CG Balancing

[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.

An update to my own post: Maiden flights of my Hellcat went just fine. I'd say it is slightly nose heavy, interestingly enough! I probably could have just balanced it at 75mm back with the gear DOWN and it would be about perfect, but I like my planes just a little nose heavy anyway. All's well that ends well!

[WintrSol]

RE: CG Balancing

An update to my own post: Maiden flights of my Hellcat went just fine.  I'd say it is slightly nose heavy, interestingly enough!  I probably could have just balanced it at 75mm back with the gear DOWN and it would be about perfect, but I like my planes just a little nose heavy anyway.  All's well that ends well!

Easy way to tell: trim for level flight at about 3/4 throttle (or whatever if comfortable). If it is nose-heavy, the nose should rise when you advance the throttle, and drop when you cut the throttle. Neutral trim should stay about level, depending on the design of the airframe. But, if it starts to dive when you go full throttle, you're tail heavy, which is why tail heavy makes for difficult control - a shallow dive turns into a steeper dive with a tail-heavy airplane.

[SR71CC]

RE: CG Balancing

An update to my own post: Maiden flights of my Hellcat went just fine.  I'd say it is slightly nose heavy, interestingly enough!  I probably could have just balanced it at 75mm back with the gear DOWN and it would be about perfect, but I like my planes just a little nose heavy anyway.  All's well that ends well!

Easy way to tell: trim for level flight at about 3/4 throttle (or whatever if comfortable). If it is nose-heavy, the nose should rise when you advance the throttle, and drop when you cut the throttle. Neutral trim should stay about level, depending on the design of the airframe. But, if it starts to dive when you go full throttle, you're tail heavy, which is why tail heavy makes for difficult control - a shallow dive turns into a steeper dive with a tail-heavy airplane.

Thanks WintrSol! I've never seen that method but will give it a try (on all my planes!) next time out!

[WintrSol]

RE: CG Balancing

An update to my own post: Maiden flights of my Hellcat went just fine.  I'd say it is slightly nose heavy, interestingly enough!  I probably could have just balanced it at 75mm back with the gear DOWN and it would be about perfect, but I like my planes just a little nose heavy anyway.  All's well that ends well!

Easy way to tell: trim for level flight at about 3/4 throttle (or whatever if comfortable). If it is nose-heavy, the nose should rise when you advance the throttle, and drop when you cut the throttle. Neutral trim should stay about level, depending on the design of the airframe. But, if it starts to dive when you go full throttle, you're tail heavy, which is why tail heavy makes for difficult control - a shallow dive turns into a steeper dive with a tail-heavy airplane.

Thanks WintrSol!  I've never seen that method but will give it a try (on all my planes!) next time out!

Note that this isn't a perfect method, as motor thrust line also has a part to play. My T-28 came with too little down thrust, and the throttle would make it rise or fall, too; once I adjusted the motor to ~5 degree down, it made the CG the primary factor.

Another way is the dive method: from a steady, moderate speed, tip into a dive, maybe 30-40 degrees. The aircraft will speed up, but motor thrust line is a weak contributor. If you are flying a 3D or mid-wing plane, you do this inverted, too. Same rules apply; a stable (very slightly nose heavy) airplane will start to pull out.