I’ve been thinking about this aircraft idea for awhile, so I thought I’d just put it out here for public ridicule, er, comment.

In a nutshell: A small, single-seat, twin engine, centerline thrust airplane powered by small, inexpensive 4-stroke industrial engines. I’ve been thinking of it as the “Micromaster.” (For those who remember the Beetlemaster discussion, this may look a little familiar--two inexpensive engines in a centerline thrust plane, Cessna Skymaster style. In reality, the planes would be very different. The Beetlemaster is an idea I still I think about all the time . . .)

The HP-per-buck of these little engines is attractive, and of course there have been several single-engine designs that use them successfully, including the Colomban MC-30 Luciole and the Minisport SD-1. These airplanes get good performance and reports indicate they are fun to fly.

So, why make a twin? The objective of adding a second engine would be:

1) To safely remain airborne in the event one engine experiences a loss of power

2) Improved climb and cruise performance in normal operation (i.e. with both engines running)

3) Enable the use of heavier but simpler, easier, and more robust construction (e.g. maybe solid-foam core wings and a welded-tube cabin rather than “a thousand sticks”)

These sturdy (but heavy) little engines are quite a bargain at prices ranging from $750-$1500 each. Even if a bolt-on additional bearing or a simple redrive is used to take prop loads, it’s possible the hardware for the whole twin-engine setup could cost less than $5000. “Hop-up” parts and shops who will do enhancements are out there. Increasing the output of the stock engines by 30+ percent is not expensive, and the reliability issues/concerns of doing these "enhancements" might be reduced in a twin engine installation

The numbers below are a quick, rough estimate of the two-engine and single-engine performance we might expect from a Micromaster. I made a lot of assumptions and did not do a complete drag buildup on the Micromaster (I just based things off the SD-1, increasing the induced drag due to the amount of extra lift we'd be needing). Despite the low HP/weight of these engines, it does look like it may be possible to use two of them to get good “normal” (twin engine) climb rates, and also still have

There's nothing here about configuration (high vs low wing), construction material type, etc. Just some back-of-the-envelope figuring.

Supplied as grist for the discussion mill.

The logical

Notes:

1: Computed from stated L/D, MTOW, and assumed 60 kt airspeed for best glide distance

2: At stated climb rate and MTOW assumed

3: Estimated lbs of drag based on MTOW, wing span and area, and resultant required Cl. Corresponding HP estimates are rough, depend on propeller efficiency at that airspeed and other factors.

4: The SD-1 has several engine options. Specs here are for the 28 HP 4-stroke B&S engine

5: Sum of the two cells above.

6. (deleted)

7. Empty weight buildup: SD-1 weight of 276 lbs + another engine (75 lbs), + prop and hub (12 lbs) + 38 lbs additional structure (engine mount, larger&stronger wing, etc) = 401 lbs

8. Useful load: 250 lb (pilot + baggage) + 66 lbs fuel

9. Required HP derived from sink rate at assumed 13:1 L/D at 60 kts and acft at MTOW.

10. With 22’ span, 70 sq ft wing, MTOW of 717 lbs.

11. Assumed “clean” Max 2D Cl of 1.4

12. Max and cruise speeds will be highly dependent on propeller efficiency at higher airspeeds. Fixed pitch props will likely have to be chosen to produce acceptable single-engine performance rather than top speed. If we simply apply the “speed increase = power increase ^ 0.33” equation and use the SD-1 as our “base case,” then the Micromaster with two 25 HP engines would have a top speed of 141 MPH. In reality, it will be appreciably less than that.

In a nutshell: A small, single-seat, twin engine, centerline thrust airplane powered by small, inexpensive 4-stroke industrial engines. I’ve been thinking of it as the “Micromaster.” (For those who remember the Beetlemaster discussion, this may look a little familiar--two inexpensive engines in a centerline thrust plane, Cessna Skymaster style. In reality, the planes would be very different. The Beetlemaster is an idea I still I think about all the time . . .)

The HP-per-buck of these little engines is attractive, and of course there have been several single-engine designs that use them successfully, including the Colomban MC-30 Luciole and the Minisport SD-1. These airplanes get good performance and reports indicate they are fun to fly.

