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Jacques Arnould's HM 293 Sea plane from Tahiti
Photo: J. Arnould
Source: L' Amateur Formule Mignet
Newsletter of the Association of the Flying-Flea lovers
FLOATS FOR HM-293
BUILDING INSTRUCTIONS
Translation by: Paul PONTOIS
1-Cut the center rib in a sheet of 4 mm plywood. Glue the top and bottom longerons on each side (The upper longeron could be straight. It is just a matter of aesthetics.
If you experience difficulties when you bend the longeron to follow the contour of the stem, just go as far as you can and cut the laths. You will make the nose afterwards, but do not cut the plywood rib.
2-Draw the bulkheads location and cut out lightening holes (optional).
3-Draw and cut bulkheads # 5, 6, 7 and 8 which are identical. (3mm plywood). Also cut bulkhead # 9 for this one, check the height before cutting, as I may have made a small measurement mistake on the plan. From the top, it is easy to draw, as all angles are identical.
Cut all bulkheads in two, along the vertical axis, taking into consideration that you have to remove 4mm for the thickness of the center rib.
Glue these 5 bulkheads in their location with one strip of 10 mm x 10 mm on each side for more resistance. (before doing this, you have to notch the plywood for the angle members, Position and glue the angle members, keeping enough length to the front and to the rear. Bend member A (on page 5). Strain it with clamps and wedges to obtain a smooth curve, as close as possible to the shape shown on the plan of sheet 1 (the front must be round enough). Measure width and height at each bulkhead location, then draw and cut up the bulkheads. Same procedure to the rear, after bulkhead #9. All slopes are the same. Then, position and glue bulkheads and members.
4-When this frame is completed, you can cover the deck (top of the float) with a 4 mm plywood sheet, which will stiffen the structure. Glue the support brackets plywood reinforcements.
5-Glue the 3 mm plywood vertical side panels. (as the plywood sheets should be too short, joins will be needed. Splices are not compulsory, the sheets may be glued end to end, on a strip of plywood, preferably between two bulkheads to prevent excessive thickness) .As the oblique sides are not yet in place, it is possible to use clamps or large size clothes pegs.
Next step is to plane the plywood sheets at the level of the longerons and to position and glue the oblique side sheets (made of 3 mm plywood they will have to be stapled or nailed).
Placing the float upside down, it will be easy to position and glue wooden reinforcement blocks for the tubes in which the base bars will slide. When dry, drill the holes, then install and glue (epoxy resin and fiberglass fabric) the tubes (25 mm dia), previously sanded for a better grip. Bolt the deck's brackets. (check alignment) .
At this step, it is possible to close the floats by fixing and gluing the bottom panels (4 mm plywood).
For bulkhead # 8, glue two support members to fix the covering of the rear part, at the rear of the step (see sheet 3). For the stem, stop at bulkhead if you cannot bend the longerons more. Then, on each side of the front of the center rib, glue two pieces of styrofoam. Sand them to shape cover with 2 or 3 layers of fine fiberglass fabric (120 or 150 gram/square meter)
Conclusion:
As they are, these floats work fine but can be improved. These notes must be considered only as a starting point.
For instance, it is possible to install front to back strakes to improve Squat and lift-off. The floats can also be built in styrofoam and resin/fiberglass carbon fiber, Kevlar etc.
The base bars can be fixed on the deck, which is easier, but not as beautiful and adds drag. etc. etc.
Cheer up and happy flights !
ADDITIONAL COMMENTS
These floats have enough buoyancy for a pilot weighing as much as 80 kg (my weight is 67 kg). A friend of mine, who weighs 80 kg, flew my HM- 293 on floats. They weigh about 12 kg each. I do not know the influence they have on the flight characteristics, as I never compared with a HM-293 on wheels under the same conditions.
The engine is a Rotax 503, single carburetor, air cooled, single ignition, with points, reduction gear 2/1 with notched belt. The plane cruises at 90 km/h at 4,500 RPM (prop diameter: 150 cm). Fuel consumption is 8 liters par hour with the rotary slide valve opened between 1/4 and 1/2 (I made marks on the throttle handle). The engine is mounted in the normal position, plugs on top. The exhaust manifold is made in duraluminum 4mm thick, welded by a professional welder. The muffler is a big pot, made of thick aluminum and is installed between the engine and the firewall. Nothing is outside the engine cowling but the bulky intake silencer which I unfortunately had to place on the left side. (the engine could not digest the sea spray which were not stopped by the classical air filter). Now, with the silencer, everything is fine. My tank held about 16 liters. I made a new one in 1.5 mm thick plywood + epoxy/fiberglass. The new one holds 26 liters, which gives me a satisfactory range.
I start the engine with the manual starter and a large diameter return pulley ear of the cockpit. Generally, the engine starts after pulling one or two times on the rope, if I inject some gas in the engine through a primer fixed on the instrument panel which I recuperated from an outboard Johnson motor.
As my plane is put away in my house's garage, I can easily check the various components between flights (spark plugs, driving belt, gas lines, electrical wiring, rinsing, etc.) Hurrah for the folding wings!
I am using the original landing gear brackets (or their location). This way, the Pou could be put back on its wheels without modifying the frame. The only point as shown on my drawings is to prepare the reinforcement blocks for the float legs brackets (20 or 22 mm dia tubing) during the fuselage building process.
The great advantage of the Mignet formula on floats, is that as the longitudinal control is made by the up and down movements of the front wind around its axis. The angle of the fuselage with the water surface is very small when taking off or landing and consequently the floats are very little nose up.
Take off and landing, so critical on the classical floatplanes are nothing but a mere formality with the HM-293.
When landing, earthling onlookers believe that I am a super pilot even the plane touched down by itself!
As you may have seen on the photographs, I placed flaps on the rear wing, as M. Croses did on his Criquet. I knew that they would be of no use for a HM-293 on wheels, but I was afraid of a possible nose down tendency due to the distance between the water level and the propeller axis, and to the drag floats in the water. My fear was unjustified and the flaps stay all the time in neutral position. They are totally unnecessary for a HM-293 on floats. A useless building difficulty. I am now sure of it.
Jacques ARNOULD
TRANSLATION OF THE COMMENTS WRITTEN ON THE PLANS
From left to right and from top to bottom
SHEET 2
-Ferrures 1 et 2. Les embouts des tubes sont faits de fafon á s 'adapt á ces ferrures .
-Brackets 1 and 2: the ends of the tubes are made to fit the brackets. -Ferrure du train
-Landing gear bracket
-Entretoise
-B ushing
-Rond en dural
-Alu rod
-La ferrure3 refoit deux tubes: I vers I' avant, I' autre vers I' arrie ( voir feuille 3 )
-Bracket #3 takes 2 tubes. One to the front, one to the rear (see sheet3)
-fixé dans le flotteur
-embedded in the float
-Solutions for
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