c
RAFTSMAN'S CORNER ••0 By Ben Owen, EAA Technical Counselor Administrator
WEIGHT AND BALANCE When you finish your airplane, if you're lucky the designer will have provided you with complete weight and balance information on the aircraft. If not, the builder is on his own. At this point, it is probably a good idea to talk to your local Technical Counselor, or anyone in the Chapter who has built and flown an airplane, to get an idea of how to do weight and balance. In the old days, the primary reference point was the leading edge of the wing, as it is then pretty easy to determine the fore and aft limits in percentage of the Mean Aerodynamic Chord (average chord) of the wing. This becomes a little more difficult on biplanes. (A reference mailout on this is available from EAA.) You could look at the rules for the International Formula Midget Racing Aircraft showing that required center of gravity location at racing gross weight must fall within 8% forward and 25% aft of the Mean Aerodynamic Chord of the wing. On the Aero Sport II, the range is 14% to 37%. In these days, to avoid having to calculate some of the lever arms in negative moments, we place the datum line well forward of the front face of the propeller, an arbitrary distance, measure all the lever arms from there to the weights (airframe, crew, fuel, etc.), multiply times the weight to get the mo-
42 JULY 1986
ments, and divide the total moment by the weight of the aircraft to get the location of the center of gravity. This is a fairly simple procedure and EAA also has a mailout on weight and balance that you can request when you reach this point. Basically, what the designer should have designed into the airplane with a proper CG location is a forward CG that is not so far forward that there is any danger of your being unable to pick up the nose to flare when landing at slightly above stalling speed. As far as the aft location goes, you don't want the center of gravity to move so far aft that the airplane becomes uncontrollable with your hand off the stick. An easy way to get a highly sensitive airplane is to move the CG aft for aerobatic use, but there is a point beyond which you don't want to go as far as longitudinal stability goes on the aft limit. One of the things you have to determine is the center of gravity of individual weights, engines, etc. Most engine manufacturers have done this for us. I constructed a swing of sorts that I hung from the children's gym set to determine my specific center of gravity, which I found to be at the front of the hip joint in a sitting position. This will be fairly close for most people to determine pilot and passenger weights.
In most cases, we have to hope the designer did things right and placed the fuel tanks near the gross weight center of gravity so that as fuel burns off, it has little effect on the center of gravity of the airplane. In weighing the airplane, the convenient points are the main gear and the tailwheel or nose wheel. Since most builders don't have access to aircraft weighing scales, bathroom scales can be used. If your weight per wheel goes much over half the bathroom scale capacity on each wheel, it would be a good idea to use two bathroom scales on each wheel, run a board across those scales and place each wheel on top. (Borrow them from your friends!) Of course, when calculating weight and balance, the weight of the board and other objects placed on the scale to hold the airplane in place ("tare" weights) are determined and subtracted from the total weight. Once you have the empty weight, it is pretty easy to calculate the gross weight. Another solution is simply to have people sit in the airplane, fill the fuel tank and put suitable baggage weight in the back compartment to get your loaded center of gravity. An important point to remember is that the aircraft is always weighed at the cruise angle of attack and, in the majority of biplanes, this means the top longeron is level. Other aircraft may have a small leveling scale glued permanently in place on the airframe, making it easy to block the airplane up on scales in a level flight, wings-level condition. In calculating weight and balance, you'll want to determine the maximum forward center of gravity, usually with a very light pilot, and all weights forward of the normal empty CG at their maximum and all weights aft at their minimum. The reverse is true when you go for the aft center of gravity; you usually want to use the heaviest pilot you think will fly the aircraft, the maximum baggage capacity and maximum weights for those weights aft of the empty CG of the airplane. By this means, you develop two sets of figures. It is a fairly simple matter to transfer the fore and aft limits to a distance from the leading edge of the Mean Aerodynamic Chord (MAC), and from that point determine the percent of MAC for your forward CG and aft CG limits.
CRAFTSMAN'S CORNER
Ben Owen
WEIGHT AND BALANCE When starting your weight and balance you determine a datum line or the place where you make all your measurements from nose to tail. If you use the front face of the propeller
wind
hub, all of the measurements from
the nose to tail will be positive. If you use a place like the CG of the aircraft, as they did in the old days, all moments forward of that CG would be negative and all aft would be positive, which led to arithmetic mistakes. There are two limits on the CG.
PILOT AVIONICS
CPI EDITION PA 11-20 PA-9 Electret Mic Telescoping Flex Boom Air-Gel Earseals
Brass Molded Plugs
Detent Volume Control
5-Year Warranty Super-Cushion Headpad Weight 15.9 oz.
