HM-8
How I Designed My Little
Plane
How I designed my little plane? Flying was the obsession of my life.
The sight of a flat plain, a beautiful cloud, a crow taking off, made me sick. Something
had to be done. Buy an airplane? Flying lessons? A vacuum cleaner for 600 francs, okay,
but 60,000 for an airplane? Nothing doing! I want a wheelbarrow, they offer me a truck. A
flier spends 40,000 francs a year.
Build a glider? Here are plans for a Zogling I bought for 100 marks in Germany. Junk, no
esthetic quality, no technique. Is that the result of vulgarization efforts? Streamlining?
Purety of lines? What is that poor carcass, that shadow of a glider? And then, what could
I do with it? It needs people helping, a winch or a car to pull it. Quite an organization.
But I am alone. Why not build a real airplane? Is it possible? Two problems, difficulty of
construction and budget.
I know that ordinary amateurs, with precise plans, build gliders perfectly strong. If there is a problem it will be adapting an engine. Could not a garage mechanic do it? The financial problem scares me more. Since manufacturers ask a fortune for airplanes, they must be made of very exotic materials, very expensive, and have Baccarat flower bases. Actually, there is not that much material involved and except for the engine it is only carpentry work. Let's suppose the problem is solved and look at some figures.
The airframe weighs about 165 lbs, an average Frenchman about the same, let's allow 110 lbs. for mechanicals. That's 440 lbs. Let's look at the chart compiled from 80 different post-war planes. To be able to take off from a rough field, fly with questionable gasoline, we want a 4,000 meter ceiling (13,100 ft.) For example, a wing loading of 15 kg. per square meter (3 lb./sq. ft.) needs enough power to not exceed 13 kg./hp (28.6 lbs.). So for 440 lbs. (200 kg.) we need 13 sq. meters (140 sq. ft.) and an engine of 15 hp. Maximum speed will be 100 kmh (62 mph), cruise 44-50 mph. From experience, such a plane takes off at 25/30 mph. This corresponds to a glide ratio of about 8 to 1. That's mediocre, but can be handled by a beginner. The expert can get better performance, but let's play safe and just "play aviation."
The engine? That's the big question mark. Without engine, the best airplane is an inert carcass while a bad glider can fly well if it is pulled by an engine. Let's look around. Where? 15 hp? Is not that the power of a 500 cc modern motorcycle engine? Well, now we get performance ... a 500 cc engine gives 15 to 17 hp at a very high speed, 3,000 to 4,500 rpm. It is useless to consider a direct drive prop at those speeds. Why? Because to overload the engine at a slower speed, it would take a small prop, less than 1 meter. Experts will tell you that at 125 mph such a prop would have an excellent efficiency, but I tell you, an airplane is not only supposed to fly, it must take off, too. If the field is not perfect it will never take off. So, we must mount the prop on a jackshaft, with a transmission, so that the engine can give its power at 3,000 to 4,500 rpm and the prop be around 6-1/2 ft and turning at 900 to 1000 rpm. The ratio will be 3:1 and sprockets of 16 and 48 teeth will do. The chain will be from a big motorcycle. The prop and its sproc- ket will be bolted to a Harley Davidson sidecar hub unless we prefer a 30 x 35 mm tube and ball bearings. We will connect the prop shaft to the engine with 21 x 24 mm tubing, eventually to the engine head if possible. This way our "power plant" will respond to our wishes: Take offs in 200 ft. and fast climb. Long live the slow, large prop.
Budget! Budget ... that's right. I almost forgot. It is
marvelous. How much does a motorcycle engine cost, new? 3,000 francs. Used, 500 to 1000.
(Note: At the time Mignet was computing these costs, the franc was worth about five
cents). As an example, a Harley Davidson engine weights 110 lbs., but gives 20 hp. There
are 7,000 of them in France.
About the airframe, here are the bills for new items:
2 wheels 440 francs
32 sq. meters fabric 260 francs
Cellulosic varnish 200 francs
Hardware 200 francs
Plywood 300 francs
Wood 400 francs
Timber for prop 30 francs
Miscellaneous 170 francs
Total 2,000 francs
Without going to extreme economy and using very good quality items, an overhauled, used
engine, our little airplane will cost us well under 4,000 francs ($200 at that time).
The Airframe
The airframe. The point is to design something simple and strong, while
having real aerodynamic qualities. I have my pride as an amateur. I like things clean,
simple, pleasing. I don't want people making fun of me. I am French; I want to adapt in a
very personal way what scientists have recently designed.
I will get a good documentation, magazines and books and will extract whatever I can use,
according to my skill. I may not have a super airplane, but I will have a good little
airplane.
First, since I may have to wait for an engine either because of my pocket book or lack of
a good bargain, my bird must be able to glide without an engine and be easy to set up,
regardless of the engine weight.
