Forward Sweep & The Great Tire Crisis
This month I'd like to explain what happened to me, and the approach I took in relofting my homebuilt.
I could see up as well. The choice of high wing versus low wing for me was easy, because I wanted to retract the landing gear. While Cessna did this quite successfully on some high wing airplanes, no one would call the system simple and trouble-free. It's a lot easier to retract the gear on a low wing airplane. Last time we talked about the Mean Aerodynamic Chord (MAC), which slices through the center of aerodynamic forces on your wing. A good starling point is to try and keep the center of gravity between 15% and 30% of chord along the MAC. On my plane, I basically have shoved the wing root aft. One of my design goals was that I wanted the leading edge of the wing root to be no further forward than the pilot's kneecaps. This would give me a nice view downwards in flight. The wing root, though, ends up behind the center of gravity. In order to keep the center of lift of the wing, which is located roughly at 25% of the MAC, in the
that may be difficult to fly! There are also advantages to having a forward-swept wing. Besides the better visibility, the forward sweep pretty much guarantees that the wing will stall root first, which is a desirable handling trait. Also there may be less interference drag. The forward sweep tends to unload the wing tips, permitting the wing to be more tapered, and helps keep the ailerons working even after the inboard part of the wing has stalled What makes forward sweep possible is the emergence of composite structures. A material like aluminum has equal strength in all directions - it is an isotropic material. Unidirectional fabric has its strength along the direction of its long fibers - it is an anisotropic material. Knowing these words will make you the center of attention at your next hangarflying session. By using unidirectional fibers and controlling the direction they're pointed, the structural gurus can tailor a forwardswept wing to reduce its tip incidence when the wing bends under load. If you want to learn more about structures, I strongly recommend Gordon's book, which is listed at the end of this article. It is the best introduction I've ever seen, and any beginner can follow it. Because of the structural divergence problem I mentioned, I hesitated to use a forwardswept wing. My salvation is that I have friends who are structural wizards. Rob Schirtzinger has graciously volunteered to make a finite-element model of my wing, which will then predict the deflections under load. This is normally a very expensive task which I couldn't afford to have done other-
Since I showed you revision 9 of my plane
right place relative to the center of gravity, I
wise. Burt Rutan is flying a couple of
in a previous article, you probably noticed that the airplane uses a forward-swept wing. I did this for one reason: visibility. If you study most low-wing homebuilts,
needed to put the wing tips relatively far forward There is a problem with forward-swept wings that you should be aware of. As the wing bends upward under high-G loading, the wing tips tend to increase their angle of attack. This increases the lift at the tips, and in response to this the tips try to produce
airplanes with forward-swept tails, and Jon Karkow at Scaled Composites did a lot of structural work looking at the divergence problem. I figured that I would just go ahead and design the structure myself, then run it by all my talented friends. My hope is that they will be so horrified at what I've done that one or more of them will redesign the struc-
even more lift. The end result of this is that
ture to keep me from killing myself. If that
the wing twists itself right oft the fuselage, leaving you with a lighter airplane, but one
doesn't work then I will try moderate amounts of crying and begging among all
DMtey/LomtMrd/Roncz Homebuilt - Revision 14
and there were plenty of times that I wished by JOHN G. RONCZ, EAA 112811
15450 Hunting Ridge Tr. Granger, IN 46530-9093
You have now arrived at a preliminary layout for your airplane. You should have your wing and tail areas pretty much under control. You have designed a nice package to
stuff the people, luggage, radios and engine into. You have added up all the wetted areas and made a SWAG (Scientific Wild-A.. Guess) at the performance of your fledgling airplane. You even know the wing's angle of incidence needed to have a relatively level fuselage at the design point I reached this point last July on my homebuilt design. In the last six weeks, though, I've basically relofted the entire airplane.
you'll see that the pilot and copilot end up sitting on the rear half of the wing. This guarantees that you can't see anything when you look down. One of the things I liked about flying Cessnas was that you could sight-see a lot when you were flying around. I never did like high wings in a turn, though.
SPORT AVIATION 43
my best friend of 27 years, both climbed on the boarding steps of my Rockwell 112A at the same time - at which point the Rockwell ended up parked unceremoniously on its tail
skid! (Larry and I both played linesmen on our high school football team). So figure out where the center of gravity is with luggage on board, people in the back seat, nobody in the front seat, and whatever fuel load puts the center of gravity at its rearmost location, then put one person's weight on the boarding
step and write down the fuselage station of the center of gravity for that loading case.
