7
$\begingroup$

Modern aircraft are capable of carrying a lot of fuel to achieve extra-long range; we're at a point where nearly every pair of destinations can be reached non-stop. Carrying so much fuel imposes limits on take-off and landing weight, so the proportion of fuel to airplane weight is not negligible.

I'm wondering how the handling characteristics of airplanes change as the fuel is used up. Do the pilots feel that the airplane is lighter towards the end of the flight? Can they do faster maneuvers with nearly empty fuel tanks?

$\endgroup$
  • 2
    $\begingroup$ Do you mean airplanes in general, or just airliners? Certainly my Cherokee climbs better with partly full tanks. (Indeed, I almost never fill them completely, just "to the tabs" which is about 80% of capacity.) I wouldn't care to depart from a high elevation field on a hot day with full tanks. $\endgroup$ – jamesqf Jun 14 at 17:29
  • 1
    $\begingroup$ @jamesqf As I was unable to find much information on either, both are applicable! :-) $\endgroup$ – JohnEye Jun 14 at 17:33
  • $\begingroup$ Datapoint for interest: NOT at all a modern aircraft :-). The Me262 could fly on one engine only when close to out of fuel Losing an engine when more fully fuelled meant 'going down" was imminent. || This based on comments from Pierre Closterman in one of his books. PC was not the most reliable of sources so that MAY not be true, but probably is. [He says he chased an Me262 and shot out an engine (or it had lost an engine) but it managed to return to base and evade him despite briefings stating it could usually not fly on one only engine. $\endgroup$ – Russell McMahon Jun 15 at 10:33
  • 1
    $\begingroup$ The later part of the question "How do handling characteristics depend on flying with ... an empty tank?" brought me here as I wondered if it was related to Running out of fuel. $\endgroup$ – chux - Reinstate Monica Jun 15 at 20:51
  • $\begingroup$ @chux-ReinstateMonica Well, if it was the case of completely empty tank, there would be some obvious differences such as that the engines would not work ;-) $\endgroup$ – JohnEye Jun 18 at 11:58
14
$\begingroup$

All aircraft are subject to performance changes due to fuel burn, and since weight, lift, thrust and drag are all tied together for every aircraft in the same way the changes scale nicely. Let's look at some extremes:

One of the most interesting examples of this was the Concorde. Since the Concorde flew ~20K above every one else in the sky in those days it didn't have the same altitude holding restrictions that regular airliners (or any IFR airplane) has. The Concorde was give a block altitude basically to do what it wanted up around 50,000 feet (ca. 15 km). Since it burned quite a bit of fuel at Mach 2.1 and gradually became lighter during the flight the pilots let the plane slowly drift up to gain efficiency as well as gliding distance if needed.

The Concorde also used its many fuel tanks to trim the aircraft as well as cool the nose cone. Fuel burn was closely monitored and constantly pumped around to ensure the aircraft remained in trim.

Even the dinky Piper Archer I fly is affected by partial tanks. If I fly the tanks from full to VFR legal reserves, the CG moves forward a full inch. Granted this does not seem like a lot you will need to trim to compensate for this throughout the flight. You will also be landing a full 240 LBS lighter which is about 10% of the total weight from takeoff. If you come in hot and high you can get yourself into trouble.

Yes you can feel it, generally pilots are not going to do “faster” maneuvers but the maximum G load may change (see your aircraft's POH for more info).

Any smart pilot should practice flying their aircraft at various fuel loads to understand the differences it makes in performance and handling. Some aircraft with things like tip tanks or auxiliary tanks even have fuel burn limitations that define where the fuel is to be burned from and in what order, again going from something as small as a Bonanza all the way up to the Concorde as discussed above.

Just how much can the handling change? Assuming you did your due diligence in pre-flight and calculated your W&B on both ends of the flight accounting for fuel burn to ensure you are within your CG limits the flight should occur with in the limits of the aircraft's POH and generally speaking within operable limits of the airframe. FAA certified aircraft have control force limits so even at the extents of the CG range you should be able to trim the aircraft to handle “normally”. You will likely notice pitch differences (at either extreme) as well as various performance number differences.

| improve this answer | |
$\endgroup$
8
$\begingroup$

Modern gliders are an interesting data point regarding this. A competition glider carries water in its wing tanks to increase weight. This betters performance at high speed, which is essential for competition when conditions provide strong lift.

The pilot can release the water at will, and the tanks can be emptied in a few minutes. This happens in competition every day, either when you are arriving (to land without the extra weight) or sometimes mid-flight when conditions are worse than expected.

So, taking as an example a Ventus 3:

enter image description here

This glider, with pilot included, weighs around 380kg. It can carry up to 212 litres of water in its wings, for a maximum gross weight of 600kg (it also carries a few litres in the tail for cg purposes).

In summary, the pilot can change the weight of the glider from 600kg to 380 kg in some 4 minutes (meaning a 38% change in weight!). This makes the change very noticeable. When empty, the glider is more "feisty", both in that it reacts more to turbulence, and in that the commands (the ailerons, especially) are way more effective.

| improve this answer | |
$\endgroup$
3
$\begingroup$

Aircraft performance does change with weight. Not only does takeoff weight and balance need to be considered, zero-fuel weight and balance does as well. Many key performance metrics depend on this, like takeoff and landing speeds and distances. Climb performance and range are also affected. Inflight, Va (maneuvering speed) is very dependent on weight. The speed at which you can fly while putting extreme loads on the airframe from abrupt maneuvers or turbulence actually decreases as weight decreases.

| improve this answer | |
$\endgroup$
1
$\begingroup$

Something else that changes with a change in CG and/or weight is your stall speed. There is a good article from AOPA here: https://www.aopa.org/news-and-media/all-news/2020/march/flight-training-magazine/aerodynamics-stall-speed

| improve this answer | |
$\endgroup$
1
$\begingroup$

Alone in an A36 Bonanza from central Florida to North Central Indiana, I flew on one load of fuel by decreasing the throttle gradually as I burned fuel to maintain a constant airspeed. The aircraft wanted to speed up as it lost weight. So, yes the aircraft will fly differently as the fuel is used.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.