[FONT=&]I've been intrigued for a long time about the idea of an inline (push-pull) VW-powered two-seat homebuilt. The layout would be like the Cessna Skymaster (aka Cessna Model 337, USAF O-2, etc) or its little brother, the Powers-Bashforth Minimaster.
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Pops mentioned in an earlier thread that he'd sketched out a VW inline twin at one point in the past, so that helped me think the idea might not be crazy.
The goal would be to have twin-engine reliability (esp useful for operations over rough terrain, water, at night, etc--places where a forced landing would be especially hazardous). A "plus" would be relatively high cruise speeds from approx 140 HP continuous (or over 70 HP at 10K' MSL for some respectable TAS cruise speeds). The excess power could also be used for climb, too, but not for critical situations where the climb is needed for obstacle avoidance (because the climb rate won't be there if either engine quits--so it would have twice the risk of "critical engine failure" as a single engine airplane).
The main challenge would be achieving safe single engine performance.
Some back of the envelope estimates of weights/dimensions:
Empty weight: An empty Sonex weighs about 650 lbs, an empty Thatcher CX5 weighs about 721 lbs. Add 180 lbs for a second VW engine, 10 lbs for engine mount, 10 lbs for another prop, maybe 15 additional pounds for the dual tail booms and tail (vs conventional empennage), 30 lbs for more wing (see below) = approx 895 to 966 lbs empty. Say 930 lbs.
Max gross weight (what we'd need to have some utility): 930 lbs (empty weight) + 400 lbs (two people) + 50 lbs (baggage) + 150 lbs fuel (ideally tanks large enough for 300 lbs fuel in for solo flight) = 1520 lbs For comparison, that's 150 lbs less than a C-152, but with about 40 more HP (both engines) and more wing (see below).
Wingspan: It's no trick to get a good rate of climb with an 80 HP engine, even if the plane is relatively heavy--it just takes a lot of wing (especially, span). A fully loaded Dimona H36 motorglider weighs 1698 lbs and the single 80 HP Limbach gives a climb of 532 ft/min: But it has a wingspan of 52 feet and a wing area of 154 sq ft. A little closer to "conventional" airplanes, the Thatcher CX5 (80-85 HP VW) claims a 1000 FPM rate of climb at 1320 lbs and with a wingspan of a wingspan of 28' (area of 126 sq ft). So, if, for emergencies, we want to have approx 300 FPM ROC on a single engine at our 1520 LB target max TO weight (approx midway between the Dimona and the CX5) it looks like we'd need something like a 35-40' wingspan a wing area of about 135-140 sq ft. So, about the span of a C-172, but less wing chord.
Layout: A tandem seating arrangement has a lot to recommend it for this concept. There would be less "blanking" of the rear prop by the fuselage. Also, if the "mission" of this plane is to use that second engine to generate some impressive cruise speed numbers, then minimizing frontal area is a big plus. On the other hand, if the plane is just to be used to have get two people over inhospitable terrain with a bit more safety, then a side-by-side configuration mights work (with careful attention to the shape of the rear fuselage to help get air to that rear prop). High wing/low wing--probably not much difference (though gravity feed from a high wing is a very reliable thing, would work great here).[/FONT]
[FONT=&]Other:
Props: Controllable pitch props would be very handy in order to facilitate safe single-engine operation (fine pitch, low airspeed, let the engine go to 3400 RPM to get all the power available) as well as high cruise speeds at reduced RPMs in normal operations. The Ivoprop adjustable pitch units look "reasonably priced" (i.e. about 1/3rd as much as each engine), but I don't know anything about their reliability. The Hoffman props used on the Grobs and Dimonas would be ideal, since they can be feathered (which would significantly reduce the drag from a dead engine), but I'll bet they are very expensive. The Revmaster R2300 engines, with that big front bearing, can take props weighing up to 22 lbs (IIRC), and are even still plumbed to provide oil pressure for a controllable-pitch prop.
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[FONT=&]I wonder how a person, having built such an airplane, could economically get checked out for twin-engine, centerline thrust.
