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Testing cg options ?

Larry DeCamp

Well Known Member
Recently an RV8A question was raised regarding simulating w&B response with weight added to pax seat, baggage etc. I cannot recover that thread.

However, some time ago I suggested a weight on the tail wheel stinger would be an easy way to optimize W&B location at minimum weight. I suggested a weight on the stinger that would be much less than weight in PAX seat or baggage. This was challenged with reference to concerns about W x r squared inertial differences that might affect spin recovery.

So, I calculated the tail wheel additional weight that would yield the same W&B as a 150lb pax. The w x r squired were the same. Can any aero eng or other smart guys comment ? A 5 lb weight on the tail wheel for optimized solo flight would be great, vs sand bags in the baggage 😒
 
Yes Bob

The formula referenced is Weight times the distance from the rotational center of mass squared. My ipad does not generate power of two symbols ��

So, can a small tail weight achieving the same weight and balance as ballast in ACCEPTED locations for weight be applied with equal safety ?
 
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The formula referenced is Weight times the distance from the rotational center of mass squared. My ipad does not generate power of two symbols ��

So, can a small tail weight achieving the same weight and balance as ballast in ACCEPTED locations for weight be applied with equal safety ?

No.
Example: 5 lbs 20' back from the datum or 50 lbs 2' back from the datum will both "balance" (e.g., put the cg in the same place). But if the center of rotation is the same as the datum, the 5 lbs in the tail has an added moment of inertia of 2000 (5 x (20 squared)); the 50 lbs adds 200 (50 x (2 squared). Big difference. (of course, you need to compare it to the whole plane. Maybe it's a small change overall).
 
Center of mass = center of rotation ?

One more question. Is it correct that center of mass , the datum, is the location of the cg balance point in the calculation? If correct, I will post some calculations for reference that may be helpful, or confirm sand,dog food or whatever must be strapped in the pax seat or baggage 😊
 
Update

Well I did the math again using RV4 arms from Vans manual. In general terms, 150lb pax would be equivalent to 29 lb tail weight yet produce the same CG. But, ( my bad in prior post) the w x r^2 values 760 thousand inlb ( tail weight) compared to 145 thousand inlb for pax. So where the weight is makes a significant difference. Thanks Bob Turner and Bill Boyd for challenging me.

However, this brings to question the guidance in builders manual. Up to 100 lb allowed in baggage ( structurally ) as long as the rear CG and gross weight limits are not exceeded. Whoa, this leaves a lot of room for big differences in W x r^2 outcomes .

I can only assume that Van has considered the extreme limits of these guidelines and they are safe. So just be aware if this is new information for you.
I will shut up :)
 
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However, this brings to question the guidance in builders manual. Up to 100 lb allowed in baggage ( structurally ) as long as the rear CG and gross weight limits are not exceeded. Whoa, this leaves a lot of room for big differences in W x r^2 outcomes .

I can only assume that Van has considered the extreme limits of these guidelines and they are safe. So just be aware if this is new information for you.
I will shut up :)
My understanding, which is extremely limited, is that the main time you need to worry about the increased moment of inertia is when doing a spin. Recovery can be hard/impossible if you have too much rotational inertia.
 
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