Figure 1 - Axle Must Be Behind this Line
this talent to get a structural design that
works. I will, however, proof load the wing before attempting to fly my homebuilt. I will be measuring the twist of the wing very carefully during this test.
In any event, I proceeded to design airfoils and model my homebuilt in three dimensions
using the VSAERO computer program (if you want it, call Analytical Methods at 206/643-
9090 and be prepared to part with at least $25,000 for the license, half that amount for a computer big enough to run it, and a few years to figure out how to use it). To make a long story shorter, I did 13 iterations of the
full size drawings I made on my computer and plotter. They were out in California sitting in the cockpit while I was in Indiana responding to their feedback. Based on their
observations, we moved the firewall forward a couple of inches, and made a couple of other small changes.
One of the decisions we made early on
was to use 6.00 x 6 tires, rather than the tires
from Tonka toy trucks, as many homebuilts do. Combined with the 55 knot stall speed, this would let us fly into grass and dirt strips. Now, where you put the main wheel's axles is not arbitrary. To determine this, you
aerodynamic design, after which the beast was looking extremely good.Time to start
do several weight and balance cases for
While all this was going on, Mike Dilley
you would likely encounter on loading your airplane. I remember an occasion in Knox-
your homebuilt. The object of the game is to
building, I thought.
find the aftmost center of gravity position that
and Larry Lombard, my partners in this adventure, built a mockup of the fuselage using
ville, Tennessee, when I and Larry Tharnish,
1 2
4
5 g
7
8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26 27 28
A B DLR2 WEIGHT AND BALANCE
9-18-89
! FSi
51.5!
124.0! 124.0! 31.8!
282.8:
273.4! 120.0! 121.5! 151.5! 94.5!
119.1! 63.0! 125.0! 110.0! 168.0! 72.5! 101.5! 69.6! 74.0! 142.0! 151.5! 75.0!
= D28/C28!
E
D
C
angle of attack), and draw a line vertically
from the center of gravity down to the runway (see Figure 1). The axles must be behind the line you just drew or the plane will stay on its tail when you land! For my homebuilt, this gave me the worst case location for the
axles, since it was a bit further aft than for the level loading case. Remember, you would like to put the main wheel axles as far forward as possible, since this enables you to rotate the plane soonest on takeoff, and prevents the plane's nosewheel from slamming down after landing. After I determined the location of the main gear axles, I found that if I retract the wheels
F
! WL MOMENTS ! DESCRIPTION FS MOMENTS =C5*B5!ENGINE (IO-360 180 HP] =A5*C5 =C6*B6!WiNG =A6*C6 -14.62; 155.71 =C7*B7!FUSELAGE =A7*C7 -4.91! 133.60 =C8*B8;PROP =A8*C8 6.00! 55.00 =C9*B9!VERTICAL TAIL =A9*C9 24.63! 18.50 =C 1 0* B 1 0!HORIZONTAL TAIL =A10*C10 2.15! 33.38 =C1 1*81112 PEOPLE =A11*C11 -8.66; 306.66 =C1 2*B1 2!MAIN LANDING GEAR =A12*C12 -26.80! 70.00 =C13*B13;BAGGAGE =A13*C13 -8.661 20.66 =C1 4*B1 ^INSTRUMENTS & RADJOS =A14*C14 6.41! 45.00 =C15*B15!FUEL =A15*C15 -16.50! 245.78 =C16*B16;ENGINE ACCESSORIES =A16*C16 -2.75! 27.40 =C17*B17!SEATS =A17*C17 -15.00! 21.00 =C1 8*B1 8!COCKPIT FURNISHINGS =A18*C18 -9.40! 68.00 =C19*B19;STEPS =A19*C19 -27.66; 5.66 =C20*B20!LIGHTS =A20*C20 -10.00! 2.00 =C21*B21!LIGHTS =A21*C21 -7.00! 2.00
WU
WEIGHT
-7.00!
300.00
-31.50!
22.66
=C22*B22;NOSE LANDING GEAR =C23*B23!ENGINE MOUNT =C24*B24!REAR SEATS =C25*B25!HYDRAUUC PUMP FOR GEAR =C26*B26!BATTERY
=A22*C22
-6.00!
=A23*C23 12.00
-15.00! -13.00! -12.00!
