LeSving wrote:
The Risen is a motorglider with a big’ish engine.
Look at it without prejudice. wingspan 8.2 meters, purpose built for the 915 engine.
And yes, as I already wrote, the Darkaero is the exact same textbook design approach – airframe, engine and prop as a coherent system.
Having listened carefully, this “young” :-( man still cannot make sense of the Onex’s relevance to this thread. A single seater kit aircraft with no Rotax engine?
From the text you posted it also looks like the one in the video is a dangerously overpowered one-off dragster with insufficient rudder authority.
Just for clarity.
I am not a Rotax fanboy – not even a potential customer. And I don’t like the sound of Rotax’es,
I just think they are doing an interesting engineering job in the small displacement/high boost vision.
I find it amazing that they still claim 2000 hours tbo for this little engine.
Citing the Risen (and Darkaero as a matter of fact) was to illustrate how no-compromise design can leverage a lightweight/high power engine to create a remarkable aircraft that defines the state of the art.
The Blackwing is also a very nice design (cool or not is not relevant ), but I believe the airframe was not “retuned” between 912 and 915 . they just bolted a 915 with 40% more power to the 912-optimized airframe. Same for Tarragon, except 80% more power?
Now let’s add some more chili in this discussion :-)
I am willing to bet that the Risen outperforms both the Blackwing and the Tarragon in apples to apples comparison (same engine same altitude, stock airplane) .
Why? because it was fastest with a 912 AND is the only one that was redesigned around the 915.
From the text you posted it also looks like the one in the video is a dangerously overpowered one-off dragster with insufficient rudder authority.
And you think the Risen with the 915 isn’t? It depends what you are used to. Lots of “fast” ULs have “less than optimal” rudder authority for slamming the throttle when at standstill, even with just 100 hp. This is a function of torque forces from the engine and weight of the aircraft. A tail dragger will emphasise this a bit more, and fixed pitch will also have an effect. There are certain things you get away with with low power to weight, that you have to be aware if with high power to weight. You can of course increase rudder authority, but that would probably also decrease Va unless some structural changes are done.
The Risen is efficient aerodynamically, but so what? It’s the performance that counts, and the same performance can be achieved by everyone today building an aircraft in the garage. The only difference will be a bit higher fuel flow.
And there are other things. I have spent a few evenings now over a longer period doing the 5 year Rotax replacement of hoses and cables on the club’s Pipistrel Alphatrainer. The Alphatrainer is also high efficiency aerodynamic carbon thing. Changing all the hoses and wires have been a PITA. It’s so cramped and so “factory production efficient with little regard for maintenance” that it’s unbelievable. On my own Savannah (STOL, aluminium, Rotax) everything is dead easy and accessible and done at a fraction of the time.
A Rotax has more than enough stuff on it in terms of wires, cables and hoses. Making that super compact is more like evildoing 
An RV is probably the optimal balance of performance and practicalities of any aircraft today. That requires 160+ hp.
It’s the way it is. You gain some here, and lose some there. There exist aircraft for every taste (almost) But when you take it to the extreme in one direction, you lose in some other ways.
