Has anyone be thinking of designing a more modern, light weight powered sailplane ?

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Aesquire

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The Xenos motor glider from Sonex sure a VW type engine ( or Jabiru ) with a "toothpick" short chord prop. Fairly low drag when stopped compared to a Cub prop. Better is a folding prop but the Sonex folk went with simple and light. Props on a tractor plane are at the extreme end when figuring weight changes.
 

lr27

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The VW engines do best with regular solid wood props. If I owned a motorglider with a VW engine, I think I'd be content to just let it run at a very moderate throttle setting, the prop just keeping pace with the airflow, while I soared around. It will sip fuel, the engine will be ready when I need it, cabin heat will be there if I want it, and it avoids the complications of any kind of special prop. Of course, I guess the magic of silent flight wouldn't be available, but I like motor noises just fine.
So we're talking about an efficient airplane as opposed to a sailplane.
 

Pops

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Why can't it be both ? A sailplane is just an efficient airplane without an engine. So we make it both by installing an engine, ( or motor).
 

Jay Kempf

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Like Topaz asked: is this thread about a cruising glider/airplane or about a self-launching sailplane?

https://www.aeros.com.ua/structure/al/index_en.php

Before these sorts of things started appearing and before Greg Cole built a whole 13.5 M high performance sailplane for 150 lbs I was looking at something in between. The AL12M is near perfect and claims 27:1. They have updated to AL21M with I think a better motor with higher TBO.

But what about taking the same approach and making a 15 meter real self launching sailplane along these lines but with simpler construction and a simple trailer. As long as it stays under the 103 limits it would be a great around the patch sport self launching sailplane.

With more aspect ratio it would have a higher useful load and higher L/D. You can copy an existing sailplane for an already figured out OML to start with. But it would have to be really light. Shrink wrap covered wing and tail. Aluminum extrusion tail boom tube and rivet and gusset cockpit cage with non structural composite shell and canopy, yadda. Simple retractable motor mast with belt drive and motor buried in the pod. If you extended the upper D tube skin you could get quite a bit of laminar flow and you could have camber changing. It won't be 40:1 and it won't be a severe weather machine but would be better than a 1-26 and self launching and Part 103.
 

BBerson

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Need to define efficient. Most airplanes are efficient at design cruise speed.
Motorgliders are heavier than gliders and have a poor payload to weight ratio. More suited to thermal speed than high cruise speed. In high speed cruise the excess span is a problem for drag and comfort. Dick Rutan wanted to burn the Voyager after a rough flight in Kansas on the flight to Osh.
Need to determine the design airspeed and purpose.
 

Pops

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Purpose for me .. Efficient airplane for working thermals and also a cruise at cross country speed of at least 80 + mph. So will have to work on the drag for the higher cruise speed.
 

Topaz

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Purpose for me .. Efficient airplane for working thermals and also a cruise at cross country speed of at least 80 + mph. So will have to work on the drag for the higher cruise speed.
Shouldn't be a problem. You're not asking for the world here. Span is your friend, you'll find a low-ish wing-loading helps, and pay a lot of attention to airfoil selection, such that the low-drag region of the CL versus angle-of-attack curve includes both your thermalling speed and your intended cruise speed. If you can't find one that does that (and it get harder to find one, as your powered-cruise speed gets higher), you might have to consider a camber-changing full-span flaperon/flap arrangement such as Victor Bravo described from the Schleicher ASW 20, to move the low-drag "bucket" higher or lower on the curve to suit a given phase of flight. If that's too much complication, you'll find that you'll need to limit your powered cruising speed to whatever speed keeps the cruise CL inside the "bucket", because there will be a drag rise once you're out of the "bucket" and it'll take quite a bit more power to go much faster.

Parasite drag reduction for the fuselage, landing gear, and tails will help as well. Drag is a total-sum game, so reduce it wherever you can.
 
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Pops

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Shouldn't be a problem. You're not asking for the world here. Span is your friend, you'll find a low-ish wing-loading helps, and pay a lot of attention to airfoil selection, such that the low-drag region of the CL versus angle-of-attack curve includes both your thermalling speed and your intended cruise speed. If you can't find one that does that (and it get harder to find one, as your powered-cruise speed gets higher), you might have to consider a camber-changing full-span flaperon/flap arrangement such as Victor Bravo described from the Schleicher ASW 20, to move the low-drag "bucket" higher or lower on the curve to suit a given phase of flight. If that's too much complication, you'll find that you'll need to limit your powered cruising speed to whatever speed keeps the cruise CL inside the "bucket", because there will be a drag rise once you're out of the "bucket" and it'll take quite a bit more power to go much faster.

Parasite drag reduction for the fuselage, landing gear, and tails will help as well. Drag is a total-sum game, so reduce it wherever you can.
I don't know what airfoil Fisher used on the Koala 202 or the Super Koala , but its a 16% airfoil used in the 202 and the prototype Fisher Super Koala next door. Easy to tell when its out of the low drag bucket.
Since simple is my goal, I will not have a camber-changing full-span flaperon/flap arrangement. I'll take what I get with the airfoil used. On the JMR, I spend 2 winters playing with NASA's on line wind tunnel trying many different airfoils and having fun. For my mission, I kept coming back to the 2412 not knowing what airplanes it was used on. It was a surprised to me that it was used on all the single engine Cessna's and many more. I changed and went to the 2414 for the added max thickness for the size on the spar I wanted to use. Yes, no matter what airfoil I use, low wing loading and span loading is desired.
 

