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

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Pops, Nov 11, 2019.

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  1. Nov 13, 2019 #61

    Vigilant1

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    Pops,
    Flaps or no flaps? With that wing and the low weight, you probably aren't looking for the improved Cl flaps offer. And they do add complexity to the build, add weight to the plane, add drag when they aren't being used, and make it a little harder to mount and remove the wings.
    There should be an easier, lighter way to increase drag a lot when needed (so you can lower the nose without picking up airspeed). Maybe a big speed brake under the fuselage might be enough? Or maybe just make the rudder big enough (and the stall characteristics of the wing right) to allow fairly strong forward slips? That wouldn't add much weight, cost, or complexity and a highly effective rudder has other benefits.
     
    Last edited: Nov 13, 2019
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  2. Nov 14, 2019 #62

    Aesquire

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    Split flaps?

    A drag device is a good idea on a high L/D craft.
     
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  3. Nov 14, 2019 #63

    Vigilant1

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    If wing flaps are needed, split flaps would have some advantages in this case compared to plain flaps.
     
    Last edited: Nov 14, 2019
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  4. Nov 14, 2019 #64

    Pops

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    Been thinking about that. We must be on the same railroad track :) Sailplane= spoilers. I have the same problem with the SSSC. With the 1835 cc, 60 hp VW engine with the 60" dia x 26" pitch prop, even at idle rpm of 900 its producing enough thrust for a shallow decent. You can hold altitude with 1400/1500 rpm at about 55 mph. But I built a large aero-balanced rudder and does a very good hard slip, so plan on slipping it all the way down on final for a normal decent. Any more wing on it and you would have to have a lot more drag. The way it is you just do a very hard slip and watch the runway come up to you out the side window.
    Like you said, flaps adds a lot of weight and complexity so on the SSSC I added a large effective rudder. On the JMR, I built flaps and had to work to get the controls for the flaps and ailerons around the wing fuel tanks. Not much space left over and it isn't easy. So, I was thinking on this airframe of using spoilers and running the torque tube out the wing in the middle of the D section with a handle hanging down by the windshield post on the left side. ( also using wing fuel tanks of about 6 gal each). Same as the flap handle in the Fisher Super Koala. IF I don't make the spoilers large enough, just add a simple belly flap on a piano hinge with a handle beside the seat. But from flying sailplanes I know that a little spoiler goes a long ways.
    Lots to think about isn't there ?
     
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  5. Nov 14, 2019 #65

    Topaz

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    Flaps on a sailplane or motorglider are a little bit different beast than they are on power planes. Yes, they do increase lift coefficient for landing, but that's not the primary purpose. Sailplanes and motorglider (except for "airchairs" like the Sandlin GOAT, the older Superfloater, and the like) generally use airfoils that provide some extent of laminar flow for drag reduction.

    Laminar-flow airfoils have a specific lift-coefficient range where the drag is significantly lower, perhaps half as much as at lift coefficients outside that range. This is called the "drag bucket." It tends to be near the middle of the lift coefficient range for most airfoils. Older style sailplane designs tried to find airfoils with very broad drag buckets, so that the drag was as low as possible at both the (slow, just above stall) thermalling speed, and also at the higher best-L/D speed for flying quickly between thermals. That means the wing lift coefficient for both conditions have to be within the drag bucket. But a "broad" drag bucket tends to be a "shallow" drag bucket, whereas airfoils with really low-drag "buckets" tend to have buckets that cover a narrower range of lift coefficients.

    Putting plain flaps up or down (10-15 degrees, either up or down) moves the drag bucket lower or higher on the overall lift coefficent versus angle of attack curve. Putting the flaps down a bit means the bucket covers a range of lift coefficients at the higher-lift-coefficient end of the range, putting the thermalling lift coefficient inside the bucket, while reflexing the flaps (negative flap deflection) moves the bucket to lower lift coefficients, putting the bucket around the best-L/D lift coefficient.

