Weight and Balance Question

[Bill Gorin]

Ok, I’m the new guy in the group so I have a question that has probably been asked several times before so please bear with me.  Why does the L-5 Type Certificate list the datum as the wing leading edge, but T.O. 01-50DB-5 L-5 Weight and Balance Manual revision date 30June1950 show the datum line as 72 inches forward of the wing leading edge (almost the end of the prop shaft)?  More important than why, but which is the correct datum?  My gut says to go with the T.O..

Thanks,

Bill Gorin

L-5E-1 N66535

[admin]

Hi Bill,

Great question. Actually, in the 20 years that I’ve been running the club no one has asked that question before. I’ll try to be thorough but concise with my answer and will use bullet points to help me stay organized.

* As you probably know, aircraft reference datum lines are arbitrarily chosen by the manufacturers. The primary datum reference for measuring all stations and the C.G. on the L-5 is at the tip of the crankshaft.

* In the side view illustration in AN01-50DB-5 you’ll notice that the datum line is shown  1.44″ from the end. That is because it is at the tip of the crankshaft, not at the tip of the prop nut which extends that little bit farther.  Lycoming calculated the C.G. of the O-435 without a prop nuts installed and the Stinson engineers used that data for their subsequent design calculations.

* All of the arm lengths shown in the L-5 W&B charts and diagrams are measured from that primary datum. When weighing the aircraft and calculating the empty C.G. location, that is the point that should be used. Of course, leveling the aircraft fore-aft is important when weighing it since the arm of the tailwheel hub changes slightly with the tail raised to a level flight attitude.

* On the FAA’s Type Certificate Data Sheet the C.G. limits are given as +13.0 to +18.2 inches, which is in relation to THE LEADING EDGE OF THE WING, not the primary datum. Why they did this is unclear but I’d guess that it makes calculating C.G. as a percentage of the Mean Aerodynamic Chord a bit quicker. While we as pilots generally don’t do % MAC calculations for simple airplanes like the L-5, engineers do and the military does. If you look closely, the MAC percentages are given in the L-5 W&B handbook.  In your 1950 version of the T.O. the range 20.5% to 32.0% of the MAC, although in earlier versions of the handbook the figures are slightly different.

* So, the answer to your question is that it’s probably wise to do both. Calculate the C.G. first by using the primary datum per the T.O. and then show conformance with the TCDS by simply subtracting 72 inches from the result — i.e. the distance from the primary datum to the wing leading edge.

* Additional observations…

1. The 1950 T.O. gives a C.G. range of 83.7 to 90.2 inches aft of the datum. Subtracting 72 inches yields 11.7 to 18.2 inches aft of the wing leading edge as opposed to the 13.0 to 18.2 inches on the TCDS.  Oddly, an earlier version of the T.O. gives an allowable C.G. range of 83.3 to 90.9 inches , yielding 11.3 to 19.9 inches aft of the wing leading edge. This may account for the oversize trim tab the military fitted to some airplanes that allowed a wider C.G. range.  It’s odd that none of the T.O. figures match the TCDS values.

2. For those interested in the % of MAC figures, you’ll find that for most conventional aircraft the center of the C.G. range is very close to 25% MAC, give or take 1%.

4. It is quite easy for an L-5 to exceed the forward CG limit on the “observer” model if a pilot weighs in excess of 225 pounds and the rear seat is empty, particularly if a controllable prop is installed.  Ambulance models are more tail heavy and therefore more forgiving in that regard. Also, a center of gravity that is well behind the forward limit provides more positive steering control, especially in crosswinds.

I hope that covers it. Comments???

Jim Gray

 

 

[Craig Cantwell]

Something else to remember, is that there is a minimum of about 65 pounds of weight that would have been removed when the a/c was licensed civil.  The radio in the front cockpit, mooring kit, engine cover kit, tool kit, most likye the fire extinguisher too. B’s and on would have lost even more weight when the SCR-274 radio package was removed and the camera stuff on C’s and on.

[admin]

Hi Craig. While that isn’t really pertinent to the question asked by Bill, what you say is absolutely true and it is something that should be kept in mind in the context of restoring an L-5 to an historically authentic configuration. That stuff was heavy (!!!) and definitely reduced the performance of the planes. The more (ahem) “portly” owners like myself definitely need to consider that when contemplating a full-on restoration.

Your comment is no doubt in reference to the military Basic Weight Checklists that are shown for each model in the L-5 W&B handbook, so thanks for reminding me to upload a copy to the website for those owners who do not have one. Those checklists are very useful when reinstalling the original military equipment because they give the precise weight, arm and moment of each item which makes recalculating the CG and preparing a new equipment list a much easier task.

[Sam Taber]

Hi Bill,

Short answer- L-5 is  now certified under  US FAA ( former CAA) Type Certificate Data Sheet ( TCDS) 741. (I am going by memory here so I may be off on the number).    We are flying our L-5s  under FAA rules / guidelines / certification now and no longer military data.

The TCDS also gives the “arm” length values using the wing leading edge datum point.    It also gives the CG limits which we are approved to comply with today.   That is the legal set of values which are recognized by the US Feds. Period.

Now, the military had values which were different when they ran the L-5 series.  Gross weight, maximum airspeed, Flap speed, CG limits were all different in the military ops.

Why did the FAA change them?   Different reasons have been put forward, but I have yet to see a published document stating the actual criteria.  In looking at other ex WWII planes , it appears that the feds may have lowered speeds and gross weights on some to  increase a safety margin.  Some they prohibited maneuvers ( like intentional spinning) on aircraft because they exhibit “ unusual spin recovery inputs” or “do not recover quick enough”  to control inputs.

So why the datum point change?  Hell if I know 🙂  But I made my W&B report out in two versions.  One FAA which I use in my logs and carry on the plane and the other is done to Military datum calculations  so I can use the cool loading charts in the Tech Order.

Do not mix the arms or datums or gross weight load values between FAA and military.  Only the FAA info ( on TCDS) is valid.

Hope this helps a  little,

Sam Taber

[Bill Gorin]

Thanks Sam, Craig and Jim.  I appreciate your answers.  I recently weighed my L-5E-1 and she actually lost weight compared to the last W&B and that was after I installed original WWII receiver and transmitter, a modern fire extinguisher and new ELT that weighs more than the one I took out.  I can’t find any record of the last time she was actually on a set of scales, so all the past W&B info is only as good as the person’s math skills that did them.  Current empty weight is 1554.5 lbs CG is at 14.65, comfortably within limits.  I go by the FAA TCDS info since that is what is legal.

Thanks,

Bill Gorin

[admin]

Bill,

I just came across a copy of the original military W&B data report for L-5E-1 s/n 44-17922, dated June 11, 1945. The empty weight was 1,604# including oil, 8.50 tires, and all fixed and removable equipment except the canvas covers and pyrotechnics kit. The CG was  located at 13.76″.

If the other “furnishings” are taken out that the military included in their empty weight calculation (tool kit, tie down kit, microphone and headphones, data case, report holder, message container,  checklist, fire extinguisher, stowage box, misc. equip. bag, sun curtain, crash pad, first aid kit, and mooring kit), we can subtract 34.6#, giving a true empty weight of 1,569.4#. That’s still 15# heavier than where you’re at and the difference is probably accounted for in the original radio package that weighed 27.4 pounds.  Of course, cotton fabric plus the dope finish probably weighed a bit more than Ceconite and modern paints do.

All this shows that your plane is in the right ballpark. I’ve seen ambulance models with empty weights of over 100# more than yours.

Jim

[Author]

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