balance Handbook states that repairs and alterations are the major cause of basic empty weight and balance changes. This is also the reason why most official weight and balance calculations (the legal record kept in the aircraft) are off. Aside from normal errors in weight and moment arm estimation during removal of old equipment and installation of new equipment, the FAA allows the official weight and balance record to remain unmodified if the change is considered negligible. According to the AC43.13-1b, a change is considered negligible if the change in weight is 1 lb or less for aircraft under 5,000 lbs empty weight and the CG change is less than 0.05% of the mean aerodynamic chord. It doesn't take a lot of repairs and alterations before all of these negligible changes add up.
Over time, dirt, trash, oil, birds' nests, etc. can accumulate in the aircraft and affect the weight and balance.
After a sufficient number of repairs or time and anytime after repainting an aircraft, the aircraft should be reweighed with certified scales. Both the AC43.13-1b and the FAA Weight and Balance Handbook describe the procedures for calculating the new empty weight and CG. Usually the aircraft must be leveled using the leveling points described in the maintenance manual. For Grummans, the leveling points are the canopy rails, and they should be placarded as such. The same basic CG formula is used except that the weights read off of the scales are used in the CG formula instead of component weights.
Impact of Center-of-Gravity Location
- The more forward the CG, the more stable the aircraft.
- The further away from neutral stability (positive or negative), the larger the tail has to be or more elevator deflection is required in order to trim the aircraft.
- The larger the desired CG range, the larger the tail has to be. This is the primary reason the Cheetah and Tiger got a larger tail than the Traveler.
- For a given tail size, a less stable aircraft is more sensitive to elevator input.
- Trim drag increases with increased stability.
- A more forward CG results in a higher stall speed. Because more elevator deflection is required at a forward CG, the wing has to be at a larger angle-of-attack for a given speed to compensate for the higher negative lift of the tail compared to an aft CG condition.
Operational Considerations
There are two basic operational considerations, one safety-related and the other performance-related.
The 4-place Grummans can be out-of-balance on the ground if the rear seat passengers load first. The usual sign is the aircraft tipping back onto its tail and possibly doing damage. The normal procedure for loading rear seat passengers is for the pilot and front seat passenger to stand forward on the wing walk while the rear seat passengers board. Then the pilot and front seat passenger can take their seats.
The weight and balance while within the CG envelope will affect aircraft performance. A nose heavy aircraft requires additional trim, which causes trim drag. Some Grumman pilots have reported that securing a gallon of water at the rearmost location of the baggage compartment will noticeably improve cruise speed. Before doing this, a weight and balance calculation should be done to ensure that you are not putting your aircraft outside of its CG envelope for your normal flying routine.
Weight and Balance Spreadsheets
Weight and Balance spreadsheets are relatively simple to construct. However, several templates for Grummans are available.