    Pilots of gliders with such "camber changing flaps" change the flap deflection as the airspeed changes, keeping the low-drag bucket centered on the lift coefficient (speed) at which they want to fly at that point in the flight. Need to thermal, put the flaps down some. Mid-speeds searching for thermals in the near area? Flaps zero. Flying fast between thermals, trying to make speed or distance at best-L/D? Flaps reflexed a bit. Landing? Flaps way down for drag and lift, like a power-plane. There are sailplanes that put the flaps down 90 degrees for landing.

    A motorglider is the same, since it has the same performance requirements as a pure sailplane, albeit with the towplane "bolted to the nose."

    A low-performance sailplane or motorglider with purely turbulent-flow wings won't benefit much from "typical" sailplane camber-changing flaps.

    A medium- or higher-performance sailplane with any significant extent of laminar flow on the wings should either select an airfoil with a very broad drag bucket encompassing both the thermalling lift coefficient and that for best-L/D, or use camber-changing flaps to move a narrower (but lower drag) drag bucket up or down the overall CL versus angle-of-attack curve, as needed.
     
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  6. Nov 15, 2019 #66

    bifft

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    The flap discussion does bring up the question: How much laminar flow can you get out of wood or aluminum D-cell fabric covered wing? For those of us who hate working with composites. And if as I suspect the answer is "very little", how much performance can you get with a turbulent foil but still with plenty of span (I've been sketching 42').

    I've got a spreadsheet I threw together to estimate power requirements for a motorglider, it is more along the lines of "If it glides as well as an XXXX and weighs ####" estimator, so I don't know how to answer the questions above.
     
  7. Nov 15, 2019 #67

    Topaz

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    A wood-skinned wing can be made to produce quite long laminar runs, chord-wise. In fact, with a lot of care, they can be almost as good as composite wings. The big issue you'll have is a high maintenance workload over time - wood wings tend to "live and breathe" a bit with changes in humidity and other environmental factors, so you'll end up stripping the surface finish and "reprofiling"' the wing back to the correct airfoil as much as every season, depending upon the kind of soaring performance for which you're looking and where you live. Utah tends to have a nice dry climate, so wood might serve you well if you take modest care of the airplane.

    Fabric-covered wings are not compatible with laminar flow. They can be very light, however, so if you're looking at a low-wing-loading machine best suited for thermal flying in fairly light lift. At the low end of the performance spectrum we have craft like the Sandlin GOAT and the other "airchairs," which are generally flown in ridge lift or modest-strength thermals.

    If you're absolutely averse to composites, an all-aluminum wing is probably the best compromise between ease of maintenance and performance. You'll need to invest in the effort to do flush riveting, at the very least forward of the spar. Most (I think all?) of the Schweizer sailplane lineup was all-aluminum construction.

    Span and wing loading will be the most powerful drivers of your soaring performance. Remember that, prior to the 1960's or so, really extensive laminar flow was a thing of the future, and even competition soaring was done with what we would today call turbulent-flow wings.
     
  8. Nov 16, 2019 #68

    bifft

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    That's good to hear. I'm pretty comfortable with flush riviting, I find flush riviting dimpled skins the easiest riviting of all. Performance goals would be for better than an airchair, emphasis on low sink rate for local flying. Being in Utah means ridge lift is pretty much always available and can get pretty strong (I've actually soared my RV once).

    Thinking of fabric covered was for weight reasons. Using 120 ft^2 for wing area with 0.020 skins you end up with ~70 lbs of just skin.
     
  9. Nov 16, 2019 #69

    Jay Kempf

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    Dick Schreder used foam ribs and metal bonded skins on the HP line of sailplanes. Other than some urban myth it was a very successful design. Most of the wing designs were hybrid with machined aluminum spars, fittings, closeouts, etc... then tightly spaced foam ribs to keep the skin from oil canning, and an aluminum skin bonded over the top. Made for a very smooth wing. A friend in high school, his dad built an HP 11. Always loved the high angle approaches with 90° flaps.

