datum measurments questions

[n502pd]

OK, I am now trying to convince myself that I understand the plans references, and the POH diagrams. Please correct me...gently of course....if I am wrong.
1) POH shows distance from datum..spinner tip...to lowest point on fire wall as 54 in. I have a Prince P tip prop that measures 1/2 in less thickness at the hub than what is shown on several pages of the plans. When I measure the datum -to-firewall, I am short by 1.5 inches. This means that the engine isnt far enough forward(??), engine mount not far enough forward? OR is the 54 inch mentioned just an example value and one should use the actual measurement from the airfame? Doing that and the rest of the empty CG location calculations I end up with the empty CG at 14.6 in aft of the leading edge. My MAC is on spec, and this CG location is nearly at the center of the CG range per the POH. Is this correct and am I good with this, or should the empty CG be located torwards one end of the range?
A) W\hat are some representative empty CG locations from some of you fellows?
B) How important does the 1.5 in difference make as long as empty CG is located properly, AND what is the proper location for empty CG?

2) IF the 1.5 in is vitally important, and I am off of the nominal empty CG appreciably, how do I look at fixing the problem?
Weights are; L=311, R=322,T=53. Total empty=686. ( I do have strobe powersupplies in the tail cone, and a larger tailwheel frame and tire.) How do these weights compare to what is seen by you guys?

thanks for any and all responses!

[gammaxy]

I believe the POH actually shows the spinner to firewall as 47.5". Keep the fuselage corner defined as 47.5" (and wing leading edge as 53") and you'll be alright. What really matters is the CG location relative to the wing, so you really don't want to change your datum to match your different spinner location or else you'll also have to change the CG limits Sonex publishes.

I checked your math with my measured wheel locations and I got a CG location of 15" aft of the leading edge, so pretty close. Looks like you did it right.

My empty CG is at 65" (12" aft of the leading edge). Your extra 15 pounds at the tailwheel accounts for the CG difference. You might find that your CG is a little too rearward if you have a lot of weight in the baggage compartment with low fuel. I'm kindof surprised how much heavier your tail is--most builders are closer to my weight: https://docs.google.com/spreadsheets/d/1slAK-g5bNygzQvq-7c_lP8WH4g2uJCgVtNiy4AmjqCg/edit#gid=753129627

[n502pd]

Thanks for the instant reply, Chris!
I didnt have the POH in front of me when I wrote, so you are most likely correct on the measurment. Sorry about that...poor memory late at night!

I went to the page you sent and made a calculation on the tail weight with VW engines. I am indeed almost 21 lbs more than average. However, with the heavier wheel assembly, the tail strobe and powersupply and the mid point belly strobe and powersupply, and wires, I might hedge a bet and say they almost make up 3/4 or all of the additional weight. I want to see what I can find to verify that is the case. I would be happy with that as a reason.

I do have a baggage sling, but am considering limiting the weight to only 20 lbs via plackard so as to not comprimize aft cg limits unnecessairly. I will probably be the sole occupant as family dosent like to fly. I also plan to always fly with as much fuel as possible in the tank as a routine.

I have all steam gauges and old Narco com radio which adds to the empty weight. However, I am very pleased with the weight overall at 686 considering everything. I do have a slight imballance L to R main gear value that I have not figured out yet, except for the battery and all firewall penatrations on the R side of firewall, and most of the pannel wireing, including ground and +12 volt buss bars on the R side too. I do expect that my postierior will adiquately compensate for that nicely!

I am going to make much more critical measurments today using stricter methods.

And lastly, when doing the weight, should the airframe be in a level attitude, or just sitting normally in the three point attitude?

More suggestions are always welcome, from anyone!

Thanks!

[gammaxy]

You should make the measurement as shown by Sonex with the upper longerons horizontal. My inspector required me to use wording that explained the attitude of the airplane when then measurements were made. You'll notice when you lift the tail, it feels noticeable lighter the higher you lift it.

You might think that ~8 degrees shouldn't matter because it's easy to forget that the CG location also has a vertical component that we don't calculate. It's probably somewhere between the crankshaft centerline and the top of the wing spar. The higher the CG location is, the more sensitive your measurements will be to lifting the tail--once this CG has been rotated above the main landing gear, you will feel no more weight on the tailwheel.

If you made the measurement in the three point attitude, I think you'll find a reduction of about 25% when measured correctly. This would make sense because I doubt your strobe alone accounts for the tailwheel weight difference.

[n502pd]

You are exactly correct! Yesterday when I weighed it I was so interested in the empty weight that I just didnt consder the facts that are obvious now! I actually had just that discussion this PM with another flyer who is very interested in Sonex building and we came to exactly the same conclusion. Minus 25% puts it right in the 40 lb range. I shall attend to this again tomorrow, and will await what Kerry says about the short datum measurment.

Thanks much!

[sonex1374]

Joe,

The datum is set by you to a convenient position. The tip of the spinner is convenient, or the leading edge, or anything, but you have to use all your own measurements in the calcs or it won't turn out correctly. My advise is to use the leading edge of the wing as reference point, measure to the tip of YOUR spinner (whatever that is), then the Datum becomes defined as "XX inches forward of the leading edge". This keeps all the numbers positive, as well, and simplified the math. You'll also want to measure and verify the distances to your main gear tires contact point, and tailwheel distance as well. They very well could be different than the book.

Jeff

[n502pd]

thanks for the reply, Jeff!
I did use the numbers as published in the POH, and did use the numbers I actually measured. There were differences, but the outcome of the calculations varried only by about 3/8 of an inch as to the empty center of gravity location aft of the leading edge. You say to use my measured values. Is there a reference for doing so that I am unaware of, as I am not aero engineer! I may need that reference as support for the DAR visit coming soon!

Thanks much!

Joe

[peter anson]

Just a small extra detail on using the leading edge of the wing as a reference - because the spar webs overlap one wing is slightly forward of the other. In theory it's 1/8" but in practice it's probably a bit more than that. I know it's not much but I bet you take measurements to that order of accuracy.
Peter
Sonex 894

[n502pd]

Thanks Peter! I am aware of the offset of the wings. I defured to the drawing in the POH, as it shows all measured data taken from the left side, and that is what I have done consistantly. I also have drawn the location of each CG value on the top of the wing, just to get a visual on what was happening. I was surprised that the locations were relatively close to the spar cap line of rivets! Kind of neat to see.

Thanks much!

Joe

[sonex1374]

Joe,

Small variations in each build can affect the numbers used for CG cals. It is possible to have a slightly longer distance to the tip of the spinner because of a thicker/thinner prop, different engine mounting bushings or extra washers, or a slightly different alignment of your engine mount. Engine mount alignment difference would also place the main gear legs and thus the tires in a different spot as well. It's all subject to minor variations.

Having said that, in practice, you're not likely to see much change in the CG when you run the numbers. Even so, when you are bumping into the aft limit, an extra 0.5" might make the difference between needing to keep a few gallons of fuel in the tank for ballast, or being able to burn the tank all the way to empty without exceeding the aft limit. In these situations, being precise pays off.

Jeff