Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Pops, Nov 11, 2019.
Maybe half the people on this forum. My avatar shows some of my early thoughts on the subject.
All other factors equal, or even if all other factors are not equal, when it comes to soaring performance... wingspan trumps everything else by a large margin.
The Moni, the KR-1B, the BD-5S, and a whole lot of other airplanes with "a little longer wing and the word 'glider' somewhere in the brochure" are not really usable as gliders and motorgliders. For a conventional aircraft (i.e. not a hang glider), an extra two or four feet of wingspan added onto a small single seat sportplane is not enough.
Yes you can soar any airplane in the right conditions. But there is also a realistic minimum threshold, somewhere in the neighborhood of 200-240 feet per minute minimum sink in a 30 degree banked turn, where an aircraft becomes soarable on a daily basis.
L/D helps a lot, and is a primary contributor to cross-country gliding performance. But the minimum sink rate in a 30 degree bank is what allows you to do the soaring part, meaning to climb in lifting air.
As we witnessed at the ESA panel discussion, it's hard for a group to come to a consensus on which direction to go. An RF3 or RF4 similar design appears to be a reasonable compromise. If three or four individuals couldfocus their talents on such a endeavor, what could they produce in a couple of years?
Thanks , I remember that. I was at a fly-in at Springfield, OH when Ken Rand flew the Turbocharged VW powered KR-2 nonstop from CA to OH. On the way home was when he got killed. I was building a KR-2 at the time.
I ridge soared my 1959 C-172 from Selinsgrove, Pa to Morgantown WV on a very windy day. 30+ knots on the ground from 330 degs.
Also worked thermals with the SSSC on a good day.
IF the SSSC had the wing extended from 30' to 38' and the drag cleaned up as much as possible, it wouldn't be a bad motor glider.
I like the Morgan Motorglider in that he took a Sierra and simply added longer wings.
I think it used the 2200 Jabaru engine.
My point is there's a number of unloved aircraft/unfinished projects out there, a KR2 for example, you could pick up and engineer in new wings with a VW engine maybe.
What are you in for, a single seater as the RF-3 or the double seater RF-4?
A wing with a CF D-tube and spar and covered by fabric aft would be very light and would let you avoid the cost of a wood spar. The CF/pultruded graphite spars are so light that it's easy to end up the same or less weight than with a strut-braced wing, and of course it is less draggy. That geodesic design does look labor intensive, but you've done it before and liked it, so that's worth a lot. Do you have to rib stitch ("strip stitch"?) everything? I don't know about the aerodynamics of the fabric covering over geodesic vs ribs, it would seem that the flow might be smoother if all the fabric attachment points were parallel to the chord and the airflow (ribs or geodesic).
It looks like a fun project, and all the needed skills are already in your kit.
Mcbj posted this back in 2017 or 18;
On the geodesic design the strips are below the surface of the wing and do not touch the fabric. I always rib stitch everything. An all CF D-tube with the CF spar would be nice but its a little over my head in designing for the strength needed, I would need help.
On building the Geo wing, I start with the spars and ribs all lined up true and epoxied. Have to be sure of any twist you want in the wing because the geo strips locks the wing solid. Think of a geo wing as a fully sheeted wing with lighting holes. I do the flat bottom first and run one row of strips from the root to the tip. Maybe takes a couple hours, then the next day run the geo strips in the opposite direction (lots of cloths pins ) then the next day turn the wing over and do the top. So 4 days working about 1.5 to 2 hrs each day, the geo strips are finished. Really the extra work of varnishing the strips takes longer. That is the extra work.
IF that question is for me, single seat. Small and light weight with a high AR and low drag as possible. Simple and easy to build.
Motorgliders are not often used for soaring on a daily basis. Only when conditions are best and fun.
In most regions it's all powered flight most of the year and occasional soaring a few times.
The Moni was an ARV (air recreational vehicle)
To make a motorglider with a sailplane performance is possible but only at greater size and expense.
So almost no one does.
I think you are advocating self-launching sailplane performance.
Yes, Sonya did put in 5000 building hours and still for medium soar-ability. How many hours do you want to put in for occasional soaring in weak conditions?
Got it. So it is there to stiffen the wing in torsion, take the place of a drag strut, etc.
Jim Marske's book has some good guidance on the spar itself (I have that book). Some people have critiqued aspects of his approach, and there are other loads to fret over, but somebody here surely knows how to get it done. Hey, you don't want to stay entirely in you comfort zone, right?
My old flight instructor was also a sailplane instructor and I used to go to contest with him and help in the ground crews and also worked at the local glider club were him and a couple others instructed. At one time he built the Moni and didn't like it , said it didn't handle good and was a poor design , so he sold it. He built a Cherokee but cleaned up the drag and different wing tip, etc. Also had a K-6.
I would like sort of a motorglider with maybe 20 to 1 performance. Low drag for a good cruise speed with the engine, but still have fun working thermals. You would be surprised how the SSSC did in the thermals. 480 lbs EW with 120 sq' of area and a good high lift airfoil. Believe a SSSC with increasing the span to 36-38' from the 30' and build as light as possible and try and clean up some of the drag, it would be fun in the thermals.
Simple is good and I will not build something that is complex and take forever to build, but I don't count hours when I'm having fun building.
Didn't know that I even had a comfort zone
I don't count hours either. But size and weight of the wings, trailer, etc., are inconvenient. I am seeking the smallest size possible around L/D 17. Same as the 2-22 I learned in.
The Moni has some undesirable features I want to avoid.
Same here, smallest size is lighter weight, easier to move and store, lower cost, etc.
I'm still very much interested in a "new" motorglider of my own design. Now that things in my life are settling down just a littlest bit, I've been starting to do some work again on the first of the two projects that are on my personal bucket list:
Single-seat touring motorglider. Goals of 28-30:1 L/D max, minimum sink of approximately -144 fpm, powered range about 300 nm + 50 nm reserve. Baseline engine is a modified Predator 670cc industrial V-twin. If you can imagine a little single-seat touring motorglider in the vein of the original Rutan Quickie, but with a conventional tail and long wings, you're in the ballpark. See my design threads linked in my sig-line.
Two-seat touring motorglider. Goals of 30-32:1 L/D max, minimum sink of approximately -144 fpm, powered range between 300 nm and 500 nm plus reserve. Baseline engine is a 65 hp VW conversion. This project is for "down the road," after I've learned from the small single-seater.
For both projects, I want full trailerability in the same vein as a standard unpowered sailplane, with the same modus operandi - the airplane essentially "lives" in its trailer and can be taken home and stored without the costs of a hangar or tie-down.
In the Fournier information in my first post , it states that the VW engine has a de-compression device that pushes on the exhaust valve rocker arms so the VW engine will windmill for a restart.
Like to learn more about this.
Sounds very good. Hope you are successful in both projects.
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