CG Balancing

[thom1050]

When balancing an airplane and checking the CG I have seen photos of the plane both being upright and inverted. What is the proper procedure and does it vary with different planes? Thanks.

[Baron]

Re: CG Balancing

I also have seen balancing the CG both ways. On my biplanes I balance them upside down but thats just me. I usually balance all others right side up unless the manual says other wise.

[seldredg]

Re: CG Balancing

On the T-28 1400 I just received, the instructions say specifically to balance the plane upside down, with the retracts down. With tricycle gear, I would wheels up or down would have a noticeable affect on the CG. Wouldn't it make more sense to balance with the wheels up? Seems then that a slight forward shift on landing with flaps would be OK, like a slight down mix on elevator. But I'm just thinking, here; no experience beyond the trainer I'm flying now.

[seldredg]

Re: CG Balancing

On the T-28 1400 I just received, the instructions say specifically to balance the plane upside down, with the retracts down. With tricycle gear, I would think wheels up or down would have a noticeable affect on the CG. Wouldn't it make more sense to balance with the wheels up? Seems then that a slight forward shift on landing with flaps would be OK, like a slight down mix on elevator. But I'm just thinking, here; no experience beyond the trainer I'm flying now.

[Randol]

Re: CG Balancing

In my experience;
Low wing, up side down
High wing, right side up
Mid wing, could be either depending on whether the mass is above or below the wing.
But hey, I'm just the guy that lost his plane in the corn :)

[Rocketeer]

Re: CG Balancing

Greetings,

I have always followed the routine outlined in Alpha's and Randol's posts and it seems to be the most consistant method. Gear up on tricyles, personal preference on tail-draggers.

It seems I did read somewhere that if/when using a balancing stand like the great planes unit, it's best (more accurate) to remove the pivoting pads. While the balls will leave a slight divit on foamies, it doesn't bother me because if I should need to re-test CG in the field I can see/use the little divits quickly.

Clear Skies

Bill L.
in Okla.

[CT]

Re: CG Balancing

Good one Randol!
Let us know how your search and rescue attempt go. I was thinking of adding this bird, what are the flight characteristics, retracts, ect.?
CT

[Randol]

Re: CG Balancing

CT the plane flies great. The most difficult part is getting the radio set up for the 3 different modes of operation for the AS3X. There are no retracts. I have been flying in precision mode. Had I been as precise as the airplane, I wouldn't be looking for it<smiley image="anim_confused.gif"/>

[Lon]

RE: CG Balancing

I just purchased and received the Great Planes CG Machine.  Up to this time, I have been doing the ole finger method for checking CG on my planes.
Now with the CG machine, I have found there is a significant difference on my planes CG I have been flying. By that I mean, with my fingers I had about a 20-25 degree nose heavy.  But then measuring immediately with my CG machine, maybe a 5-10 degree nose heavy. Now I've been happy with the way my planes fly and handles.  Wondering if because of the surface contact area of my fingers is a lot less than the pads on the CG machine, that I have such a difference in indication of balance??

[WintrSol]

RE: CG Balancing

I just purchased and received the Great Planes CG Machine.  Up to this time, I have been doing the ole finger method for checking CG on my planes.
Now with the CG machine, I have found there is a significant difference on my planes CG I have been flying. By that I mean, with my fingers I had about a 20-25 degree nose heavy.  But then measuring immediately with my CG machine, maybe a 5-10 degree nose heavy. Now I've been happy with the way my planes fly and handles.  Wondering if because of the surface contact area of my fingers is a lot less than the pads on the CG machine, that I have such a difference in indication of balance??

Not so much the contact area, as the variation in pressure in your fingertips over that area; the CG machine uses a bearing that doesn't cause a difference in pressure, front-to-back, across the contact pad.

[Grossman56]

RE: CG Balancing

Balancing a Biplane?  What is everyone's opinion on this?  Upside down, right side up?  Bottom wing or Top?

Grossman56

[WintrSol]

RE: CG Balancing

Balancing a Biplane?  What is everyone's opinion on this?  Upside down, right side up?  Bottom wing or Top?

Grossman56

I should think that it depends on whether the CG, vertically, is closer to one wing, so the weight is suspended. If neither, it would be like balancing a mid-wing, in that it's dealer's choice.

[Loneflier]

RE: CG Balancing

Balancing a Biplane?  What is everyone's opinion on this?  Upside down, right side up?  Bottom wing or Top?

Grossman56

On the biplanes that I have, I have balanced them from the bottom of the top wing, with the plane right side up per the instructions and not had any issues. The only two larger ones that I have are the Horizon Albatross, and the Art Tech Pitts Special. Both are nice flying planes.

[Pheonix]

RE: CG Balancing

I just purchased and received the Great Planes CG Machine.  Up to this time, I have been doing the ole finger method for checking CG on my planes.
Now with the CG machine, I have found there is a significant difference on my planes CG I have been flying. By that I mean, with my fingers I had about a 20-25 degree nose heavy.  But then measuring immediately with my CG machine, maybe a 5-10 degree nose heavy. Now I've been happy with the way my planes fly and handles.  Wondering if because of the surface contact area of my fingers is a lot less than the pads on the CG machine, that I have such a difference in indication of balance??

I have been using this CG machine for some time now and am really pleased with it.  it is very accurate and works for most planes.  It does not or is difficult to use on the A-6 or maybe a problem with the A-5 coming out.  A friend showed me how to use even on planes with very large CD measures.  I think this is a very good investment.