So: Independence between fuselage and wing. The wing will mount on the
stand about the fuselage so that the vertical of the center of gravity is at the right
place in relation to the center of lift of the wing. Then my airplane will fly as well as
the best of the industry designed by "Mr. Chief Designer of the Big Company. . . .
" It is a matter of plumb line, shims, bolt in one of the holes provided on the
stand. There will be one set of cables for the glider and one for the airplane if needed.
The Wing
The wing? Of course it will be a parasol. A logical and reasonable man would not consider
a low wing which hides the most essential sight the ground below. Moreover, no one, not
affected by "professional deformation" would conisder a light structure in
complete cantilever. I will reinforce my wing with some strong cables. I will fly with
peace of mind and enjoy the scenery below my wheels.
The wing structure? I want a wing quickly built. I am an amateur, I am
in a hurry. Standard structures include too many fittings, bolts, diagonal bracings, etc.
Too many adjustments.
I will copy the German method (let's take from others what's good), a
"monolongeron," but instead of using the monospar as leading edge, which
involves curves, delicate gluings, I will bury my monospar within the wing. It will be
more "amateur." This spar, an enormous box of 7" x 7", weighs 33 lbs.,
is a beam of incredible rigidity. Three reinforcements allow mounting the wing on the
stand and the side flying wires.
The airfoil is determined by 24 ribs 12-5/8" apart, set exactly with wooden shims. The shape of the ribs will determine the stability of my bird. I want a stable plane, which flies well right away. To hell with hollow airfoils ... I prefer a rather flat one, less lift but better stability. I choose a double curvature,the neutral fiber convex forward, concave behind. I give the necessary thickness for the spar and thin out the ribs tails so that the top and bottom air layers are parallel at the trailing edge, like the birds. I imagine it will not be bad. 15-3/4" from the trailing edge I insert a strip which will receive the ailerons hinges and give rigidity to the wing with a small extra flying wire to avoid any torsional stress on the spar. The spar is located ahead of the center of lift of the wing, regardless of the angle of incidence. The leading edge is pointy to make it easier to build and lighter. A round one is necessary for a powerful, fast airplane, but not for our low power, with an incidence usually close to the economical range.
The laboratories have shown us that elliptical wings are better.
Square ones? Horror, finished, forgotten. The chord length must decrease considerably from
the center to the tips. What a difficulty ... I want easy work, a "standard"
construction. Thanks to my thin rib tails and full span ailerons, I can make 18 ribs on
the same jig and only 6, different two by two, on three jigs.
I don't have to worry about the aspect ratio since my plane has plenty
of area and is slow. It is a continental bird, more a vulture than an albatross, 26 ft. 3
in. wingspan and 6 ft. 3 in. chord will be enough and my wing will not be cumbersome.
I will have an excellent stability and eliminate any tendency to spin if I reduce the
incidence toward the wing tips. For that, I will set the last ribs so that their tail be
1-5/8" higher than the central ones. Straightedge, shims, string will line up the
other ribs and give me a perfect symmetry of the two wings. If my "monobloc"
wing is slightly twisted, the rear little flying wire will correct it. If it is not
enough, the ailerons will take care of it. The aileron ribs, extremely simple, will be cut
to form the elipse of good performance. These ailerons, running the whole wingspan, can
modify the curvature of the wing by pulling both cables at the same time. By the way, I
only want outside cables, visible, easy to lubricate and inspect. Pulleys? Bellcranks? NO.
Control will be direct, straight from the stick home to the ailerons, like a flying wire.
This way I avoid the terrible wing vibration and my roll control will be smoother than the
best ball bearing set up.
Finally, when I turn the wing lengthwise over the fuselage for road
travel behind my motorcycle, I will told the ailerons over the wing and the total width
will not exceed 5 ft.
Complete, strong as reinforced concrete, my wing is all wood, without
any other metal than the many small nails used to hold the plywood while the glue dries.
Here is an ideally simple wing, without mechanical parts, only wood and fabric, that I
build in ten days. Long live the "monolongeron."
Materials? Standard, standard. Let's simplify everything. Wing,
fuselage, control areas, consist of wood of only two dimensions: 3/4" x 1-3/16"
for the structures, 1/4" x 1/2" for the small parts, ribs, etc. There might be a
couple of other parts made out of hard wood, but all the rest will be pine. A carpenter
will cut it for me on a band saw, from a clean board without knots and with straight
grain. It the blade is sharp the strips will not have to be planed, the glue will be that
much stronger. The plywood will be Okoume 1/16" for the wing and the rear of the
fuselage, 1/8" for the front and for reinforcments. For the fun of it, I figured out
all my dimensions, one third over 7 Gs so as not to be in danger, should there be a knot
or a defect in the wood that I would not have noticed.
