Shifting CG forward

Chrisgoesflying

Chrisgoesflying

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Chrisgoesflying
Not sure if this would go into Maintenance Bay - if not, please let me know. I have a very skinny Cessna 150. Last W&B last year came back at just over 1,000 lbs so I have nearly 600 lbs useful load. My usual mission is 200 - 300 nm weekend trips with my wife and our dog.

The Cessna 150 has baggage compartment 1 (right behind the seats) and baggage compartment 2 towards the back. The dog occupies baggage compartment 1 weighting about 60 lbs, leaving us with up to 40 lbs for baggage. I'd like to put our bag in baggage compartment 2, however, anything over 15 lbs puts the CG too far aft. The thing is, even with full fuel, we have plenty of useful load to spare (we're pretty light weight ourselves) but the CG doesn't work out with anything over 15 lbs in the very back of the plane in baggage compartment 2.

So, my question is, what can be done to shift the CG more forward?
 
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The only thing I can come up with is the seats position. The range is 33 to 41. Move the seat positions full forward,
 
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As others have found, make sure that your current W&B is actually true and accurate first. Consider getting the plane on the scales by someone that will follow the correct procedures. You may not have as big an issue as you believe.

But on the other hand, your only recourse may be to get a strap to secure the luggage in the front right seat.
 
Matthew Rogers said:
As others have found, make sure that your current W&B is actually true and accurate first. Consider getting the plane on the scales by someone that will follow the correct procedures. You may not have as big an issue as you believe.

But on the other hand, your only recourse may be to get a strap to secure the luggage in the front right seat.
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And his wife can walk?
 
Chrisgoesflying said:
what can be done to shift the CG more forward?
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Was the last weight and balance an actual or calculated result? Regardless make some removeable ballast bags that will fit on the floor right in front of your seat(s). Include a strap that will wrap around a seat leg to hold it in place up. Calculate how much weight you need and if it is a bit high look to use a dense material like lead or inconel steel vs sand to keep bag size compact. Just be sure to remove them as needed. There are some helicopters that have a minimum cockpit weight which requires a light-weight pilot to augment his presense with additional weight. We would fabricate ballast bags for these challenged pilots so they could fly any assigned aircraft in the fleet that had that requirement.
 
Is the 15# limit due to the envelope or is there a baggage 2 limitation? Put some weight in main baggage with the dog. Get a box that fits the area and let the dog sit on top.

Cessnas fly better with the CG aft. Faster, too.
 
Matthew Rogers said:
As others have found, make sure that your current W&B is actually true and accurate first.
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That there. I have seen way too many W&Bs that were way off. Too many calculation errors It happens easily in airplanes that don't use a theoretical datum somewhere in front of the airplane. If the firewall or wing leading edge is used, any changes forward of that have an opposite effect to any changes aft of it, and one has to make sure he's using + or - appropriately.
 
Not a lot of room under the seats in a 150, but maybe you could slide one or two 10 or 25# weights underneath the seat(s)?
 
Oddly enough when reweighing my plane for the first time in probably 50 years, it didn't change the W&B paperwork much. There was a bigger discrepancy when we reweighed it about ten years later.
 
David Megginson said:
And with less pitch stability (so turbulence will seem a bit worse). It's a tradeoff.
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I never understood why guys say that. If you want a twitchy Cessna (or Cub) fly it at the forward limit of CG. You'll be busy. They're more docile with aft loading. Not as quick into the air but more comfortable once you're there. Both of my planes (Cessna and Cub) are easily within the CG envelope when empty and solo. I keep 40-ish pounbds in the back of both to calm them down. Without the ballast they aren't much fun.
 
Jim Carpenter said:
Not a lot of room under the seats in a 150, but maybe you could slide one or two 10 or 25# weights underneath the seat(s)?
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Not only is that space so tight that I can't get my hand under the seat to remove seat rail stops, but the seat is aft of the empty CG. Sticking weight under there would be counterproductive. Empty CG is typically around 35 inches aft of datum, and the seats slide between 35 and 41 inches.

The W&B paperwork is likely off. Without a reweigh, the OP might do something that results in the CG beyond the forward limit when running light.
 
Stewartb said:
I never understood why guys say that. If you want a twitchy Cessna (or Cub) fly it at the forward limit of CG. You'll be busy. They're more docile with aft loading. Not as quick into the air but more comfortable once you're there. Both of my planes (Cessna and Cub) are easily within the CG envelope when empty and solo. I keep 40-ish pounbds in the back of both to calm them down. Without the ballast they aren't much fun.
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It's the physics of flight. Assuming a non-canard/non-delta-wing plane, the further back the CG (and the closer to 0 load on the horizontal stabilizer), the weaker the airplane's aerodynamic tendency to revert to its trimmed pitch after a disturbance. If you move the CG forward, the horizontal stabilizer will have a bigger AoA (in the opposite direction of the wings), causing the plane to snap back to its trimmed AoA faster. The problem is that if the horizontal stab is applying (say) 150 lb of downwards force to stabilise the plane with a far-forward CG, it's also creating more drag, and that's why you lose a knot or two with that far-forward CG.