Performance: I wonder what cruise speeds of such a pane might be. If it could be kept aerodynamically "clean", the 140HP (or so) available at SL might provide quite a push, but all the wing needed for acceptable SE climb might slow her down a bit (but, it should fairly leap off the ground with both props turning!)[/FONT]
[FONT=&]Thoughts, nasty comments, corrections are solicited. Crazy idea after all??[/FONT]
Mark
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Pops mentioned in an earlier thread that he'd sketched out a VW inline twin at one point in the past, so that helped me think the idea might not be crazy.
The goal would be to have twin-engine reliability (esp useful for operations over rough terrain, water, at night, etc--places where a forced landing would be especially hazardous). A "plus" would be relatively high cruise speeds from approx 140 HP continuous (or over 70 HP at 10K' MSL for some respectable TAS cruise speeds). The excess power could also be used for climb, too, but not for critical situations where the climb is needed for obstacle avoidance (because the climb rate won't be there if either engine quits--so it would have twice the risk of "critical engine failure" as a single engine airplane).
The main challenge would be achieving safe single engine performance.
Some back of the envelope estimates of weights/dimensions:
Empty weight: An empty Sonex weighs about 650 lbs, an empty Thatcher CX5 weighs about 721 lbs. Add 180 lbs for a second VW engine, 10 lbs for engine mount, 10 lbs for another prop, maybe 15 additional pounds for the dual tail booms and tail (vs conventional empennage), 30 lbs for more wing (see below) = approx 895 to 966 lbs empty. Say 930 lbs.
Max gross weight (what we'd need to have some utility): 930 lbs (empty weight) + 400 lbs (two people) + 50 lbs (baggage) + 150 lbs fuel (ideally tanks large enough for 300 lbs fuel in for solo flight) = 1520 lbs For comparison, that's 150 lbs less than a C-152, but with about 40 more HP (both engines) and more wing (see below).
Wingspan: It's no trick to get a good rate of climb with an 80 HP engine, even if the plane is relatively heavy--it just takes a lot of wing (especially, span). A fully loaded Dimona H36 motorglider weighs 1698 lbs and the single 80 HP Limbach gives a climb of 532 ft/min: But it has a wingspan of 52 feet and a wing area of 154 sq ft. A little closer to "conventional" airplanes, the Thatcher CX5 (80-85 HP VW) claims a 1000 FPM rate of climb at 1320 lbs and with a wingspan of a wingspan of 28' (area of 126 sq ft). So, if, for emergencies, we want to have approx 300 FPM ROC on a single engine at our 1520 LB target max TO weight (approx midway between the Dimona and the CX5) it looks like we'd need something like a 35-40' wingspan a wing area of about 135-140 sq ft. So, about the span of a C-172, but less wing chord.
Layout: A tandem seating arrangement has a lot to recommend it for this concept. There would be less "blanking" of the rear prop by the fuselage. Also, if the "mission" of this plane is to use that second engine to generate some impressive cruise speed numbers, then minimizing frontal area is a big plus. On the other hand, if the plane is just to be used to have get two people over inhospitable terrain with a bit more safety, then a side-by-side configuration mights work (with careful attention to the shape of the rear fuselage to help get air to that rear prop). High wing/low wing--probably not much difference (though gravity feed from a high wing is a very reliable thing, would work great here).[/FONT]
[FONT=&]Other:
Props: Controllable pitch props would be very handy in order to facilitate safe single-engine operation (fine pitch, low airspeed, let the engine go to 3400 RPM to get all the power available) as well as high cruise speeds at reduced RPMs in normal operations. The Ivoprop adjustable pitch units look "reasonably priced" (i.e. about 1/3rd as much as each engine), but I don't know anything about their reliability. The Hoffman props used on the Grobs and Dimonas would be ideal, since they can be feathered (which would significantly reduce the drag from a dead engine), but I'll bet they are very expensive. The Revmaster R2300 engines, with that big front bearing, can take props weighing up to 22 lbs (IIRC), and are even still plumbed to provide oil pressure for a controllable-pitch prop.
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[FONT=&]I wonder how a person, having built such an airplane, could economically get checked out for twin-engine, centerline thrust.
Performance: I wonder what cruise speeds of such a pane might be. If it could be kept aerodynamically "clean", the 140HP (or so) available at SL might provide quite a push, but all the wing needed for acceptable SE climb might slow her down a bit (but, it should fairly leap off the ground with both props turning!)[/FONT]
[FONT=&]Thoughts, nasty comments, corrections are solicited. Crazy idea after all??[/FONT]
Mark
