We have had many discussions over the years on Steve Wood's Sky Pup ultralight for its unusual wood-fabric-foam-construction and its rudder/elevator two-axis controls. One thing I have often daydreamed about is a Super Sky Pup...not a direct derivative of Wood's design, but a new original design using the same basic design principles (but not construction) to create an affordable, easy to build and easy to fly plane in the U.S. light sport or European microlight category.
Here's what I came up with for a single-seater assuming a 38 hp Polini Thor 303 two stroke with dual ignition and liquid cooling as the base engine. I actually increased the aspect ratio from 7.4 to 8.0 to get nice round numbers for the span and chord.
| PRELIMINARY PERFORMANCE ESTIMATE FOR PROPELLER DRIVEN AIRCRAFT | ||||
| Crew weight | 200 | lbs | ||
| Fuel + baggage weight | 100 | lbs | ||
| A/C empty weight | 360 | lbs | ||
| Total weight | 660 | lbs | ||
| Stall speed (flaps up) Vs1 | 36.01 | mph | ||
| Climb airspeed | 46.813 | mph | (climb speed = 1.3 x stall speed) | |
| Climb airspeed | 68.7 | ft/sec | ||
| CLmax | 1.55 | (flaps up) | ||
| CL at 1.3 Vs | 0.92 | (at 1.3 Vs, CL = CLmax x (1/1.3²)) | ||
| Wing area required | 128.3 | ft² | (L=1/2 rho V² S CL) | |
| Propeller efficiency at take-off | 65 | 70% | ||
| Propeller efficiency in the climb | 70 | 75% | ||
| Propeller efficiency in the cruise | 80 | 85% | ||
| Cd (profile) | 0.02 | 0.01 | ||
| Cd (parasite) | 0.03 | 0.015 | (Speeds between 1.1 Vs and 2.7 Vs) | |
| Aspect ratio | 8 | |||
| Induced drag factor K | 1.2 | (Cd (induced) = K CL²/π A) | ||
| (K = 1.0 elliptical wing, 1.1 moderately tapered, 1.2 rectangular) | ||||
| Wing span | 32.0 | feet | ||
| Mean wing chord | 4.00 | feet | ||
| ESTIMATED PERFORMANCE WITH AN ENGINE OF YOUR PROPOSED RATED POWER | ||||
| Selected rated shaft power | 38.0 | BHP | ||
| Effective flat plate area | 6.69 | ft² | ||
| Take off run | 400 | feet | ||
| Rate of climb at 1.3 Vs | 925 | feet per minute | ||
| Max level speed | 87 | mph | ||
| Cruise speed at 75% power | 79 | mph | ||
| Flaps up stall speed | 36.01 | mph | ||
| Glide descent rate at 1.3 Vs | 405 | feet per minute |
I could see something like this going together as a simple aluminum tube-and-gusset fuselage with plywood seat and firewall with the same lines as the original Sky Pup but tricycle gear with a free-swiveling nosewheel and differential brakes to simplify landing in a crosswind. Wings would be aluminum spars and perhaps Ercoupe-style Warren-truss plywood ribs for torsional stiffness and a single strut each side, all covered in Oratex. Cockpit would keep the Sky Pup lines but optionally enclosed and include a single door for easier access.
Two-axis controls would, of course, remain with the builder's choice of stick and rudder bar/pedals (stick moves fore and aft only for elevator, bar/pedals for rudder) like the Sky Pup or stick only (stick fore and aft for elevator, left and right for rudder) like a Pou-du-Ciel, the latter along with dual hand lever brakes providing a great option for anyone with limited or no use of their legs. For this light a design with generous wing area so it should slow down in a hurry, I think you could get by without any sort of flaps or air brake even without the ability to slip it in, will just take some practice to get the approach right.
A two-seater of about 900-1000 lb gross weight could potentially be an option with a reasonably affordable and reliable current-production two-stroke of 50 hp or so (Hirth, Simonini, or...?) or maybe a minimalist, belt-drive 1.0 liter Suzuki G10 three-cylinder four-stroke conversion. That might require flaps or an air brake for glide slope adjustment.
Thoughts? Pros? Cons? As usual, I've got my fireproof undies on. ;-)