=A24*C24 0.00 =A25*C25 0.00 =A26*C26 25.00
=E28/D28! =sum(C5..C26) =sum(D5..D26)
=sum(E5..E26)!CENTER OF GRAVITY
Weight and Balance - Formulas 44 MAY 1990
You need to have the main gear axles further aft than this point or your plane will also be parked on its tail. The other loading case is the combination of pilot, passengers, fuel and baggage that would put the center of gravity furthest aft in normal use. Mark this center of gravity location on the drawing, at the correct fuselage station and waterline, then rotate the plane to its landing attitude (I used 12 degrees
inboard, the tire ended up mostly flying in the
breeze out ahead of the root part of the wing!
ters around to face forward. Guess what? The center of gravity barely moved. So much
around 25% chord, since this is where the
of gravity close together. Now if I moved the main spar under the forward part of the front seats, the back seaters could park their Hush Puppies under the front seats as well. I determined that I could get the tire to fit
for it.
for my earlier notion that keeping everybody's heads together also kept the centers
This didn't seem to be a particularly low drag arrangement. The wing is too thin to try retracting the gear outboard, so that wouldn't
work either. A scouting trip to the airport soon revealed that most factory-builts don't do a very good job of getting their tires inside the wings, either. Most protrude at least a bit below the wing. If you make this pilgrimage to the airport, take a look at all the bumps and lumps different manufacturers have put on their airplanes to try and fair the protruding tire into the wing - it's a real lesson in the power of positive desperation. This convinced us that the place to put the tire was under the pilot's seat. Now, even using a low-profile 6.00 x 6 tire, when you add the brakes and gear strut the assembly ends up being 15 inches in diameter, and 8-1/2 inches thick. I drew this up on my CAD program, allowed some thickness for the pilot's seat and upholstery, parked a 97.5 percentile US Government approved adult male on the seat, and gave him some headroom. Yipes! To make all the above fit, my fuselage was 53 inches tall. It was time to rethink the problem. I had wanted to put the spar behind the front seats, since the pilot faced forward and the backseaters faced aft. To make room for the back-seaters' feet, I had to keep the fuselage flat for a while. Using the mirror-imaging tool in my CAD package, I turned the back sea-
into my root airfoil if I made it a bit thicker and used a 50.5 inch chord at the tire location. I also needed the spar to be straight as
it ran across the fuselage, so I decided to keep the spar straight across the fuselage, then start the forward sweep of the spar at
*he fuselage sides. To make the tire fit, I had to keep the chord length constant across the tire - thus making a straight leading edge. But the wing looked strange unless the trail-
ing edge was swept all the way to the fuselage sides. Being the founder of the Attila the Hun School of Airfoil Design, I decided to mathematically stretch the root airfoil starting
at the thickest part of the airfoil section. This
kept the fat part of the airfoil the same for parking my tire inside, while making the airfoil section's thickness-to-chord ratio smaller
for less interference drag at the fuselage
intersection. A few trips through my army of airfoil performance codes revealed that surprisingly, this appeared to be acceptable aerodynamically as well. Lastly, to make this whole scenario play,
I would have to put the main spar at 21% of chord. Usually, you want the spar to be
0.675; FSi PERIMETER:
)bs/ftA2
49: 8 10 11
12 13 14
15 16 17
18 19 20
21 22
51! 53! 55: 57! 59! 61! 62.5! 67.5: 72.5! 77.5! 8"6T
TOTAL WEIGHT:
tail at the fuselage side, and relofled the fuselage to match the shape. The result is re-
vision 14 of my design, which will be the final version, since Larry and Mike have threatened to have me killed any day now so that they can finally build the airplane. One final trip through the weight and balance is necessary to finalize the design. The
best way to determine the weight of your plane is to calculate the weight of each and
every ply of skin, the weight of the foam or honeycomb, and the weights of each and every fitting, pulley and bolt that goes into it.
I didn't do that. I used a statistical basis for
the weights of each element of the airplane's
structure. The weights I used are as follows: cowling: .675 pounds per square foot
i
MOMENTS!
AREA!
0.528!
23.779:
0.78!
83.475! 93.113; 99.310! 103.477! 107.667; 111.146! 114.597! 117.608! 119.825!
0.709! 0.828! 0.902! 0.951! 0.990! 1.026! 1.058! 1.088; 0.835!