The airplane's W&B CG limits are set so that you won't go to neutral or negative pitch stability at the aft end, and won't exceed the horizontal stab's ability to provide counteracting downward force at the fore end. But you get closer to one of those as you move the CG towards either limit.
 
Perhaps the dog could ride in your lap? They like having their head out the window too.

If you can do it safely I'm sure the dog would be quite happy strapped to the cowling. That would move the CG forward. 8~)
 
Not much you can do in a 150. If your CG paperwork is correct, you’re stuck with the 15 lbs max unless the dog is willing to share space.
 
Snowmass said:
Not as long as my first true cross country: 1200 miles in a 65HP Aeronca Chief
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23 gallons at a bit under 4 gallons per hour in my buddy "Chief" Daniel's 11AC... Fun, took along a bottle of spring water, not coffee.
 
Assuming you actually have plenty of useful load, can you add weight to the nose and/or helium balloons to the tail? The balloons are a joke, but I have flown planes in the past which had permanent lead weights installed for envelope purposes. The problem is usually too much forward CG, but it could be anything.

As others have pointed out, you should verify your W&B. It's unusual for a 152 to be this skinny and I'd hate for you to get into an overload/out of envelope situation. None of our 152s have more than 500 lbs useful load. With some, myself and a skinny CFI means we get about enough fuel to take off and land with reserves.
 
bflynn said:
Assuming you actually have plenty of useful load, can you add weight to the nose and/or helium balloons to the tail? The balloons are a joke...
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I have 285... pop... 284 helium balloons in the back of my Luscombe right now :) It's an elegant solution, they don't have to be tied down if they fill the available area and they stay up off the control cables on their own. I just have to make a trip to the party supply store every couple weeks.

Heck, its worth it just to see the expression on the OI's face as he conducts a ramp check and my tailwheel is floating four to five inches above the tarmac.



EDIT: Be sure to use HELIUM balloons :) no matter how much cheaper the hydrogen filled ones are.
 
David Megginson said:
It's the physics of flight. Assuming a non-canard/non-delta-wing plane, the further back the CG (and the closer to 0 load on the horizontal stabilizer), the weaker the airplane's aerodynamic tendency to revert to its trimmed pitch after a disturbance. If you move the CG forward, the horizontal stabilizer will have a bigger AoA (in the opposite direction of the wings), causing the plane to snap back to its trimmed AoA faster. The problem is that if the horizontal stab is applying (say) 150 lb of downwards force to stabilise the plane with a far-forward CG, it's also creating more drag, and that's why you lose a knot or two with that far-forward CG.

The airplane's W&B CG limits are set so that you won't go to neutral or negative pitch stability at the aft end, and won't exceed the horizontal stab's ability to provide counteracting downward force at the fore end. But you get closer to one of those as you move the CG towards either limit.
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You guys are talking about two different things. With a far aft CG, the plane is less stable ("mushy"). With a far forward CG, the plane is more sensitive ("twitchy").
 
MauleSkinner said:
Not much you can do in a 150. If your CG paperwork is correct, you’re stuck with the 15 lbs max unless the dog is willing to share space.
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If that 150 doesn't have some weight in the tail that doesn't belong there. the W&B paperwork must be off. I've flown and maintained 150s and never had CG problems. A reweigh is in order. Or the OP is miscalculating his loading somehow.
 
Sluggo63 said:
You guys are talking about two different things. With a far aft CG, the plane is less stable ("mushy"). With a far forward CG, the plane is more sensitive ("twitchy").
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As CG moves aft the total moment for a given surface deflection *increases* (since the restoring moment, stability, decreases) and the airplane becomes more responsive. As the CG moves forward the total moment for the same deflection *decreases* (since the restoring moment increases) and the airplane becomes less responsive. The more stable airplane will return to a trimmed condition faster than a less stable airplane, but it will be more difficult (and slower) to displace the more stable airplane from that trimmed condition. "Twitchy" and "sluggish" are qualitative and have different meanings to different people. I would hardly describe an unstable airplane as "mushy", which to me implies sluggishness or lack of response - highly maneuverable airplanes use relaxed stability to increase the responsiveness; and I would definitely not describe a very stable airplane as 'twitchy', which to me implies overly responsive but my definitions may differ from yours.

Nauga,
control powered
 
With respect to the average pilot? The aviation definition of stability often isn't what they think it is.

