Question for RW GA pilots: Weights and Balances

[Howellerman 104]

Hello all, 

I have a question for real world GA pilots about weights and balance, or more accurately, changing the balance once underway at cruise altitude.

For instance, I like flying "with a mission". This can be summarized as going "somewhere" with "somebody" to do "something" once we get there. And example would be flying from Portland-Troutdale in Oregon with friends and family to visit the Pacific Northwest (maybe Sequim W28) to look at places to retire. Another would be to load up the family, including doggos, at Reid-Hillview (San Jose, my local GA airport) and fly to visit friends in Lake Tahoe (KTVL). 

Given such a mission, I know I want to be below MTOW and that the CG of the aircraft should be forward versus aft at takeoff and climb. The problem is, taking the Carenado TBM 800 as an example, the seats to place the heaviest people are rear facing Row 2. If I fly with my children, 160 and 240 lbs respectively, they can't stand sitting in rearward facing seats - I know this because it drives them buggy on the train IRL. That leaves my two dogs, 20 and 100 lbs, leashed to the Row 3 forward facing seats respectively. So, at takeoff, I have from front to back, left to right, 220/190, 160/240, 100/20. The aircraft is balanced well enough side to side, and all the heavy loads are up front.

The question is, is it realistic to have them swap positions once we have attained cruise altitude? Unleash the dogs from Row 3 and put them in the rear facing Row 2 and put the kids in forward-facing Row 3? The dogs don't seem to mind rear facing seats, although I do have to admit when they stick their heads out the car window it is always forward facing...  

So, is it "permissible" to change seat assignments once at cruise altitude?

Thanks!

[DavidP 1,408]

As long as you remain in balance then it should be no problem.

[Chock 17,769]

Providing you are still within the CoG limits, then yes, you can move things around, and it is probably worth doing too from a fuel efficiency standpoint, providing you can do so safely, because the further back your CoG is, the more efficient your aeroplane will be and the faster it will cruise. But, you should be aware of all that is going on aerodynamically when and if you do choose to play around with the CoG, because it can have serious consequences in regard to stall speeds, longitudinal stability, and spin recovery. It might, therefore, be prudent to put the aircraft in a slight descent at a high speed when moving things around to ensure you've got plenty on the clock.

When moving the CoG, you are moving the aeroplane's balance point either nearer, or further away from the wing's centre of pressure, which is the central concentration point of the lift, although it is worth noting here that the centre of pressure for a wing is not a static location, it moves back and forth a bit with variations in speed/AoA. 

If you have a CoG which is forward, you'll have more longitudinal stability and this in turn means you'd be able to recover from either a stall or a spin more easily, but the bad part is that you're going to be at more risk of stalling with a forward CoG, because you need a lot of lift to counter the large amount of tail down force the elevator has to provide when there is a forward CoG, so you have to fly at a higher angle of attack, thus you're closer to the wing's critical angle.

With a rearward CoG, you've got less longitudinal stability, and a slower stall speed, but it will be much more difficult to recover from a stall or spin. You can fly with a lower angle of attack with a rearward CoG because there's less tail down force required from the horizontal stabiliser to counter the lift, with less aerodynamically induced drag because of this, you can of course fly faster.

It's worth bearing in mind too that the position of the CoG affects the effectiveness of both the elevator and the rudder. This is because the CoG is basically the 'pivot point' of the aeroplane, or the 'fulcrum' if you want to use the proper name for a pivot point. Just as is the case with a pry bar, spanner (or wrench if you're an American) or any other kind of lever for that matter, the longer the lever, the more force it can can exert, since a long lever puts the work effort through a longer arc of movement. Thus the further the control surfaces are from the CoG, the more effective they will be, since they are working as a lever against the GoG. This explains why it is harder to recover from a spin or stall with a rearward CoG; the control surfaces cannot exert as much force as they could if the CoG (i.e. the fulcrum point) was further away from them.

So yeah, you can switch things around, but be aware of all this stuff when doing so, because it will affect how your aeroplane flies.