33.312! 40.560; 46.001! 50.380! 54.436; 58.464! 62.432! 66.396! 52.170!
1.05! 1.23; 1.34! 1.41! 1.47; 1.52! 1.57! 1.61; 1.24!
126.248:
2.884: 3.019:
194.648:
4.27; 4.47: -631
'l31L378r 1
35.28 if
136.824:
"3-l'25i ..„....__
218.881! '242'."l80l 127^49!
19.5.36.
657668 WETTEDAREA:
which meant squaring off the lower corners of the fuselage to avoid interference drag. Then I made the decision to start the 4-1 '2 degrees of dihedral at the fuselage sides, in the same place that the spar begins its forward sweep. Any kinks in a spar create local stress in the spar, but we will carry that stress into the large fuselage shell at the fuselage sides, which can handle the load. Then I lofted the airfoil with the stretched
"WEIGHT!
-" • • • • • • • • • • " -4 gp
"45!
47!
Lastly. I lowered the root airfoil section to the bottom of the fuselage, to get the spar as low as possible across the fuselage,
B
ENGINE COWLING WEIGI-rf:i "4 g
lift load is applied. A couple of phone calls to structural designers convinced me that we could do this and not pay a severe penalty
!ft A 2
Fuselage Weight Breakdown - Example SPORT AVIATION 45
1
2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 18 19 20 21 22
B
A I ENGINE COWLING WEIGHT:! FSi 43!
C
0.675 PERIMETER 45 =49.3136+18.41444 =68.82101 + 14.65372 47! =80.02707+13.08544 49! =86.01677+13.2935 =92.83727+10.6402 53! =97.05622+10.61082 55! 57j = 102.8652+8.280828 = 106.3253+8.271292 59! .................................J51.L.. = 111.5025+6.105335 = 113.722+6.103294 62.5! = 122.2204+4.027367 67.5! =131.3776 ...............................72,5]..... 135.281 77.5! 136.824 . . 80]
...........:...:i5j... ::::::::::M:
TOTAL WEIGHT: ! =sum(C5..Cl8_) .F:S,= ...................J..... =sum(D5..D18)/B20 =sum(E5..E18) WETTED AREA: !
D
I
E
lbs/ftA2
WEIGHT
MOMENTS!
AREA!
-E5*$B$2 =E6*$B$2 = E7*$B$2 =E8*$B$2 -E9*$B$2 =E10*$B$2 =E11*$B$2 =E12*$B$2 =E13*$B$2 =E14*$B$2 =E15*$B$2 -E16*$B$2 -£17*$B$2 =E18*$B$2
=C5*A5! = C6*A6! = C7*A7! = C8*A8| =C9*A9i =C10*A10i =C11*A11: =C12*A12! = C13*A13i = C14*A14! =C15*A15^ =C16*A16 = Cl7*Ai7i =C18*A18!
=0.5*(B4 + EJ5)*(A5-A4)/144! =0.5*(B5+E!6)*(A6-A5)/144! =0.5*(B6+E!7)*(A7-A6)/144! = 0.5*(B7+E!8)*(A8-A7)/144; =0.5*(B8 + E(9)*(A9-A8)/144| =0.5*(B9+B10)*(A10-A9)/144! =0.5*(B10+B11) *(A11-A10)/144; =0.5*(B11+B12) *(A12-A11)/144! =0.5*(B12+B13) *(A13-A12)/144] =0.5*(B13+B14) *(A14-A13)/144i =0.5*(B14+B15) *(A15-A14)/144! =0.5*(B15+B16) *(A16-A15)/144; =0.5*(B16+B17) *(A17-A16)/144! =0.5*(B17+B18) *(A18-A17)/144!