In discussions about CG and aircraft control? Speed needs to be considered. In a high power cruise I prefer an aft CG. For slow ops I prefer forward CG. Get the aft CG airplane slow and it gets difficult to control. Get the forward CG airplane going fast and it gets difficult to control. That's my layman's take on it regardless of what the books say.
 
Sluggo63 said:
You guys are talking about two different things. With a far aft CG, the plane is less stable ("mushy"). With a far forward CG, the plane is more sensitive ("twitchy").
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That's an interesting distinction.

Maybe to be more specific, with a foreward CG, it will take more control force (or turbulence) to raise the nose from its trimmed position, and it will return to that position more quickly when you release pressure (if you want to call that "twitchy", fair enough). With an aft CG, the pitch will move more easily (via control force or turbulence), but won't snap back to its trimmed position as quite as fast when you release pressure (or the jolt of turbulence ends).

If you moved the CG even further back, beyond the W&B limits, you might enter the realm of negative stability, where if you pull back on the yoke and then release pressure, the nose would just going up on its own.
 
40YearDream said:
Actually, isn't the issue typically with the CG too far forward at max gross?
View attachment 100567
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Note the x-axis on that graph. It's not CG. However, it is true that many airplanes have a more rearward front CG limit when heavy. The aft limit doesn't usually change.
 
David Megginson said:
That's an interesting distinction.

Maybe to be more specific, with a foreward CG, it will take more control force (or turbulence) to raise the nose from its trimmed position, and it will return to that position more quickly when you release pressure (if you want to call that "twitchy", fair enough). With an aft CG, the pitch will move more easily (via control force or turbulence), but won't snap back to its trimmed position as quite as fast when you release pressure (or the jolt of turbulence ends).

If you moved the CG even further back, beyond the W&B limits, you might enter the realm of negative stability, where if you pull back on the yoke and then release pressure, the nose would just going up on its own.
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In the first 10 hours of flying my exp Cub I described it as riding a saucer sled on ball bearings. The plane was all over the place. Not uncontrollable but I couldn't relax for 2 seconds. It was within the forward limit of CG but not by much. It was better at slower speeds but that's not practical when breaking a new engine in. When I tossed my 65# dog in back things calmed. I expanded on that. When I shoved 50# of tools and survival gear all the aft, aft of the bulkhead marked 10# maximum? It flew more like a Cub. Wing slats expand the aft envelope by one inch. I contend that they may move the forward envelope aft an inch, too, but I'm not going to be the guy to test it to limits. Forward CG is no fun. The faster you go, the more work it takes. Aft CG in my Cessna is comfortable in cruise and in turbulence. The only time it gets my attention is when slowing to land short. Get it slow enough and the tail drops. Not a disaster at a foot or two above the ground but cause for disaster much higher than that.
 
Stewartb said:
Get it slow enough and the tail drops. Not a disaster at a foot or two above the ground but cause for disaster much higher than that.
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That sounds a bit like a tail stall, which is extremely dangerous, and shouldn't happen within the normal W&B envelope unless you have ice contamination or similar.

Perhaps, though, it's just a matter of being trimmed too far back and releasing forward elevator pressure at a bad moment. You might not even be conscious of doing it.
 
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In the RC community, where the risks are lower but the physics is the same, there is a saying. If it's nose heavy, it flies badly. If it's tail heavy, it flies once. This taken to the extreme that if you don't have enough elevator authority to bring the nose down, you're potentially in a really bad spot right after liftoff.

I've flown one cub that was supposed to be a bit tail heavy. It flew fine, and it was probably a little bit quicker than the average cub, but it was miserable to land. I finally got used to it, but had to come in a bit steeper and faster than I was used to, and flare closer to the ground. It did not want to be low and slow at the same time for whatever reason.
 
Wow, thanks for all the responses. Super helpful. I won't address all of them, just those that kind of stuck out. The plane was weighted a year ago. Not calculated W&B but actual weighted W&B - unless the bugs on the leading edge had too many burgers, I would say the W&B should still be pretty accurate today. I do like the suggestion of putting some of the baggage on the floor on the front seats. Since my wife doesn't operate the plane, she has lots of leg room on her side and as long as I can somehow secure the baggage from sliding forward towards the rudder pedals, that's probably the best solution, simply putting the flight bag and computer bag right there.I'll have to calculate it, but it might be enough to potentially put our small travel back all the way in the back of the plane while the dog keeps his space right behind our seats.

300nm is a long flight in a Cessna 150, but it's not as long as driving and a whole lot more fun. I upgraded to the 150 from what we call "advanced ultralights" in Canada so it does feel like a big plane to me, at least for a year or two - at which point I'll probably have to upgrade again, in small steps to always get that "big plane" feeling :)
 
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