Aft CG=higher cruise speed?

jfrye01

jfrye01

Pre-takeoff checklist
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Jacob Frye
I'm heading out for San Antonio on Thursday, forecast to have at least 10 kt headwinds the whole way, and I'd like to save fuel...I'll spend the night in Denton on Thursday, and head out the next morning for the hop to 1T8. I'm thinking about throwing some extra weight in the baggage compartment (not enough to be unsafe, just enough to balance out the CG)...would this make a difference?? Any thoughts??
Blue skies,
Jacob
Edit: Planning on cruising at 8500, if that makes a difference...
 
jfrye01 said:
would this make a difference?? Any thoughts??
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Probably not significant. If you have a choice between loading toward a forward CG or an aft CG with the SAME WEIGHT, loading to the aft is more efficient.

However, if you are adding weight to get an aft CG, the added weight will increase your angle of attack resulting in a less efficient flight.

All things considered, take the forward CG and leave the weight at home.
 
Well I am sure that i notice it on a 172. I always put bags as far back as possible.
 
Oh and on the flybaby on a long flight the CG shifts dramatically with fuel burn. Indicated airspeed would be almost 5 mph faster at the end of a leg than at the beginning. Trim drag is real and you can reduce it. Racing gliders all now have tail ballast tanks to compensate for fat pilots and keep the CG back where it should be. Heavier pilots use brass tail wheels and fill the tire with foam instead of air. Some gliders have a battery on the tail too for CG help.
 
I am blessed/cursed with a header tank in the Lancair, but it does give me significant control over the CG once in flight. The difference between having the header full vs running low is worth about 3-5kias. If you're flying at 11k and dealing with headwinds, it's worth it.

As for adding weight, I guess it depends how much you'd need to add.
 
Better to take off with less weight that try to fight for the perfect aft CG in a small plane
 
If you want to add weight to change CG effectively for efficiency, you want to add it behind the horizontal stab. Only there will the positive effect of CG change outweigh the negative of the extra drag from the weight.
 
In my experience, moving the CG aft but still in limits can be good for a few knots.

In really calm air, get all trimmed up and check your speed. Then if you can safely do so, slide your seat back. You'll need to trim the nose down so as not to climb and see a couple knots when you do.

Legend has it that Mooney salesman would slide their seats back all the way, then lean back when they said, "She's all yours!". Like I said - good for a few knots when speed is of utmost importance.

All due to less downforce on the tail, and so less drag if it hasn't already been mentioned.
 
I wouldn't add 'ballast' so to speak, in a small powered plane but in a 172, I've noticed it flies all around better if you move someone to the back seat and put the bags in the baggage area instead of the seat. That includes the fact that it seems to land better as well.

That being said...

Should I stir the pot and bring up the centuries old debate of "Getting on the Step?!" I assume that's already been debated at length in the forums though. LOL
 
TriGear28 said:
I wouldn't add 'ballast' so to speak, in a small powered plane but in a 172, I've noticed it flies all around better if you move someone to the back seat and put the bags in the baggage area instead of the seat. That includes the fact that it seems to land better as well.

That being said...

Should I stir the pot and bring up the centuries old debate of "Getting on the Step?!" I assume that's already been debated at length in the forums though. LOL
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Lol, there s one time that you 'get on the step' with an airplane and that is after you've exceeded critical altitude.
 
Besides the obvious mention of prevailing winds blowing SW > NE, you can't change your True Course, but you can stay as low as possible.

I did a flight once and all the data said that I could gain about 2-3 KTAS by climbing another 2,000 feet on a SW heading. It wasn't intuitive, but all my data suggested that there was a shear layer that allowed this. I didn't trust my interpretation and in hindsight wish that I had, it was the only thing that prevented me from having that elusive (near) perfect flight that derived from hours of data collection and interpretation.

Keep in mind also, that you can step climb and descend also. It would be nice if some of the EFB tools included vertical profile planning, so we don't feel "stuck" at one cruise altitude.

Wear your wheel pants if you have them for the plane. Very much worth it in speed on the C-172.
 
Greg Bockelman said:
Probably not significant. If you have a choice between loading toward a forward CG or an aft CG with the SAME WEIGHT, loading to the aft is more efficient.

However, if you are adding weight to get an aft CG, the added weight will increase your angle of attack resulting in a less efficient flight.

All things considered, take the forward CG and leave the weight at home.
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Not necessarily. A friend of mine who does air racing in his Tiger found through experimentation that adding a certain amount of weight as far aft as possible did indeed result in greater speed. However, it's not likely the OP will want to go to that effort, in which case leaving the weight at home is best. Note also that while this increases speed, it does not increase climb performance.
 
Ron Levy said:
Not necessarily. A friend of mine who does air racing in his Tiger found through experimentation that adding a certain amount of weight as far aft as possible did indeed result in greater speed. However, it's not likely the OP will want to go to that effort, in which case leaving the weight at home is best. Note also that while this increases speed, it does not increase climb performance.
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That makes sense. Air racers use a lot of elevator in those turns.
 
If it is a nice clear day, you can pick up 5 knots just by staying down low and using the thermals. I did it on a flight from Tennessee to Kansas. Ok, I did it for half of the flight. I had to give up and go higher because I was getting air sick from the turbulence. Every time I hit a thermal it was catching a 500+ fpm elevator. That free lift can be traded for airspeed.

Jim
 
Kiddo's Driver said:
If it is a nice clear day, you can pick up 5 knots just by staying down low and using the thermals. I did it on a flight from Tennessee to Kansas. Ok, I did it for half of the flight. I had to give up and go higher because I was getting air sick from the turbulence. Every time I hit a thermal it was catching a 500+ fpm elevator. That free lift can be traded for airspeed.

Jim
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We can do that all summer in South Texas -- but it sure ain't worth it.

We climb high where its cool and smooth, no matter what the speed penalty.
 
Kiddo's Driver said:
If it is a nice clear day, you can pick up 5 knots just by staying down low and using the thermals. I did it on a flight from Tennessee to Kansas. Ok, I did it for half of the flight. I had to give up and go higher because I was getting air sick from the turbulence. Every time I hit a thermal it was catching a 500+ fpm elevator. That free lift can be traded for airspeed.

Jim
Click to expand...

Anytime I am level or nose down and I see my VSI climbing, I feel like I just found a twenty on the ground. Free lift ;)
 
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