Even professional pilots can fall foul of not knowing exactly how all that stuff works and what affect it can have on handling. The most famous example of this is probably a 1979 incident with TWA Flight 841, which was a Boeing 727. At the time it was rumoured among pilots of the 727, that if you partially deployed the flaps to a couple of degrees, you'd get a more efficient cruise because of the change in the centre of lift's position in relation to the CoG, but since you couldn't do that without also deploying the leading edge slats, crews would pull the circuit breaker for the slats and then drop a small amount of flaps. Although this wasn't allowed, many crews did do this regularly, and it is thought this is what the crew of TWA Flight 841 were trying to do (they denied this) but they either forgot to pop the slat circuit breaker, or didn't do it fully, and so the slats deployed at high speed, and one of them was damaged and ripped off, leaving it ineffective whilst on the other wing, it's counterpart slat was still intact. The disparity in lift between the wings at cruise speed caused the aeroplane to roll, then stall and enter a dive, where it passed the speed of sound, losing 34,000 feet of altitude in approximately one minute in the process. Fortunately the crew were able to recover the aeroplane. It was significantly damaged, although eventually repaired and returned to service.

The moral of the story is, yeah, you can play around with the CoG, but you'd better be sure you know what you're doing!

Edited October 15, 2018 by Chock

[scottb613 1,188]

Hi John,

Hah - imagining people and dogs moving around in a virtual plane - now THAT's immersion...

Well - it comes down to - you need to be familiar with your safe operating envelop of the aircraft you're flying... I don't know how accurate Carenado is in modeling this... Some RW planes are pretty particular - others have wide margins... Bigger planes tend to have wider margins - as you may have noticed you are never really weighed when getting on an airliner - they just average the weights...

Typically we calculate for "Full Fuel" and "Empty Fuel" conditions in GA - it's usually the heaviest dynamic weight on the aircraft - if you move people around frequently - you may be want to perform some additional calculations...

You want to be inside the safe operating envelop during all phases of your flight...

 

Here's a RW full fuel vs empty fuel example that I use before a flight...

 

Regards,

Scott

Edited October 15, 2018 by scottb613

[FDEdev 955]

At no time the airplane must be operated outside the CG limit. Especially if it's the aft limit! At high cruising altitude at low IAS you definitely don't want to fly an airplane that's unstable in pitch!

If you encounter turbulence you might drop out of the sky in an unrecoverable spin really quick.

Below MTOW the CG can move forward quite a bit on the TMB850 24% > 14% but not so much concerning aft movement 35% > 37%

You need to know the actual inflight weight during cruise and calculate (before the flight) how much you can move your Pax etc. around at what weight. 

 

[FDEdev 955]

21 minutes ago, Chock said:

... but the bad part is that you're going to be at more risk of stalling with a forward CoG, because you need a lot of lift to counter the large amount of tail down force the elevator has to provide

I don't agree. The stall speed is almost always given at the most forward CG, which means that the further the CG moves aft, the lower the stall speed becomes.

Since you need already a lot of downforce on the horizontal tail you need a lot more additional elevator force to be able to stall the airplane than with an aft CG.

Some airplanes even can't be stalled at a forward CG.

The wing stalls at the same AoA regardless of CG. With a forward CG the IAS will be higher at the same AoA, nothing else.

Edited October 15, 2018 by J35OE

[FDEdev 955]

18 minutes ago, scottb613 said:

I don't know how accurate Carenado is in modeling this

Just read the complaints in the Carenado 1900D forum how awfully unstable the plane is in pitch...turns out it's always the fact that the pilots are loading all the baggage into the aft compartment 😉 

[scottb613 1,188]

17 minutes ago, J35OE said:

Just read the complaints in the Carenado 1900D forum how awfully unstable the plane is in pitch...turns out it's always the fact that the pilots are loading all the baggage into the aft compartment 😉 

Hah - oops...

I don't know if you remember Cantu's freeware DC-10 in FS9 - but it was impossible to land for many - turns out the plane was GROSSLY overweight for flight - I'm not sure why the developers made that the default state - but you could barely get off the ground on a 10,000 ft runway and landings were virtually impossible... Get it down below MTOW and it behaved soooo much better...

