I'm reposting this in the Design thread, hoping for more feedback. To reference the original thread, please visit here. I'm not looking for quantitative analysis, but simply some informed opinions about whether this avenue is worth pursuing and further research on my part. Point out the flaws in my thinking, please.
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Okay, as business permits I've continued tinkering with motorglider ideas. I'm working toward a decision to either focus my efforts upon one of those or continuing with my single-seat pure sailplane.
A key factor has turned out to be the propulsion system, and specifically how much of its drag I can eliminate for the times when I'm soaring the airplane. That will probably be, in terms of flight hours, as much or more of the aircraft's air time than it will be used under power, so it's important to me that the airplane be at least a decent sailplane. That makes drag reduction fairly important. Balancing that is my need and desire for simplicity and low cost. My goal is something with SGS 1-34 performance (32:1 L/D, 170fpm minimum sink) with the motor off.
The balance of this thread has shown me that a feathering prop is not commercially available at a price compatible with my budget, and that folding props in this power range (65-80hp) are simply unavailable for all practical purposes.
I start thinking about simply leaving the prop out in the breeze and closing off all the cooling inlets and outlets on the engine - until I imagine holding a prop blade out the window of a car on the freeway and get all weak and woozy from imagining all that drag. :speechles And all those little moving doors to build in the cowling...
I had a "sit bolt upright" moment earlier today, and I'd like to trot it past the group for opinions. The attached drawing is schematic, not to scale.
If you put two 'draggy' objects one behind the other with respect to the airflow - even with a gap between them - the total drag is greatly reduced, sometimes even below the level of each of the objects alone. I got to thinking about what would happen if I could arrange to have the prop blades (two-bladed prop) stop directly in front of the cowling sides of a horizontally-opposed engine (I'm baselining a VW). Would the cowling provide that 'second object' for the stopped prop blade, radically reducing the drag it produces?
Practical matters:
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Okay, as business permits I've continued tinkering with motorglider ideas. I'm working toward a decision to either focus my efforts upon one of those or continuing with my single-seat pure sailplane.
A key factor has turned out to be the propulsion system, and specifically how much of its drag I can eliminate for the times when I'm soaring the airplane. That will probably be, in terms of flight hours, as much or more of the aircraft's air time than it will be used under power, so it's important to me that the airplane be at least a decent sailplane. That makes drag reduction fairly important. Balancing that is my need and desire for simplicity and low cost. My goal is something with SGS 1-34 performance (32:1 L/D, 170fpm minimum sink) with the motor off.
The balance of this thread has shown me that a feathering prop is not commercially available at a price compatible with my budget, and that folding props in this power range (65-80hp) are simply unavailable for all practical purposes.
I start thinking about simply leaving the prop out in the breeze and closing off all the cooling inlets and outlets on the engine - until I imagine holding a prop blade out the window of a car on the freeway and get all weak and woozy from imagining all that drag. :speechles And all those little moving doors to build in the cowling...
I had a "sit bolt upright" moment earlier today, and I'd like to trot it past the group for opinions. The attached drawing is schematic, not to scale.
If you put two 'draggy' objects one behind the other with respect to the airflow - even with a gap between them - the total drag is greatly reduced, sometimes even below the level of each of the objects alone. I got to thinking about what would happen if I could arrange to have the prop blades (two-bladed prop) stop directly in front of the cowling sides of a horizontally-opposed engine (I'm baselining a VW). Would the cowling provide that 'second object' for the stopped prop blade, radically reducing the drag it produces?
Practical matters:
- The cowling isn't as wide as the prop span, but if the effect occurs over the majority of the prop span, that might be 'good enough' considering the simplicity of the system. I can't afford the drag of the entire prop hanging out in the breeze. I might be able to live with the drag of, say, the outer eight or ten inches of the tips.
- BONUS 1: The inexpensive, very basic version of the VW I'd like to use puts the prop right in front of the cylinders - very close. However, by way of turning liabilities into assets, that puts the prop closer to the cowling, enhancing the effect I'm looking for. (It also reduces prop efficiency, which may be a problem for climb, given the low installed power.)
- BONUS 2: If the prop is close to the cowling inlets, and the blade is stopped right in front of them, do they effectively provide "doors" for the inlets? In other words, would this (very) significantly reduce the airflow through the cooling plenums with the engine stopped? If so, I can limit the moving 'doors' to a simple set of cowl flaps on the cooling exhaust, and still eliminate almost all of the 'cooling' drag when the engine is stopped. This was the "aha!" moment in this scheme for me. Can I get the drag reduction on both the prop and the cooling system, for just the 'price' of stopping the prop in a specific orientation?
