Forward vs Aft CG and Cruise Speed/Efficiency

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AffordablePlanesWhen

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I am curious to know if there's any appreciable difference in cruise speeds and fuel efficiency if loading a small GA airplane (i.e. 182, Bonanza, Lance) near the aft limit vs the forward limit.

Does anyone here take this into account and load an aircraft one way or the other intentionally?
 
Not hard to test on your own aircraft, and a valuable exercise to learn how the handling is affected. Easiest way is to have a friend along and see if there is a difference between him or her sitting in the front versus in the back. Or have some weight you can move around between the different baggage areas or seats (this does take significant weight as we're talking similar to the weights of people or the baggage load limits for your plane. Cases of water work well, about 40 lbs each, and are cheap.)

Of course, follow all baggage areas limits and calculate your weight and balance carefully.
 
In the Cardinal RG I used to own I accidentally discovered it was much faster with an aft CG. I was taking daughter and boyfriend on a flight. On the leg home she decided to ride in back instead of up front with me. Immediate 10 knot increase in cruise, something which repeated every time after that.
We are all taught that aft CG is dangerous and forward CG is safe. Truth is that aft CG within the limits is far better. Faster cruise, better landings.
 
Highflier said:
I move the seat back during cruise in a 182RG and typically pick up 1-2 kts
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That's good to know. Presumably that's not putting the CG very much further aft but that's still a free speed boost.

aftCG said:
In the Cardinal RG I used to own I accidentally discovered it was much faster with an aft CG. I was taking daughter and boyfriend on a flight. On the leg home she decided to ride in back instead of up front with me. Immediate 10 knot increase in cruise, something which repeated every time after that.
We are all taught that aft CG is dangerous and forward CG is safe. Truth is that aft CG within the limits is far better. Faster cruise, better landings.
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That's a huge difference. Yes, within limits rear CG has the benefit of efficiency, no doubt. I wouldn't have expected that much though, that's incredible

EDIT to add:

Is there a standard CG %MAC for the performance data in the POHs? I.e. when I check cruise speed for 65% power and standard day, is that worst case scenario (forward CG), best case scenario (aft CG), or somewhere in the middle....or is it known to anyone other than the engineers?
 
Forward CG = Better Stability
Aft CG = Better performance.

This is for conventional rigged aircraft. What happens as you move the CG aft is that you need less downforce on the tail to maintain your pitch attitude. The guys who are serious about racing make their planes as light as possible and any weight that remains they try to get aft.
 
I installed an all-electric air conditioning system in my A36 Bonanza this past summer (the one from Kelly Aerosystems). It added 60 lbs, all in the tail, with a corresponding rearward move of my CG. Now, an A36 is by design somewhat CG-forward, and requires noticeable aft forces/trim when flown unless you fill up the rear cabin, so this was actually a good change since most of my flights are with 1 or 2 people.

I mention it here since I have noticed absolutely zero difference in cruise speed. Yes, I'm 60 lbs heavier, but the farther-aft CG offsets that for cruise performance.

- Martin
 
I put three cases of water in the back of the V35A (120 lbs).....and it handles remarkably better....much lighter on the controls. I have no idea if I'm faster.....but I do get good tail winds now and again.
 

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@Martin Pauly, I am surprised to hear your experience doesn't align with what others reported. I find gaining 10 knots hard to believe in a light single but 1-2 knots I would imagine is possible. I'm curious if you can provide what the BEW CG was before and after the A/C installation?

@Checkout_my_Six, that seems counterintuitive. With a shorter arm to the tail, you'd think the control input (rudder and elevator) would have to be more exaggerated to get the same effect.
 
AffordablePlanesWhen said:
I am surprised to hear your experience doesn't align with what others reported. I find gaining 10 knots hard to believe in a light single but 1-2 knots I would imagine is possible. I'm curious if you can provide what the BEW CG was before and after the A/C installation?
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Yes, you can see the W&B details at the end of this video:


From flying with different loads and distributions, I, too, find 10 knots difference hard to believe just from moving one person from the front to the back seats. But I admit I have not tried it in a Cardinal.

- Martin
 
Doing dual XC in Grumman Aa1b with my large instructor and as much fuel as possible,it would indicate about 3 kt faster then when solo.The spacious baggage area,in that Grumman at least,was just for show:Das I was nearing aft cg with the two of us and flight bags
 
Martin Pauly said:
Yes, you can see the W&B details at the end of this video:


From flying with different loads and distributions, I, too, find 10 knots difference hard to believe just from moving one person from the front to the back seats. But I admit I have not tried it in a Cardinal.

- Martin
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My particular Cardinal was very nose heavy (3 blade prop), and doing W&B it became obvious you would have to haul gold ingots to ever have a concern about rear limits.
I also contribute at least part of it to the stabilator instead of conventional stabilizer/elevator/trim tab.
 