So, why make a twin? The objective of adding a second engine would be:

1) To safely remain airborne in the event one engine experiences a loss of power

2) Improved climb and cruise performance in normal operation (i.e. with both engines running)

3) Enable the use of heavier but simpler, easier, and more robust construction (e.g. maybe solid-foam core wings and a welded-tube cabin rather than “a thousand sticks”)

These sturdy (but heavy) little engines are quite a bargain at prices ranging from $750-$1500 each. Even if a bolt-on additional bearing or a simple redrive is used to take prop loads, it’s possible the hardware for the whole twin-engine setup could cost less than $5000. “Hop-up” parts and shops who will do enhancements are out there. Increasing the output of the stock engines by 30+ percent is not expensive, and the reliability issues/concerns of doing these "enhancements" might be reduced in a twin engine installation

__if__it has safe single-engine performance.The numbers below are a quick, rough estimate of the two-engine and single-engine performance we might expect from a Micromaster. I made a lot of assumptions and did not do a complete drag buildup on the Micromaster (I just based things off the SD-1, increasing the induced drag due to the amount of extra lift we'd be needing). Despite the low HP/weight of these engines, it does look like it may be possible to use two of them to get good “normal” (twin engine) climb rates, and also still have

*acceptable*single engine performance. There’s no doubt that the single-engine situation would require some careful attention during the design phase, and this first look indicates 25 HP engines might not be enough.There's nothing here about configuration (high vs low wing), construction material type, etc. Just some back-of-the-envelope figuring.

Supplied as grist for the discussion mill.

The logical

__next__step would be-- 3 engines, two seats. We could call it the "Tri-Mow-tor."SD-1 Minisport | ..........“MicroMaster”......... | ............................................... | |

Length (ft) | 14.27 | ~ 16’ | |

Wing span (ft) | 19.62 | ~22 | |

Wing area (sqft) | 65.66 | ~ 70 | |

Aspect Ratio | 5.85 | 6.9 | |

Empty Weight (lbs) | 276 | 401 ( Note 7) | |

Useful load (lbs) | 304 | 316 (Note 8) | |

MTOW (lbs) | 580 | 717 | |

Horsepower (gross) | 24 hp (Note 4) | Estimates below for 25, 28, and 30 HP | |

Effective HP (80% eff) | 19.2 hp | ||

Prop diameter (in) | 46” | 46” | |

Span loading | 1.51 | 1.48 | |

PERFORMANCE | TWO ENGINES | SINGLE ENGINE (one stopped prop) | |

Cruise Speed (mph) | 81 | Unk | |

Level Top Speed (mph) | 112 | Unk (Note 12) | |

Clean Stall Speed (mph) | 50 | 52 (Note 11) | 52 (Note 11) |

Climb rate at MTOW (fpm) | 515 | 2 x 25HP (80% eff=20hp): 920 FPM 2 x 28 HP (80% eff) : 1130 2x 30 HP (80% eff) : 1280 | 25HP (20 eff@80%)= 0 FPM28 HP (22.4 eff) = 100 FPM 30 HP (24 eff) = 175 FPM |

L/D | 13:1 | 13 : 1 (estimated) | 10.5 : 1 (estimated, incl 13 lb of dead prop drag at 60 mph) |

HP reqd for level flight at 60 kts (Note 1) | 12.84 hp | 15 HP (Note 9) | 15 HP (note 9)+ 5.2 HP (13 lb drag from stopped prop) = 20.2 HP |

Addtl HP reqd for claimed ROC (Note 2) | 9.05 hp | (see climb rates above) | (see climb rates above) |

Total HP reqd for flight + climb (Note 5) | 21.89 hp | ||

Induced drag at 70 kts (SL, level flight, lbs) (Note3) | 19.45 lbs (~7.8 HP) | 22.5 lbs | 22.5 lbs (~9.0 HP) (Note 10) |

Power Loading (lb/HP) (ref: Cessna 152: 15.2) | 24.2 | 2 x 25 HP engines: 14.35 2 x 28HP engines: 12.8 2 x 30HP engines: 12 | 25 HP engine: 28.7 28 HP engine: 25.6 30 HP engine: 23.9 |

Notes:

1: Computed from stated L/D, MTOW, and assumed 60 kt airspeed for best glide distance

2: At stated climb rate and MTOW assumed

3: Estimated lbs of drag based on MTOW, wing span and area, and resultant required Cl. Corresponding HP estimates are rough, depend on propeller efficiency at that airspeed and other factors.

4: The SD-1 has several engine options. Specs here are for the 28 HP 4-stroke B&S engine

5: Sum of the two cells above.

6. (deleted)

7. Empty weight buildup: SD-1 weight of 276 lbs + another engine (75 lbs), + prop and hub (12 lbs) + 38 lbs additional structure (engine mount, larger&stronger wing, etc) = 401 lbs

8. Useful load: 250 lb (pilot + baggage) + 66 lbs fuel

9. Required HP derived from sink rate at assumed 13:1 L/D at 60 kts and acft at MTOW.

10. With 22’ span, 70 sq ft wing, MTOW of 717 lbs.

11. Assumed “clean” Max 2D Cl of 1.4

12. Max and cruise speeds will be highly dependent on propeller efficiency at higher airspeeds. Fixed pitch props will likely have to be chosen to produce acceptable single-engine performance rather than top speed. If we simply apply the “speed increase = power increase ^ 0.33” equation and use the SD-1 as our “base case,” then the Micromaster with two 25 HP engines would have a top speed of 141 MPH. In reality, it will be appreciably less than that.

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