Now the fuselage. Am I chicken or careful? I don't like those planes on
long legs. it seems that the breakage of an axle would be catastrophic. I have seen many
planes like "pylons" or even on their back. Also, some beautiful cantilever
wings without the smallest asperity, without strut or wire, but with a complicated
structure for landing gear. Oh, logic . . . my axle will go through the fuselage and only
the wheels will be in the airstream. Besides the good aerodynamic quality, I will be able
to land without risk in tall grass or wheat.
The low fuselage will not tend to flip over and an accident will
only result in dragging and no iodine.
A fuselage ... is some sort of packing crate but since plywood cannot be nailed to itself
to assemble the sides, a thicker strip will receive the nails which will clamp the glue.
Of course, to simplify, even at the cost of a slightly higher weight, the whole fuselage
is all plywood. The four strips making the edges get together at the rear, a strong point
that would take a tremendous ground loop to give way.
The pyramid behind the pilot's head reinforces the wing stand which is
supported in front by an "A" frame in tubing bolted to flat stock fittings. The
front bears a cross tube with motormounts welded by the neighborhood garage man. This tube
is isolated with rubber bushings. Until an industrialist comes up with a "power
plant" that I can mount with three bolts, not four, I will mount the prop shaft at
the top of the "A" frame of the wing, with another "A" frame bolted
somewhere on the engine.
Should I have a beautiful cowling from the engine to the windshield? We will see later,
when the wrench will stay in the too] box. It will be a slight loss, for a few mph, but
that's too bad.
The gas tank will be in the wing, behind the spar, between two ribs and will take 1 0 to
15 liters. The cap? A shaving soap tube. A float in a glass tube will show me the fuel
left. A filter, as indispensable as a crash helmet and a seat belt, just before the
carburetor. Mothers: Demand helmet, seat belts and filter.
My fuselage will be very long, 11 or 13 feet from axle to tail post.
The controls will be efficient. Tail feathers? My bird has large empennage and wing and
will not be able to taxi with an average tail wind. It will weathervane, it will make
figures eights. Instead of a tail skid, I will have steerable wheels mounted on the same
axle as the rudder which, being light, will follow the bumps while the "lentil"
wheels will guide the plane despite the wind.
I have a stable wing, long fuselage, I don't need a fixed stabilizer. I
don't care about Mr. Designer's smiles. My elevator will be lined up with cables and pivot
on a "finger" mounted at the rear of the fuselage. It won't break. A pyramid
supports the rudder. In case of rain my bird will get wet: I will paint it inside and out
with two coats of lacquer or cellulosic paint used on cars. They are not soluble in
gasoline. Just take a rag to it and it will look brand new.
"You forgot the prop, where did you buy it? How much did you
pay for it?"
Yes, I forgot that easy, pleasant little job. In fact, a prop is like a small wing turning
in circles. The speed at the tip of our prop is in the order of 125 mph, 62.5 at the half
diameter and 31.25 at the quarter diameter. The air resistance varying with the square of
the speed, the important part of that stick is the outside half. It is a wing with simple
curvatures easy to cut with a plane. The rest is hub and arms and we are only concerned
with centrifugal force.
Knowing the airplane speed and the prop rotation, the incidence is
drawn at the tip, half and quarter radiuses. Marking the beech or walnut timber is then
very easy. A few cuts with a saw, removing the extra material with a chisel, then planing,
automatically makes this delicate masterpiece of the craftsman. Cost of the timber? Less
than $2.50, including varnish. Cutting, one day, finishing another day. My last one is No.
14 and not much better than no. 1. No training needed. Not a shade of difficulty. And I
will have built my bird in forty days of eight hours each.
Epilogue
Now, dear reader, are you convinced of the possibility? Accept these
lines as simple directives. If you like my HM-8, the result of ten years of amateur work,
don't change anything. You might have some unpleasant surprises, or design yourself and
check the influence of each pencil stroke.
You have an attic, a basement, a little corner of a shop. In a box, drill, plane, file,
hammer, the tools of an amateur. Sheet steel is available at the hardware store, the
moulding maker will send you the wooden strips, you will go to the movie only once a
month. No more Pernod (a French liquor) and Gitanes (cigarettes). While the mental
faculties will improve, the $200 will fall in the piggy bank. You will become the owner of
an airplane. You will polish it. The well lubricated engine, this thoroughbred of modern
mechanic, will give it this particular personality of a live being to which your reflexes
will obey. It will be your child and master, familiar machine to be defended, cherished
more than your old camera or chrono.
Empty dreams? Slow ambitions? You will have lots of fun, you will learn
to drive an engine not only by the throttle but by the carb jets, valve lifters, spark
plugs, chain, tachometer, wrench.
You will really fly. Your engine will take you under your wing to live
the life of flight. You will learn the reactions of the atmosphere. Maybe you will meet a
flight of eagles and try to fly like they do and learn to use the turbulence of the air.