AffordablePlanesWhen said:
@Checkout_my_Six, that seems counterintuitive. With a shorter arm to the tail, you'd think the control input (rudder and elevator) would have to be more exaggerated to get the same effect.
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While that does seem counterintuitive, I can see control forces and control effectiveness being affected in opposite ways by the CG change. With the aft CG, the controls would feel lighter AND be less effective.
 
Contest racing glider pilots like their CG as far aft as possible, less elevator deflection in level flight. Optimum was to have the CG at 85-90% towards aft CG.
 
flyingcheesehead said:
While that does seem counterintuitive, I can see control forces and control effectiveness being affected in opposite ways by the CG change. With the aft CG, the controls would feel lighter AND be less effective.
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The CG to tail arm is less, but the basic airplane moment the controls have to overcome is also reduced as the CG moves aft, which results in the controls becoming *more* effective as the CG moves aft. The controls feel lighter because you need to move them less to get the similar response as the CG moves aft since there's less restoring moment from the basic stability of the airplane.

I too am surprised that a GA airplane could see a 10 kt increase due solely to trim drag while staying within the CG envelope.

Nauga,
having a moment
 
flyingcheesehead said:
While that does seem counterintuitive, I can see control forces and control effectiveness being affected in opposite ways by the CG change. With the aft CG, the controls would feel lighter AND be less effective.
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It's definitely lighter on the controls.....recall that the tail doesn't have to "make" as much downward lift....so it may also need less input. The tail surfaces might be closer to neutral....
 
Mooney pilots said you had to get the plane on a Step. One way they said was climb above your cruise altitude the let her descend gaining speed to get on the step. C182 RG's said to be slower if loaded aft because of the open rear wheel wells would catch the airflow. Any truth who knows.
 
My experience is that both handling and airspeed are best as one gets closer to the aft edge of the CG envelope. But, inexplicably, even with a 10 mph tail wind it doesn't make the Cub fly 10 mph faster.
 
I think as the CG moves aft, it moves closer to the CL.

The closer the CG is to the CL, less stick force is required to move the control surface a certain amount.
So, with an aft CG, there is less elevator stick force required to move the control surface which is referred to as 'light on the controls'.

I have experienced this in one airplane; very far aft loading with resultant light controls....but the downside was it was so far aft (CG so close to CL) that it was ultrasensitive, even "twitchy".

The reverse is a fore-CG situation in which it takes a lot to move the airplane in pitch; ie it is not light on the controls, it is very 'stable'.

In IMC or turbulence, I think I'd rather have fore CG loading, stable.

lmk if I have any of this wrong.
 
Let'sgoflying! said:
The closer the CG is to the CL, less stick force is required to move the control surface a certain amount.
So, with an aft CG, there is less elevator stick force required to move the control surface which is referred to as 'light on the controls'.
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No, at least least as far as major contributors to control feel and response are concerned. Speaking to the pitch axis for ease of discussion. The CG moving closer to the CL does not in and of itself change airflow over the tail, which is what causes the force resisting deflection of a conventional control surface ('hinge moment'). At a given dynamic pressure and relative angle of attack at the tail the hinge moment is pretty much independent of what's happening at the Center of Lift well ahead of it. What happens to the whole airplane when the surface moves changes considerably. Less deflection is required to achieve the same total moment with an aft CG, and it's this moment that moves the airplane. If you move the yoke (and elevator) the same amount with forward and aft CG the resulting pitch acceleration and pitch rate will be higher with the aft CG. If you move the yoke (and the elevator) enough to feel the same response with forward and aft CG you will have deflected the elevator less at aft CG (due to reduced stability) and as a result will feel lower force at the yoke, "lighter on the controls."

Nauga,
hinge moment limited
 
Think of the airlines. They regularly fly with aft CG to save fuel.
 
AffordablePlanesWhen said:
I am curious to know if there's any appreciable difference in cruise speeds and fuel efficiency if loading a small GA airplane (i.e. 182, Bonanza, Lance) near the aft limit vs the forward limit.

Does anyone here take this into account and load an aircraft one way or the other intentionally?
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Yes, and yes.

Although i would echo what Martin said about the cg being offset by weight, so just adding ballast in the back doesn't make the plane faster. If it's weight you're going to be carrying anyway, getting it as far back as possible is helpful. I don't notice it so much in the 4 seat planes, but it does make a difference in the Lance. My plane is nose heavy anyway (well, it was when it had an engine on it...), so I load it back-to-front.
 
AffordablePlanesWhen said:
I am curious to know if there's any appreciable difference in cruise speeds and fuel efficiency if loading a small GA airplane (i.e. 182, Bonanza, Lance) near the aft limit vs the forward limit.