Regards,

Scott 

Edited October 15, 2018 by scottb613

[pracines 823]

2 hours ago, scottb613 said:

Hah - imagining people and dogs moving around in a virtual plane - now THAT's immersion...

😂

I thought I had the immersion record for desktop flying - I even had a cousin get airsick (literally) flying my sim.

Now it will not be long before there is a utility that does this very thing real time.👍

 

[Chock 17,769]

18 hours ago, J35OE said:

I don't agree. The stall speed is almost always given at the most forward CG, which means that the further the CG moves aft, the lower the stall speed becomes.

i think you misunderstood my post, or maybe I didn't make it clear or typed something wrong (not unknown lol), here is what I was on about...

https://www.youtube.com/watch?v=a_33vmrV52s

https://www.youtube.com/watch?v=aGUBswKQUeQ

 

Edited October 16, 2018 by Chock

[skelsey 765]

24 minutes ago, Chock said:

i think you misunderstood my post, or maybe I didn't make it clear or typed something wrong (not unknown lol), here is what I was on about...

 

You are correct of course to say that the stall speed is higher with a forward CG.

However, what you said was that there is a greater risk of stalling with a forward CG. This is probably not really the case for the reasons J35OE outlined - the increased longitudinal stability and greater nose-down moment generally  makes recovery much easier and entry characteristics more docile.

So whilst the actual stall speed will be a few knots lower than the book figures with an aft CG, the reduced stability and increased difficulty in recovering probably makes a stall easier to enter whilst also making the recovery more problematic.

However, we're talking semantics really!

[Chock 17,769]

Yup, probably.

The point I was really trying to make is that, whilst in a sim it hardly matters much, the title of the thread included a reference to real world, and I'm always a bit wary of putting stuff out there without quantifying things when that kind of thing is mentioned, because you never know if someone ten years down the line will do a search on the 'net, find a quick post on some sim forum which says 'yeah it's fine to mess about with my CoG and stick fat uncle Bob on the back seat' without realising what the comment was about, and then go and kill themselves in their Cessna or whatever for real, where there's no reset option, by taking off with a grand piano on the back seat lol.

Edited October 16, 2018 by Chock

[Howellerman 104]

Hi all!

Thanks for the replies, especially Alan's detailed response. 

The last comment from Alan about Fat Uncle Bob, and the comments from J35OE and Scot (cool tool, by the way!) about the Carenado B1900 suggested a simple experiment. Take off and trim to level flight with "normal" weight and balance, and then repeat with my 6'5"/240lb son in the back seat and see what, if any, difference there is in the trim setting. If the B1900 responds to improper loading perhaps the Baron (another favorite, especially with Hank's turbo mods) and TBM will also respond in similar fashion.

As for immersion, yeah, a little bit of a mission with "somebody going somewhere to do something" really helps. When I first started simming I actually changed airplanes mid-flight just to see what the differences were (!). Now I am much more a cold-and-dark kind of simmer and I take it personally when a planeload of "somebodies" dies if I screw up. So, real weather, real flight planning, with real loads. 

Thanks again!

[fppilot 4,355]

Great discussion, but having spent a career rooted too often in problem determination and problem resolution begs a question. Given the root of your problem why did you settle on an aircraft with rear facing seats?

Edited October 16, 2018 by fppilot

[FDEdev 955]

2 hours ago, Howellerman said:

Take off and trim to level flight with "normal" weight and balance, and then repeat with my 6'5"/240lb son in the back seat and see what, if any, difference there is in the trim setting. If the B1900 responds to improper loading perhaps the Baron (another favorite, especially with Hank's turbo mods) and TBM will also respond in similar fashion.

Just tested: TBM with pilot & co-pilot +110lbs fwd baggage, clean takeoff, neutral trim, no pitch input after rotation. The TBM settles at a ~17deg nose up attitude and ~90kias.

Try again with pilot only + 1 pax in the last row +110lbs aft baggage, clean takeoff, neutral trim and again no pitch input after rotation.  She pitches up to 25deg and stalls…..