weight and balance - again

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Richard Roller
Posts: 58
Joined: Mon May 22, 2017 11:14 am
Location: Olathe, Ks.

weight and balance - again

Post by Richard Roller »

There has been an interest the last few years on the Air Camper’s weight and balance. Also it appears to me that operating behind the aft c.g. is not being taken seriously enough.

After reading and commenting on a recent post on different engine weights I thought I’d post some more information on Ken Perkin’s Piet, N34KP.
When initially finished Ken’s Piet was tail heavy. I witnessed an unintentional flight one day while Ken was doing taxi tests and getting some time on his engine. While running down the runway, tail on the ground, a gust of wind hit the Piet and it came off the ground in a nose high attitude and proceeded to zoom to an angle of about 30°. Ken had full forward stick and could not get the nose down. He wallowed around for a hundred feet or so, just off the ground in ground effect and by nursing the throttle he got it back on the ground in one piece. That scared both of us badly (obviously Ken more so), so we decided to was time for a weight and balance check.

We decided to weigh the Piet with electronic scales. The aircraft was built to the short fuselage plans with a Ford Model A engine, also modified to the plans. The engine has an aluminum head, water pump on the back of the head, a slick 4000 series mag driving off the crank and an Ole Fahlin 76x44 prop.

The aircraft was levelled in a closed hanger in the flight attitude with full oil and water. The fuel tank was drained to residual fuel and then the aircraft attitude was rechecked for level. Measurements were taken with the forward side of the lower firewall as datum. The initial weight and balance was taken in this configuration, then adding fuel, then with the pilot and then with co-pilot and no pilot. This was done to establish the actual c.g. of the pilot, co-pilot and fuel.

Initial figures were:
Item quantity wt. total wt. c.g. moments
Left main 1 328.00 328.00 16.75 5494.00
Rt. Main 1 313.00 313.00 16.75 5242.75
Tail 1 17.00 17.00 158.5 2694.50
Totals: 658.00 20.41 13,431.25
C.G. aft of the leading edge: 12.91.

Adding fuel and pilot (Ken’s weight at that time) for the worst case aft c.g. check.
Item quantity wt. total wt. c.g. moments
Fuel 12.5 6.00 75.00 29.98 2248.50
Pilot 1 236.00 236.00 60.32 14235.52
Totals: 969.00 30.87 29915.27
C.G. aft of the leading edge: 23.37.

3.37” aft of the recommended aft c.g.. Apparently too far aft since Ken couldn’t get the nose down. Now substituting the FAA “standard” 170 lb. pilot.

Item quantity wt. total wt. c.g. moments
Pilot 1 170.00 170.00 60.32 10,254.40
Totals: 903.00 28.72 25,934.15
C.G. aft of the leading edge: 21.22.

Still 1.22” aft of the recommended aft c.g..

After removing the engine it was found it’s weight, with oil, water and propeller, was 256.3 lbs.. We decided to move the engine 7” forward. Moving the engine, rebuilding the mount and cowling to match the new location added 9.00 lbs. to empty weight.

Item quantity wt. total wt. c.g. moments
Engine -1 256.30 -256.30 -11.88 3044.84
Engine 1 256.3 256.3 -18.88 -4838.94
Mount/cowl 1 9.00 9.00 -18.88 -169.92
Totals: 667.00 17.19 11,467.23

C.G. aft of the leading edge: 9.69.

Now adding fuel and Ken (who had lost down to 200.00 lbs. by this time.) we get the following.
Item quantity wt. total wt. c.g. moments
Fuel 12.5 6.00 75.00 29.98 2248.50
Pilot 1 200.00 200.00 60.32 12,064.00
Totals: 942.00 27.37 25,779.73

C.G. aft of the leading edge: 19.87.

With a 200.00 lb. passenger.
Item quantity wt. total wt. c.g. moments
Co-pilot 1 200.00 200.00 23.00 4600.00
Totals: 1142.00 26.60 30,379.73

C.G. aft of the leading edge: 19.10.

Ken uses a gross wt. of 1150 lbs. and the aircraft climbs nicely (for a Piet) at this weight. With Ken and fuel at 19.87” aft of the leading edge the aircraft flies hands off in pitch at cruise. It needs a little back stick for a lighter pilot.

I’m not trying to tell anyone where to put their aft c.g. limits. I’m putting this out to share the experiences of Ken and I with his Piet. I’m just saying everyone should be aware of the c.g. limit recommendations and give them due consideration.

One more thing. The last engine Ken put in his Piet is a Ford model A with a gleaner/combine block. The block adds 7.00 lbs. to the engine weight. That move his empty weight c.g. forward another .38 inches.

I'll be checking in from time to time for comments and questions. I apologize for the weight and balance tables, when I submitted the file everything was condensed.
Last edited by Richard Roller on Fri Sep 13, 2019 8:39 am, edited 1 time in total.

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Re: weight and balance - again

Post by taildrags »

Richard; excellent information, and a lot of it. You and Ken obviously did the W&B the right way and did it carefully. No guessing about where the pilot's belly button is or any of those other approximations. Like they say, if you do it right the first time, there will not need to be a second time. The distances of pilot, passenger, and fuel from datum will not change for the rest of the airplane's life unless something gets rebuilt drastically. Weighing the engine with accessories was not something a lot of builders would do either, so your detailed description of the engine and its weight is very useful information for other builders.

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