ZEPHYR PHC-31 PADDED HEADSET CASE • Made of 1000 Denier Cordura 1 Holds headset & Intercom in Main Compartment • Side Pocket Holds Calculators • Reinforced Top Handle • Reinforced Shoulder Strap • Measurements: 10-1/2" x 9-1/2" x 5" ' Larger Style Available 13" x 17" x 12 $24.95 (LPHC-32)
All Headset Purchases Include: ($24.95 Value)
The forward limit determines how
far forward you can move the flying weight of the aircraft. The limit here is usually how far forward you can move the CG without the airplane either tipping up on its nose or that you have insufficient elevator to flare and land. The designer sets that and the aft limit as well. The aft limit is usually the point where if the C.G. is moved further aft you may find the aircraft to be uncontrollable. I recommend that you keep two sets of weight and balance in your aircraft. Not for some strange reason, but a very scientific one. To calculate the forward and rearward CG extreme, you would use the light-
FREE Shipping in USA
FREE Mic Windscreen FREE Cloth Earseal Covers
ITEM
*42.95
Padded Deluxe Flight Case • Made of 1000 Denier Cordura •Adjustable Padded Center Dividers • Two End Pockets For Headsets/Radios etc. • Two Front Pockets
• Side Slit Pocket For Charts • Easily Holds 2 JEPPS. 2 Headsets. Intercom... • Measurements: 17-1/2" x 10-1/2" x 9-1/2""
WEST WIND
• FREE Catalog Available • S5.00/UPS Red $10.00
2. 3. 4. 5.
1-800-456-1665 Order Desk Open 24 Hours
______California 94123_______ 96 APRIL 1995
well within forward and rearward CG extremes.
There is an unusual notation on weight and balance forms called moment.. I would compare moment to a teeter-totter. If you have a 200 lb. father 2 feet away from the fulcrum and a 100 lb. child 4 feet away, both have a moment of 400 ft. lbs. and
SCALE TARE WEIGHT NET ARM
MOMENT
Left Wheel 511 lbs. - 5 506 lbs. 54 lbs. 27,324"/lb. -5 507 lbs. 54 lbs. 27,378"/lb. Right Wheel 512 lbs. 50 lbs. 190 lbs. 9,500"/lb. Aux wheel 60 lbs. -10 -13 lbs. 21 lbs. -273"/lb. Less Oil -13 (7 qts.) Totals 1050 63,929 63,929" lb. = 60.885" 1050 lbs. Notes: a) Subtract oil to get true E.W. b) "Tare" refers to lumber, blocks, clocks, etc. used to hold the wheels on the scale. Always subtract. On our example, let's use weight lifters (260 lbs.) and jockeys (110 lbs.) rather than the usual 170 lb. pilot. Also remember: 1. EW of A.C.-Forward of usual flying CG
ZEPHYR PDFC-35
1850 Union St. #1633, San Francisco
edgeable designers - the gross weight, both fore and aft, will be
EMPTY WEIGHT
FREE PHC-31 Padded Cordura Case
UPS Blue (2nd day)
est and heaviest pilot and passenger you think will be in the aircraft to get them as far forward and as far aft as the airplane will ever see. As long as these limits don't exceed the designer's recommendations, you are in good shape. I always load the airplane up with the most it is ever going to carry so that I have a gross weight as high as the aircraft will ever see. The reason this is important is that if you ever have to put it down in a farmer's field, the FAA will look at your weight and balance forms to be sure you are within limits. The usual most forward and the most rearward locations the aircraft will see is not the same as the gross weight CG. On most aircraft - thanks to knowl-
J
Oil and fuel - Forward of usual flying CG Pilot, passenger and baggage - aft of usual flying CG On your aircraft these may be different. If you are very heavy or light always do W&B before you buy the aircraft or the plans.
w i l l balance each other out. Moments on the weight and balance form are in inch/lbs. The usual sport plane will take five bathroom scales (two under each wheel with a board across so the scales
should not try to weigh a 400 lb. main wheel on it. I have tried this three times and have finally learned that it is cheaper to buy a second scale or
tailwheel or nosewheel. I strongly suggest that even though your bathroom scale may go to 400 lbs., you
and borrow the fancy weight and balance scales from your local fixed based
won't break) and the fifth under the
borrow from the neighbors if you have to! Use the best scales that you can
find. You can get more sophisticated operator.
Pacific Coast Avionics SUN N' FUN BOOTH #B14-17 CALL FOR FREE CATALOG
GARMIN GPS-95 XL
Airspace Alerts/Portable GPS with Moving Map
$1075 GARMIN GPS-55 AVD...................
GARMIN GPS-100 w/database.........
