Effect of fuel burn on a trimmed aircraft

[APenNameAndThatA]

Suppose you are trimmed and flying along. Eventually, your aircraft will become lighter. Does the aircraft maintain the speed it is trimmed for and gain height? 

[Flying Binghi]

Questions:

 

Re airframe CofG. What is the moment arm (CofG) of the fuel tank?  

 

Do the level of fuel in the tank change its moment arm?

 

How many fuel tanks ?

 

If more then one tank are there fuel pumps to ‘rebalance’ ?

 

If the engine is fuel injected where do the excess fuel go to ?

 

 

 

 

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[Downunder]

27 minutes ago, APenNameAndThatA said:

Suppose you are trimmed and flying along. Eventually, your aircraft will become lighter. Does the aircraft maintain the speed it is trimmed for and gain height? 

My opinion.....

In theory yes. In practical life, no.

There are many other changing factors like wind and barometric pressure, temperature. All changing, all the time.  

The most obvious need to retrim is from fuel usage. 

Even tanks nominally over the cofg tend to need a retrim......

My fuel use tends to drop the nose after a while.

As my trim adjustment is quite course, I trim for a slight climb at a high throttle setting,  then throttle back for level flight. Then as fuel burns and the nose dips, I just add a bit of throttle....works well, without constantly playing with the trim.

 

Edit. I'm talking very light/ultralight aircraft....

 

Edited October 17, 2020 by Downunder

[Student Pilot]

Depends on the individual aircraft, all different

[cscotthendry]

When I had the Nynja, it had a fuselage tank behind the seats. That required constant retrimming every 10 to 15 minutes.
The plane I have now has wing tanks. I don't notice needing to trim the elevator, but I have to manually manage the fuel to keep the lateral balance.

[old man emu]

15 hours ago, APenNameAndThatA said:

Suppose you are trimmed and flying along. Eventually, your aircraft will become lighter. Does the aircraft maintain the speed it is trimmed for and gain height? 

This is a good, first principle question. However, the discussion could be prevented from wandering, as it already has, if an exemplar aircraft was used. Basically, things depend on the position of the fuel tanks relative to the CofG. If the question was put with that addition, then the simple situation could be explained, leaving the hangar doors open for a second question relating to things Flying Binghi has mentioned.

[Flying Binghi]

To help explain fuel tank moment arm re airframe CofG:

The top tank in a Drifter is sloping backwards towards the top of the tank. When the tank is full its moment arm is further rear wards then when it is half full.

 

I would say though in the 3 Drifters I owned over the years that I never noticed trim changes as I flew..🙂 

 

 

 

 

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[APenNameAndThatA]

3 hours ago, old man emu said:

This is a good, first principle question. However, the discussion could be prevented from wandering, as it already has, if an exemplar aircraft was used. Basically, things depend on the position of the fuel tanks relative to the CofG. If the question was put with that addition, then the simple situation could be explained, leaving the hangar doors open for a second question relating to things Flying Binghi has mentioned.

Which is why your "Diagnose This" question was wrong.

[old man emu]

41 minutes ago, APenNameAndThatA said:

Which is why your "Diagnose This" question was wrong.

Hey! I was backing you up!

[Flying Binghi]

Seeing as no-one pinged me on the Drifter tank mathematics I doubt there’s any interest in this subject. Not really relevant to most light aircraft pilots any way..🙂

 

 

 

 

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[Yenn]

What trimming does is get the plane flying in balance. If your C of G moves aft for instance as you burn fuel, you will need to trim nose down to correct it. If you don't you will start to climb and slow down, so you will need to trim to regain speed. If C of G moves the other way so does the need to trim to prevent speeding up and descending.

I remember a Mooney pilot telling me that he had to move his charts and paperwork from the passengers seat to the top of the Panel to trim for fuel burn.

[facthunter]

Retrimming for balance or speed are different. IF you are lighter you need less power to do the same speed, so you either reduce power or go faster or you can climb to a higher cruise level at the same speed. IF a passenger moves  to the front or the rear, there's a balance change without any overall mass change.  Adjust pitch trim. Your plane is in a constant state of balance. Change one factor and others must be affected. Nev

Edited October 21, 2020 by facthunter
typo

[antonts]

On 17/10/2020 at 6:47 PM, APenNameAndThatA said:

Suppose you are trimmed and flying along. Eventually, your aircraft will become lighter. Does the aircraft maintain the speed it is trimmed for and gain height? 

In general case - not.

There are two variables - speed and height, both affect vertical speed and interdependable between each other. So the same zero vertical speed can be achieved with given mass, power setting and trim with full range of height and speed, it depends on miriads of conditions (design, temperature, current air pressure, weight balance etc). It can happen that only height increased and exactly compensated reduced weight with decreased drag, but it will be the same event like to win a lotto.

 

May be it is possible to build a plane which has stable speed in some more or less narrow range of conditions (range of power, range of trim, range of pressure, range of temperature etc), but no one will care to do this - trim is specially designed to do the task, no need to embed such a thin interdependencies in design itself.

 

[Roundsounds]

As already mentioned, there are so many variables. Assuming you had an aeroplane with the fuel load directly on the CG, you would either need to slow down or reduce the angle of attack to maintain height - both changes requiring a re-trim. 
recall in level flight lift = weight. As fuel burns, weight reduces. The only practical things a pilot can do to reduce lift is reduce angle of attack, a part of the CL and/or V (airspeed)

Lift=CL x 1/2rho x Vsquared x S

 

A bit of trivia.. The Curtiss P40 Kittyhawk has a fuel tank in the belly just below and behind the pilot. As this burns the CG moves forward significantly and requires frequent retrimming.