!pounds jft A 2
Fuselage Weight Breakdown - Formulas
fuselage
below
the
.655 fuselage as well, since this helps account for
canopy:
pounds per square foot canopy: .055 pounds per cubic inch of canopy material remainder of fuselage: .55 pounds per square foot
the tapes and other attachment parts at the intersection. In the second of these articles I told you how you can find the wetted areas along the fuselage. In the spreadsheet for this article,
wing: 1.515 pounds per square foot tails: 1.6875 pounds per square foot
you will use those numbers in order to estimate your fuselage weight. Once you've
For the fuselage components, the square footage refers to the total skin area (wetted area). For the flying surfaces (wings and tails), the square footage is the wing area as normally used, not the wetted area. The cowling has to be stiff enough to hold its shape. The fuselage below the canopy is reinforced, and has fasteners for the canopy itself. The canopy is based on the weight of the plexiglass used. The back end of the fuselage is the lightest part of the airplane, since there are no moving parts there. The wing includes the weight of the ailerons and the flaps and their actuators. The tails weigh the most per square foot, since they have proportionately
more
moving
parts,
found the total weight and center of gravity location for the fuselage, you can enter it into the spreadsheet to calculate the weight and balance for your airplane. The weights of the engines, propeller, radios, and other pieces of the airplane have to be obtained from the manufacturers. As I pointed out earlier, the weight and balance sections of the pilot's handbook for factorymade airplanes are a great source of this information. As you make this list, you enter
the weights and center of gravity locations as to Fuselage Station and Waterline into
the weight and balance spreadsheet. While you can calculate the weight of the
flying surfaces using the formulas for metal
hardware and fittings per square foot. I got these weights by checking on the weights of
airplanes, or educated guesses for composites, it is more difficult to pinpoint the center of gravity for each of these pieces. A good
use the formulas in Pazmany's book or any other design book you are using, since metal
Aerodynamic Chords of each flying surface. This will give you Buttlines for the wing and horizontal tail, and a Waterline for the vertical tail. This leaves you with the issue of what Fuselage Station to use. The answer will depend on how the weight is distributed chordwise inside the flying surface. If the wing spar is going to be located at 50% of chord, and the beasty has heavy flaps, you might guess that the center of weight will be at 58% of the Mean Aerodynamic Chord. If your spar is at 25% of chord, and you don't
known composite airplanes. If you are making a metal plane, you can
weight estimation has been perfected over
the years. You can also use the metal weights for a composite airplane, by adding 10 to 15% more weight than the formulas call for. Sorry, folks, but the average composite airplane is going to be heavier than its
metal counterpart, but will probably be
stronger also. When you figure the weights for the flying surfaces, include the areas hidden inside the 46 MAY 1990
bet is to use the location for the Mean
use flaps, then the center of weight could be at 28% of the Mean Aerodynamic Chord. So you have to be the judge. Once youUve made the decision, enter the weight and center of gravity locations into the spreadsheet. If you are really uncomfortable with your guess, then bracket the solution. For example, if you guess that the center of weight of your wing is going to be at 35%
of the MAC, but aren't too confident, go ahead and try 25% in the spreadsheet, and write down the airplane's center of gravity, then change your guess to 45% of chord, and write down what happens to the airplane's center of gravity. This is the beauty of working with spreadsheets. You can play "what if" games to your heart's content. You'll find that an error of a few inches will not be catastrophic. Since the engine is going to be the heaviest single part of your plane, you might wait until you get the airframe built before you build the engine mount. Moving the engine forward or aft a couple of inches gives you a terrific way of
adjusting the balance of the airplane, assuming that you weren't off a country mile when you started. When all else fails, you do what the factories do, and shove the battery around until the airplane balances. MAKING THE SPREADSHEET
There are 2 spreadsheets for this article, both of which are quite simple. The first one shows you how you can do a weight buildup
for the fuselage structure. I have divided the fuselage into weight groups, which are the cowling, canopy, fuselage below the canopy (which is heavier due to the various attachments), and the aft fuselage. Each element
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
B I A I DLR2 WEIGHT AND BALANCE 9-18-89 ! FS! 51.5! 124.0! 124.0! 31.8; 282.8! 273.4! 120.0; 121.5! 151.5; 94.5; 119.1! 63.0! 125.0! 110.0! 168.0; 72.5! 101.5! 69.6; 74.0! 142.0; 151.5! 75.0!
WL! -7.00! -14.62! -4.91! 0.00; 24.63! 2.15! -8.00; -26.80! -8.00! 6.41; -16.50! -2.75! -15.00! -9.40! -27.00; -10.00! -7.00! -31.50; -6.00! -15.00; -13.00! -12.00!