FORWARD CG EXTREME
ITEM A.C. EW Oil - 7 qts. Pilot
Passenger Fuel - 24 gal.
GARMIN GPS-150....................... APOLLO 920+ GPS with moving map..
1050 lbs. 13 lbs. 110 lbs.
60.885"
21" 119"
MOMENT 63,929"/lb. 273"/lb. 13,090"/lb.
144 lbs.
49"
7,056"/lb.
WEIGHT
ARM
Totals 1317 lbs. 84,348"/lb. = 64 05» 1317 lbs. O.K. if designer says 64" is Forward Limit
84,348"/lb.
Passenger
Fuel -1 gal. Totals
WEIGHT
1050 lbs. 7.5 lbs. 260 lbs. 260 lbs. 6 lbs. 30 lbs. 1613.5 lbs.
ARM
60.885" 21" 119" 84" 49" 144"
BENDIX/KING
NEW KLX-135 GPS/COM $2395 KR-22 MKR BKN............ ,..$395 KA-134 AUDIO PANEL...... ..$519 KX-125 NAV/COM........... .$1640 KY-96A/97A COM........... $919 KR-86 ADF................... .$2495 KT-76A TRANSPONDER..... ,..$940 AND 2 YEAR WARRANTY
AVIONICS
MOMENT 63,929'yib.
157.5'yib. 30,940"/lb. 21,840"/lb. 294"/lb. 4;320"/lb.
"/11* = 75.290" 1613.5 lbs. O.K. if designer says 75.3" is max aft. GROSS WEIGHT EXAMPLE REARWARD ITEM MOMENT WEIGHT ARM 60.885" 63,929"/lb. A.C. EW 1050 lbs. 21" 273"/lb. Oil - 7 qts. 13 lbs. 30,940"/lb. 260 lbs. 119" Pilot 84" Passenger 260 lbs. 21,840"/lb. Fuel - 24 gal. 144 lbs. 7,056"/lb. 49" 144" 4,320"/lb. 50 lbs. Totals 128,358"/lb. 1757 lbs. Weight O.K. if designer say 1760 lb. is gross weight limit. 12 8
TRIMBLE FLIGHTMATE PRO........... MAGELLAN SKYBLAZER.... ........... MAGELLAN SKYNAV 5000........ ...
INCLUDES PREWIRED HARNESS
REARWARD CG EXTREME
ITEM A.C. EW Oil - 4 qts. Pilot
APOLLO 360 round with moving map..
/lb .1757 7C^K lbs. ' = 73.06" - within limits We know forward CG at GW is within limits - no chart needed. Good luck!
MK-12DW1D-824 NO/GS ..52349 MK-12D+/ID-825W/GS..,..$2549 AT-150 TRANSPONDER.....5950 COM-81014 VOLT.........$1025 TX-760D/rN-200D..........S2160
TX-760D/TN 2000 W/GS.
$2360
TRT-250DXPDR............J1070
MX-300/170/3B5...........$1195 MX-11RREPLCOM
......$675
MX-11COM............. .....5745
HANDHELDS ICOM IC-A22............................... .$549 ICOM IC-A3..................................$499 KING KX-99....................................$509 AIR-960,,. ..................................$299
Headsst adapter wlth/Push-to-Talk tor ICOM and AIR-960........................$39
HEADSETS/INTERCOMS DAVID CLARK H10 13.4..................$259
DAVID CLARK HI0-13.48...............$269 DAVID CLARK H10-13.4H................$279 DAVID CLARK H10-20....................$235 DAVID CLARK H10-30....................$199 TELEX AIR 3000...........................$169 TELEX ANR 4000..........................$419 FLIGHTCOM 4DX...........................$95 FLIGHTCOM 5DX BLACKHAWK.........$135 NEW FLIGHTCOM ECLIPSE..............$169
FLIGHTCOM IISX Intercom 2-place.......599 FLIGHTCOM III Stereo 2-place...........$124 PELTOR 7004..............................$195 FALCON F-14..................................$89 FALCON M5................................$129 FALCON F-16.................................$69 FALCON F-22..................................589
PM-1000 II INTERCOM....................$259 PM-2000 STEREO INTERCOM............$339
Price and Availability Subject to Change We Check and Report Fraudulent Credit Card Users. No Surcharges on Credit Cards
WE SHIP
WORLDWIDE
Auburn Municipal Airport
FAA Repair station *OPXR455L
1833 Auburn Way N. Auburn, Washington 98002
Class 1,2/Limited Instrument Mon-Fri 8:00am - 5:30pm PST
ORDER LINES 800-353-0370
SALES INFO 206-931-0370 • FAX 206-931-0584
SPORT AVIATION 97