103

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I rib stitched top and bottom as the normal practice , but due to the geodetic strips getting in the way of the wing stitch layout, I had to use 3 different layouts in the length of a wing panel. I don't know why you would consider the SSSC Geodetic wing being easier. The 1/8" x 3/4" geodetic strips are on 6" centers.
Easier based on your picture the Rib is immediately below the fabric on a FP202/SSSC. ON the Cygnet Wing the Geodetic members ride above the RIB.
upload_2019-11-25_18-37-43.png
Matt
 

Pops

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Easier based on your picture the Rib is immediately below the fabric on a FP202/SSSC. ON the Cygnet Wing the Geodetic members ride above the RIB.
View attachment 90683
Matt
I didn't know that. I thought the geodetic strips were below the surface of the ribs. Now I understand.
 

Topaz

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I don't know what airfoil Fisher used on the Koala 202 or the Super Koala , but its a 16% airfoil used in the 202 and the prototype Fisher Super Koala next door. Easy to tell when its out of the low drag bucket.
Since simple is my goal, I will not have a camber-changing full-span flaperon/flap arrangement. I'll take what I get with the airfoil used. On the JMR, I spend 2 winters playing with NASA's on line wind tunnel trying many different airfoils and having fun. For my mission, I kept coming back to the 2412 not knowing what airplanes it was used on. It was a surprised to me that it was used on all the single engine Cessna's and many more. I changed and went to the 2414 for the added max thickness for the size on the spar I wanted to use. Yes, no matter what airfoil I use, low wing loading and span loading is desired.
Okay, gotcha. I'm getting a picture of this airplane more as a conventional tube-fabric plane, but with extra span and area for some low-performance soaring with the engine off or throttled way back. Is that generally in the ballpark?

Exactly this sort of beast was the topic of a talk by Neal Pfeiffer at the Experimental Soaring Association conference this year. Worth a watch, since you're designing in this space. One of the topics that came up in the discussion portion of that lecture is that a conventional high-wing design has really terrible visibility in a thermal, in that the wing is blocking almost all the view up and inward in a continuous turn, such as you'd consistently encounter circling in a thermal. Several glider pilots expressed some pretty strong concerns about the safety issues this would present. The consensus was that something with a cockpit arrangement rather like a Cygnet or Saab MFI-9, below, would be a better solution for a strut-braced design, or go low-wing like most motorgliders.

1258027.jpg 1280px-Malmo_MFI-9_Junior_(SE-CPG)_03.jpg
 

Pops

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Yes, you are in the ballpark. And you are correct with not seeing in the bank while working lift. Skylights is not the answer. To steep of bank required with skylights in a little 24" wide single place hi-wing airplane. The SSSC has a good roll rate with 7' long ailerons and a light feel. So I would quickly raise a wing to clear the area often. A mid wing would be a big help. Always loved the look of both airplanes. Having the LE of the wing at eye level is the best of both worlds.
 

BBerson

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The Cessna 150 manual lists the L/D of 8.8 (prop windmilling)
The Cherokee 140 lists the L/D of 10.56 (prop windmilling)
The Super Cub manual doesn't list L/D at all (probably less than 8)

Adding a few feet span to a conventional design won't make it a sailplane. Airplanes have significant prop drag, cooling holes, struts and gear drag.
Mooney claimed 15:1 with the gear retracted and prop feathered.
 
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103

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The Cessna 150 manual lists the L/D of 8.8 (prop windmilling)
The Cherokee 140 lists the L/D of 10.56 (prop windmilling)
The Super Cub manual doesn't list L/D at all (probably less than 8)

Adding a few feet span to a conventional design won't make it a sailplane. Airplanes have significant prop drag, cooling holes, struts and gear drag.
Mooney claimed 15:1 with the gear retracted and prop feathered.
My Cygnet with fixed prop gets 13:1
 

bifft

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Just as a question, how well did the SSC glide? What ratio and at what speed?
Also what was the weight, wing span and area? I'd like to put it into my spreadsheet and see what it predicts for span increases.
 

Pops

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Just as a question, how well did the SSC glide? What ratio and at what speed?
Also what was the weight, wing span and area? I'd like to put it into my spreadsheet and see what it predicts for span increases.
The SSSC glided very well. Don't know the ratio, never measured it. Best gliding speed was about 45 knots. Flying weight about 750 lbs. Wing span-30' with 120 sq ft of wing area.
Don't know the airfoil, but 4' cord with a flat bottom 16% airfoil.
Lot of fun working lift on a good day.
 

bifft

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The SSSC glided very well. Don't know the ratio, never measured it. Best gliding speed was about 45 knots. Flying weight about 750 lbs. Wing span-30' with 120 sq ft of wing area.
Don't know the airfoil, but 4' cord with a flat bottom 16% airfoil.
So I put in those numbers and calculated Cdi and Cdp to get best glide at 45 kts and 33 hp required for 80 mph cruise (80% prop efficiency) with my motorglider power required spreadsheet (see attached). It came out to 8.6 glide ratio and min sink of 456 ft/min.

If I then take those same Cdi and Cdp and put in a 38 ft span and 134 area, then it goes up to 10.9 glide ratio and 327 ft/min sink. Cruise on the same 33hp only goes up to 82 mph.

My spreadsheet isn't that sophisticated, but I find it usually works out power for speed and min sink speeds within 10-15% if I calculate the Cdi and Cdp to match listed best glide performance.
 

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