Does anyone here take this into account and load an aircraft one way or the other intentionally?
Click to expand...

It is more than cruise. You will have better takeoff, climb, cruise, landing performance and lower stall speeds.
 
Legend has it that Mooney demo pilots would slide their passenger seats all the way back and lean back in order to get the 201’s TAS close to the advertised 201 mph.

Anyway, the performance gain from moving the cg aft is easily demonstrated: in smooth air, set the power, trim, and note your IAS. Then, slide your seat back - the plane will start to climb. Forward stick/trim will be necessary to keep that from happening, and the accompanying increase in airspeed is obvious. In my experience it’s usually just a few knots, but it’s free, so why not?
 
My observation from sailplanes. We often have tail water ballast tanks that let us adjust the CG location. Yes flying at aft CG location does provide a small reduction in drag. Better glide ratio in gliders, slightly faster in power planes. BUT, and this is something I think something we don't address enough, is that stall characteristics and spin recovery get worse as the CG moves aft. All staying in the CG range guarantee's is the the airplane is recoverable.

The dis-service we often do is we are training in a 172 or similar with 2 people in front and forward CG and find it has really benign stalls and barely will break into a spin, if it will. We probably don't try power on full flap stalls enough, which are the worst case scenario in most aircraft (go around). Then the next time we practice it is in a similar situation on our flight reviews. We load up our plane to max gross and a further aft CG and since we know the plane has benign stall characteristics we are more lax than we should be about stall prevention, we can fly that approach at 60 (we did in training) instead of 65, add a bit wind shear and the plane breaks into a spin before we realize what happened.

I have a friend that flew the same sailplane 1000's of hours for over 20 years. He decided to try flying it near the aft CG limit, he flew it probably 20 hours that way. Our normal climbing mode of thermalling is steep turns just above stall speed, so he got lots of time flying it slow just above stall speed. Then totally out of the blue while thermalling it dropped a wing and went into a spin, something it had never done is the previous 1000's of hours. This particle model of sail plane has a bit of reputation for stall spin accidents. He thinks he knows why now. It is a high performance racing glider and he thinks a lot of pilots racing it have got bit by trying to fly it near the aft limit for a bit of a performance advantage. He is a very competitive racing pilot (numerous 1st place wins) and decided the performance advantage was not worth the higher chance of a stall spin.

Point being, if you have NOT tried stalls near the aft CG you might be in for a surprise when you do, hopefully not a nasty surprise.

Brian
CFIIG/ASEL
 
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bonefishpete said:
Mooney pilots said you had to get the plane on a Step. One way they said was climb above your cruise altitude the let her descend gaining speed to get on the step.
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Old wives' tale, though you can make some observations that would seem to support the theory.

Here's why: At cruise (and any time your airspeed isn't changing) there is an equilibrium between thrust and drag. If there's more thrust than drag, you accelerate, if there's more drag than thrust you decelerate.

When you first level off at cruise, there's three ways to get to that equilibrium:

1) Set cruise power immediately and wait. In theory, it would take an infinite amount of time to get to cruise speed, though in terms of any speed change being noticeable it'll only take a few minutes.

2) Get "on the step" (which is a seaplane thing) by climbing an extra hundred feet or so and zooming back down to accelerate to cruise speed more quickly.

3) Leave climb power in, accelerate to cruise speed, and THEN set cruise power.

IME, 2 and 3 take about the same amount of time to get to the condition where you're level at cruise speed with power set, and 3 is really the right way to do things. But if you only try 1 and 2, then 2 looks like a far better option.

bonefishpete said:
C182 RG's said to be slower if loaded aft because of the open rear wheel wells would catch the airflow. Any truth who knows.
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Plausible. I believe there are some planes (Swift?) where Vx changes with gear up vs down for this reason too, high AoA = high drag with gear down due to a lot of airflow hitting in the wheel wells.
WDD said:
Think of the airlines. They regularly fly with aft CG to save fuel.
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I thought that the large airliners got positive lift from both wing and tail. There's certainly a spot where you achieve the optimum L/D, but is it still at the aft end of the envelope?
 
nauga said:
The CG to tail arm is less, but the basic airplane moment the controls have to overcome is also reduced as the CG moves aft, which results in the controls becoming *more* effective as the CG moves aft. The controls feel lighter because you need to move them less to get the similar response as the CG moves aft since there's less restoring moment from the basic stability of the airplane.
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What do you mean by the "basic airplane moment"? Measured from where? Are you talking about the difference between the CG and CL?

This seems to go against everything I've ever learned, but I'm always willing to learn more. It's just breaking my brain at the moment. (Easy to do in the AM.) More below.
Let'sgoflying! said:
I think as the CG moves aft, it moves closer to the CL.