    If real light weight is the goal there are already a bunch of ultralight somewhat high performance sailplanes out there that use shrink wrap covering over an extended D tube front section and bare ribs aft. They get adequate performance and have incredibly low sink rates for what they are just because the wing loading is so low. If you manage all the other drag sources you can get decent low speed performance. Where you lose with these designs is in cross country performance.
     
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  10. Nov 16, 2019 #70

    Topaz

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    Which brings us around to the question that ought to be asked and answered before starting any airplane design: "What do you want it to do?"

    Motorgliders, like pure sailplanes or "cruising" powered aircraft, are not a monolithic block in terms of performance goals. You can't say, "motorglider" and understand the performance desired of the airplane any more than you can say, "piston single-engine land" and understand what that means in terms of design goals.

    Some motorgliders are floaters, intended for local micro-thermal or ridge-lift flying. A motorized GOAT, Swift, or Carbon Dragon would be examples.

    Some motorgliders are essentially powered airplanes with some modest soaring capability. The RF-4, Moni, or Ogar are examples of this type.

    Some motorgliders are high-performance sailplanes with some compromise towards being able to fly point-to-point under power. Stemme S-10 is the prime example of this type. Caret is another, albeit slightly tilted more towards powered flight, as is the ASK-14. It's a spectrum, not a series of "hard" categories.

    Self-launching gliders aren't intended for powered cross-country flight at all, and are exactly what their names describe: Sailplanes that don't need a tow-plane. Eta, or any of the other pure sailplanes with a retractable motor and propeller are examples of this type.

    So before you can go choosing construction methods, materials, configuration, and so on, you need to know what you want the airplane to do. You can't do something like a Stemme S-10 in aluminum, or at least it won't be a very good one. Composites are the obvious choice. You could do it in wood, but the maintenance of the surface contour and finish would be quite significant every year. On the other hand, an all-aluminum airplane in the class of the RF-4 would be perfectly feasible and reasonable. Tube and fabric is almost necessary for something like the GOAT, where the need for a minimum wing loading dominates most other design considerations. Conversely, you could decide materials and methods first, but understand that your choice will limit the kind of performance you should expect from your design.

    My own project is most like the Caret or ASK-14, so I'm using composites, in which I feel comfortable working anyway. I'll note that the ASK-14 is an all-wood design, with fabric-covered panels aft of the spar.
     
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  11. Nov 17, 2019 #71

    bifft

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    Exactly. Pops gave his thoughts back in post 41, I'll give what I'm thinking of.

    Electric self launch low/medium performance sailplane. Design more toward minimum sink rate than max glide ratio. Be able to stay up on weak lift days, but don't expect to get far from the airport. Go cross country on the trailer. Enough power so that the rate of climb is five times the min sink. Fifteen minutes of battery at full power: One five minute takeoff, one five minute save and leave the rest in the battery for longevity. That will give one hour of flight per charge even if you can't find any lift. Put solar panels on the trailer so it is charged and ready to go by the weekend even if you don't have a place to plug in.

    Weight wise, do the above carrying a 250 lb pilot, and have structural margin for a 300 lb pilot.

    Putting numbers on performance, I'm thinking 500lbs empty and 800 gross. Min sink from 150-200 ft/min. If I can get 20-25 glide ratio, those numbers work out to 30-50 hp or so needed.
     
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  12. Nov 17, 2019 #72

    Jay Kempf

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    Rate of climb 5x min sink, 30-5-hp, 500lbs empty is a tall order for one reason only. 1 hour of batteries is gonna be heavy, maybe half your overall empty weight fraction. I think the difference is that this thread isn't about a self launcher. Those have minimal batteries for say 3 launches to 2k feet altitude or say 15-20 minutes of batteries at say 3-500FPM climb with no reserve.
     