106.2:
-0.10!
c
I
D
I
E
I
F
DESCRIPTION WEIGHT! FSMOMENTS!WL MOMENTS 15450.0! -2100.00!ENGINE (IO-360 180 HP) 300.00; 155.71; 19302.0! -2276.51!WING 16560.4! -655.98!FUSELAGE 133.60! 1749.7; 55.00; O.OOiPROP 5232.2! 18.50! 455. 73!VERTICAL TAIL 33.38! 9126.9! 71.76IHORIZONTALTAIL 300.00; 36000.0; -2400. 00;2 PEOPLE 70.00! 8505.0! -1 876.00IMAIN LANDING GEAR sbsd^oT 20.00! -160.00^BAGGAGE 45.00! 4252.5! 288.45!INSTRUMENTS & RADIOS 245.78! 29260.6! -4055.37!FUEL 1726.2; 27.40; -75. 35!ENGINE ACCESSORIES 21.00! 2625.0! -315.00!SEATS 7480.0! 68.00; -639.20!COCKPIT FURNISHINGS 5.00; 840.0; -135.001STEPS 2.00! 145.0! -20.00!LIGHTS 2.00! 203.0! -14.00!LIGHTS 1531.2; 22.00; -6 93. 00;NOSE LANDING GEAR 12.00! 888.0! -7 2. 00; ENGINE MOUNT 0.00; 0.0! 0.00!REAR SEATS 0.00! 0.0! 0.00! HYDRAULIC PUMP FOR GEAR 25.00; 1875.0! -300.00!BATTERY 1561.37!
165782.7:
-14971. S^CENTER OF GRAVITY
Weight and Balance - Example
is further divided into strips, which do not
in the example spreadsheet, or however
engine cowling as an example In column A,
many you need for your structure. The $B$2 in column C tells the spreadsheet not to
have to be of equal size. We will use the
I have listed the fuselage stations (in inches)
at which I have measured the perimeter of the cowling. I entered the perimeter (in inches) into column B. Notice that in rows 5 through 15, two values are entered and added together. These represent the cowling itself (first number) and the spinner fairing (second number). Column E calculates the area of each strip by averaging with the preceding perimeter and multiplying by the width of the strip, then converting to square feet. Column C then multiplies by the weight per square foot, which is contained in cell B2. Column D calculates the moment of each strip based on its weight. The total weight appears in cell B20, the fuselage station of the center of gravity is reported in cell B21, and the wetted area of the cowling is in cell B22. Your piece of structure may have more or less strips than the cowling did. No problem. Enter the titles as shown, and the weight per square foot in cell B2. Now type the formulas into row 5. columns C. D. and E. Now use the copy command ( C for Lotus 1 -2-3) and copy the formulas in cells C5, D5, and E5. You put the copies in cells C6 through C18
change the cell address B2 when it makes
the copies. We use the spreadsheet built-in function SUM() to add up all the columns for us. If you have more or less cells for your structure, you need to change these formulas to reflect the number of rows you have. For example, in cell B20, the formula tells the
spreadsheet to add together all the numbers
from cell C5 to cell C18. If you used rows 5 to 15 for your airplane, you'd change the formula to = sum(C5..Cl5). The example shows that my engine cowl
weighs 19.536 pounds, its center of gravity
is at fuselage station 65.068, and its wetted area is 28.94 square feet. These numbers can then be entered into the weight and balance spreadsheet. The other spreadsheet calculates the weight and balance for the entire airplane. It calculates both the horizontal (fuselage station) and vertical (waterline) locations of the center of gravity. Again, type in the titles as shown, and the sample numbers from an earlier version of my airplane as shown. Type the formulas into cells D5 and E5, then copy the formulas
down to row 26 as before. Lastly, enter the
formulas in row 28 as shown. When you are done, the example shows that the airplane'Us center of gravity is at fuselage station 106.2, and waterline -.10, and the gross weight is 1561 pounds. You can change the weight of the pilot and passenger (2 PEOPLE), or put some people in
the back seats (BAGGAGE) to see how my
plane responds to various loadings. For users of Lotus 1-2-3, change all formulas beginning with an equals sign ( = ) to a plus sign ( + ), as we discussed before Change - sum to (« sum everywhere it appears. Hopefully, you're getting used to this by now.
Lastly, I'm showing you revision 14 of my
airplane, for those of you who've written asking for it. You may notice that it's got a 6 cylinder engine in it. We decided to design the cowl to accommodate an engine that large, so I digitized Lycoming's appropriate blueprint. This cowl has room for a third world family of 9 to live inside, if I build it with the 4 cylinder powerplant. REFERENCE
Gordon, J. E., Structures, or why things don't fall down. Viking Penguin, Inc., 40 West 23rd St., New York, NY 10010. SPORT AVIATION 47