[doctormike]

RE: CG Balancing

RANDOL made it about as simple as it gets. You can't go wrong with that. I have compared finger balancing to using a balancing tool on most of my planes. Tool balancing wins every time. One tends to cheat when finger balancing, giving much less than perfect results. The Doctor

[Tim]

RE: CG Balancing

Generally, low wing warbirds are best balanced upside down with their gear up (since this is where they are during flight, which is when the CG position is the most critical ;) ). Technically, a tail dragger's main landing gear on most of the Warbird series (except the Corsair and P-40) are not too steeply raked, so the difference in the position of weight distribution in their "gear up" and "gear down" states is almost negligible, but it is still common practice to measure it with gear up.

High wing airplanes like Cessnas are best balanced right side up.

Cool. Thank you. I just received the F2G Corsair high speed, and the book says to balance wheels down. I'm going to balance wheels up.

[doctormike]

RE: CG Balancing

Why not do both just to compare. It may make very little difference. Let us know your results. The Doctor

[SR71CC]

RE: CG Balancing

Generally, low wing warbirds are best balanced upside down with their gear up (since this is where they are during flight, which is when the CG position is the most critical ;) ). Technically, a tail dragger's main landing gear on most of the Warbird series (except the Corsair and P-40) are not too steeply raked, so the difference in the position of weight distribution in their "gear up" and "gear down" states is almost negligible, but it is still common practice to measure it with gear up.

High wing airplanes like Cessnas are best balanced right side up.

Cool. Thank you. I just received the F2G Corsair high speed, and the book says to balance wheels down. I'm going to balance wheels up.

Here's the thing about wheels up vs down, which can certainly have an impact to the balance point on a plane such as the Corsair, P40 or like my newest plane, an F6F Hellcat, and why I still have a real question about this:  when the manufacturer indicates the proper balance point, are they assuming you are balancing with wheels up or down?  Because in reality there is a proper balance point with the wheels up and a proper balance point with the wheels down.  My confusion comes from trying to understand what the manufacturer means when they say that the "correct CG is 75mm from the leading edge".  That could be the correct point with wheels up, or that could be the correct balance point with wheels down!  They need to define it such as: "with gear retracted, the CG should be at 75mm, but with gear extended, the CG should be at 60 mm" or something to that effect.  Unfortunately, on my Dynam Hellcat, they state CG at 75mm and, incredibly, show pictures with gear up and down and illustrating 75mm!  Can't be true with gear that retracts back like the Hellcat!  I'm still confused!  On my specific plane, has anyone had experience?

[OV10]

RE: CG Balancing

Hello Dave (SR71CC),
Trying to understand what the manufacturer meant or intended would be like doing a chinglish mind reader act. As Alpha has correctly indicated in his response earlier to Tim, CG of the aircraft would be with regard to the gear up position. This is because the aircraft is predominately in this configuration through out it's flight other than landing/takeoff. So with birds like the F4U, F6F and P40(and I have all 3 of them from FMS) whose main gear swings aft, this puts the wheels weight on average 3" aft of the attachment point. Now when the gear is put down this weight transfers forward which shifts the CG into a slightly nose heavy condition which is very manageable because you will have assistance negating this effect with flaps. Unfortunately the Dynam birds don't have flaps whereas the FMS ones do, just means you use some up elevator to offset the wheels down CG shift. If you balance the CG with gear down you are asking for a tail heavy bird when gear is retracted. Tail heavy balanced birds are not a friendly adventure. Hope this helps to give some insight on the rationale for the standard(common) practice of gear up balancing.
Best Regards,

[SR71CC]

RE: CG Balancing

Hello Dave (SR71CC),
Trying to understand what the manufacturer meant or intended would be like doing a chinglish mind reader act. As Alpha has correctly indicated in his response earlier to Tim, CG of the aircraft would be with regard to the gear up position. This is because the aircraft is predominately in this configuration through out it's flight other than landing/takeoff. So with birds like the F4U, F6F and P40(and I have all 3 of  them from FMS) whose main gear swings aft, this puts the wheels weight on average 3" aft of the attachment point. Now when the gear is put down this weight transfers forward which shifts the CG  into a slightly nose heavy condition which is very manageable because you will have assistance negating this effect with flaps. Unfortunately the Dynam birds don't have flaps whereas the FMS ones do, just means you use some up elevator to offset the wheels down CG shift. If you balance the CG with gear down you are asking for a tail heavy bird when gear is retracted. Tail heavy balanced birds are not a friendly adventure. Hope this helps to give some insight on the rationale for the standard(common) practice of gear up balancing.
Best Regards,

Thanks Charlie,
I totally understand that concept and you explain it very well.  And we know that a "correctly balanced airplane" with gear swinging aft has one point where it will balance with the gear up and another point further forward where it will balance with the gear down.  That's fine and I have absolutely no problem with that; it's just a fact.  My issue is that the manufacturer is not defining each, or indicating that their "ideal" CG point indicated is with gear up, or with gear down.  The safest assumption, of course, is to balance to their CG point with the gear up because the worst that can happen there is a nose heavy airplane.  We all know that a nose heavy airplane flies poorly, but a tail heavy airplane flies ONCE!  It's just maddening to me that they are not clear about it.  It would be most appropriate to always assume that the measured CG is with gear up for all of the reasons you mention (predominantly in that configuration throughout its flight) and that be the standard assumption.  But, to see two illustrations in the manual - one showing gear up and one showing gear down - with the exact same CG point indicated, is what causes one to just shake his head, saying "what do they mean"?  So, I'm taking the safe assumption and unfortunately adding significant weight to the nose of my Hellcat.  At this point, it balances at 75mm back of the leading edge with the gear up and back.  With the gear down, it balances at 65mm back.  Their illustration, of course, shows a 75mm point in both orientations!  I guess we'll just fly it and see how it feels!  Thanks for your excellent response.