The closer the CG is to the CL, less stick force is required to move the control surface a certain amount.
So, with an aft CG, there is less elevator stick force required to move the control surface which is referred to as 'light on the controls'.

I have experienced this in one airplane; very far aft loading with resultant light controls....but the downside was it was so far aft (CG so close to CL) that it was ultrasensitive, even "twitchy".

The reverse is a fore-CG situation in which it takes a lot to move the airplane in pitch; ie it is not light on the controls, it is very 'stable'.

In IMC or turbulence, I think I'd rather have fore CG loading, stable.

lmk if I have any of this wrong.
Click to expand...

nauga said:
No, at least least as far as major contributors to control feel and response are concerned. Speaking to the pitch axis for ease of discussion. The CG moving closer to the CL does not in and of itself change airflow over the tail, which is what causes the force resisting deflection of a conventional control surface ('hinge moment'). At a given dynamic pressure and relative angle of attack at the tail the hinge moment is pretty much independent of what's happening at the Center of Lift well ahead of it. What happens to the whole airplane when the surface moves changes considerably. Less deflection is required to achieve the same total moment with an aft CG, and it's this moment that moves the airplane. If you move the yoke (and elevator) the same amount with forward and aft CG the resulting pitch acceleration and pitch rate will be higher with the aft CG. If you move the yoke (and the elevator) enough to feel the same response with forward and aft CG you will have deflected the elevator less at aft CG (due to reduced stability) and as a result will feel lower force at the yoke, "lighter on the controls."
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What is the "total moment" you're speaking of here? Same as "basic airplane moment" above? Or just the sum forces on the entire aircraft? (or both?)

So, at a particular airspeed, a particular force on the yoke will cause a particular deflection and a particular change in down force. (Lower airspeed, same yoke force will be higher deflection but same change in downforce, right?)

The change in downforce will create a torque around the CG of the airplane. As the CG moves aft, the arm decreases and thus the torque around the CG is lower. Is the reason for the change simply that the change in downforce is a higher proportion of the total downforce when we have an aft CG?

Thanks!

Flyingcheesehead,
Insufficiently caffeinated
 
AffordablePlanesWhen said:
I am curious to know if there's any appreciable difference in cruise speeds and fuel efficiency if loading a small GA airplane (i.e. 182, Bonanza, Lance) near the aft limit vs the forward limit.

Does anyone here take this into account and load an aircraft one way or the other intentionally?
Click to expand...
Aft CG is slightly less drag.. as previously stated
 
bonefishpete said:
C182 RG's said to be slower if loaded aft because of the open rear wheel wells would catch the airflow. Any truth who knows.
Click to expand...
Nope. The pitch attitude will be the same for any combination of weight and airspeed. Angle of attack and airspeed are inextricably linked. The only difference will be less drag generated by the horizontal tail when the CG is farther aft.
flyingcheesehead said:
3) Leave climb power in, accelerate to cruise speed, and THEN set cruise power.
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From the Canadian Flight Instructor guide:

(2) To resume straight and level flight at selected altitude:
  1. Lower nose to the cruise attitude, and allow speed to increase (do not exceed red line RPM);
  2. Keep straight and maintain altitude;
  3. Reduce power to cruise RPM;
  4. Trim;
  5. Adjust attitude and power if necessary to achieve selected altitude and speed.

Anything else just results in altitude excursions and constant fooling with the trim,
 
Dan Thomas said:
Nope. The pitch attitude will be the same for any combination of weight and airspeed. Angle of attack and airspeed are inextricably linked. The only difference will be less drag generated by the horizontal tail when the CG is farther aft.
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Negative. Pitch attitude will be the same for any combination of *wing lift* and airspeed. More forward CG requires a higher AoA because you have to generate more lift with the wing to cancel out the increased tail down force.
 
flyingcheesehead said:
I thought that the large airliners got positive lift from both wing and tail. There's certainly a spot where you achieve the optimum L/D, but is it still at the aft end of the envelope?
Click to expand...
Atleast at my cargo airline, it is taken into account to try to load tail heavy for efficiency. The idea of the tail plane providing lift in the upwards direction is false - only aircraft with canards do that. It's just less downwards lift and therefore less drag overall.

This is why I was curious if anyone in light singles or twins has taken the time to see if there's any realistic benefits and the reviews seem to be mixed so far but leaning towards maybe a 1% increase in TAS.
 
I used to play with it in the Beechjet 400…fuselage fuel was in the back, and transferring it forward moved the CG forward. After leveling off at FL410, as airspeed approached the barber pole, I’d start transferring fuel to slow down a few knots, then stop transfer to let it speed up, and start transfer to keep it off the barber pole again.

Obviously it would be more efficient to reduce thrust and delay transfer, but that wasn’t as much fun. ;)
 
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