  13. Nov 17, 2019 #73

    Pops

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    I would use a 1600 -1835 VW engine. Non-electric, with a firewall forward weight of 140 lbs , I know I can do that. When I had the first set of wings on the SSSC with 4 gal wing tanks in each wing for a total of 8 gal , the EW was 450 lbs. I believe I can built the longer wings and stay under 500 lbs with no problem.
    I built another set of wings for the SSSC for the total fuel Cap of 16 gallon and went to a slightly larger spar caps and went from 5" dia wheels to 6"dia with 800x6" heavier tires and the total weigh increase was 35 lbs for the new EW of 485 lbs.
    Staying with a 500 lb EW and just using a 50 HP, 1600 engine it should have a good ROC , while burning about 2.7 GPH at cruise when under power.

    I would like to have some information on compression release with the exhaust valves. Believe a shaft could be run above the rocker arm with lobes to push the exhaust rocker down about .015" for compression release. The shaft would have to be run through the rear of the valve cover ( oil seal needed ) and extended to the cockpit with a handle for the pilot to rotate for the release. Both shafts from the cylinder heads would have to be coupled together for one release handle.
    Ideas ?
     
  14. Nov 17, 2019 #74

    BJC

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    Does anyone know the current status of the GosHawk?

    It seems to be at a standstill.


    BJC
     
  15. Nov 17, 2019 #75

    gtae07

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    I keep thinking about a self-launching Goat. Preferably electric. Not any time to work on such a project right now, though.
     
  16. Nov 17, 2019 #76

    Pops

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    Thinking of just going to a little different version of the SSSC. Very conventional looking , just the SSSC squared up a little and a full pressure cowl for less drag. Squared stab, elevator and fin and rudder. 38' wing span to fit in most hangers with a 4' cord, of 134 sq' of wing area @ 800 GW- 5.97 lbs wing loading.
    With a 170 lb pilot and full fuel of 12 gal = 5.53 lbs wing loading.
    Wish the wing loading was less.
    So, increasing the wing cord to 54" and the outer wing from the strut to the tip tapered to 42" gives a total wing area of 152 sq'. 152 sq' @ 800lb GW is a wing loading of 5.26 lbs. With the 170 lb pilot and full fuel of 12 gal = 4.88 lbs. I like this better.
    Love to keep the EW down to 470/480 lb range.
    Quick drawing of the squared off SSSC with the pressure cowl. Scanner not working so I took a picture. With a 1600/1835 cc VW engine , cruising at very low power, 40% of so, the fuel burn will be about 2 gph so with 12 gal of fuel, 6 hrs of range.


    DSCF0011.JPG
     
    Last edited: Nov 18, 2019
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  17. Nov 18, 2019 #77

    bifft

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    I am talking about a self launcher with 15 minutes of battery. 100-150 lbs or so? (still a large fraction). The hour flight is 5 minutes of climb followed by 25 minutes of glide at min sink done twice.
     
  18. Nov 18, 2019 #78

    pictsidhe

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    For low sink, you need span, not low loading. I'd be looking at removeable or folding wing tips to keep it fairly easy to hangar yet have more than 38'. Don't be afraid of airfoils up to maybe 18% to hide your spars in. Or go over to the dark side. :wonder:
     
  19. Nov 18, 2019 #79

    Pops

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    That is why the span is 38'. I want span PLUS low wing loading. Folding wings would be nice, but adds weight. I want it as light as possible and still go in my hanger without hitting the sides of the opening. Gives me 3' of clearance on each wing. I'll be using the same airfoil as used on the SSSC and its 16 %.
     
  20. Nov 18, 2019 #80

    cheapracer

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    Of course you can, not good enough for whom?

    The Morgan motorglider I showed on a previous page, is a cheap aluminium craft that the owner/builder was flying reguarly in New Zealand catching thermals, and having a great time for a tenth the price (literally).

    Morgan Motorglide 2.jpg

    Morgan motrglider 1.jpg
     
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