# Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Topaz, Sep 10, 2014.

1. Jun 7, 2015

### oldcrow

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This is what I am working on right now. It's kind of a "Taggart" and "Sandlin" mix. Powered by a 20hp PPG motor up front.
(wing ribs are not shown at this time) right now I am at 150lbs + 25lbs for motor and then control stick, covering and boat seat.
my cost estimates are;
motor- $3000.00 fuse-$ 250.00
wing- $1500.00 covering-$ 250.00

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2. Jun 8, 2015

### Topaz

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Sorry it's been so long. Things are easing up for me a bit, so I hope to get this going again on a more regular basis. Glad you're getting something out of it!

3. Jul 1, 2015

### Himat

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If memory serves me, I think it was mentioned on this forum that the Concorde did have a L/D of 7 at supersonic cruise speed. The sailplane graphed is down to same L/D when the speed reach approximately 145mph.

4. Jul 1, 2015

### Topaz

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Now imagine the L/D of the Concorde at 145 mph. :gig:

Part of design is getting L/D[SUB]max[/SUB] (or L/Dcruise) to happen at the speed that you want. Resulting in very different designs.

The graph above is for a long-winged variant of the little tailless sailplane shown in one of the albums in my user profile, so it's sacrificing a high L/D[SUB]max[/SUB] speed for a high L/D[SUB]max[/SUB] value, along with a low minimum sink rate value.

5. Jul 2, 2015

### highspeed

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Tough one, since that's below Vmc for Concorde. I'd put L/D at 145 mph somewhere between a brick and an anvil. :grin:

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6. Jul 2, 2015

### Topaz

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I think even that may be optimistic.

7. Jul 2, 2015

### Himat

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If the Concorde is on the ground at 145mph I think we have the case of L/D of a wheeled vehicle. That one is usually quite good, at least as long as the ground is flat.:gig:

8. Jul 4, 2015

### Rienk

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Re: Power to Weight Ratio

In your latest post, you talk about sizing the engine according to your weight to power ratio.
One thing I'd like to note, to those who may (hopefully) also use this design process, is a clarification of one of your comments.
In going with a lower W/P ratio, you state that "I'll also get a smaller, less-expensive engine in the bargain" - but that is rarely true.
In theory, a smaller engine should cost less - but only if an inexpensive off-the-shelf engine is actually available in the size you need. Given that, in practice, there are relatively few available engines in certain power ranges, with notable gaps in certain power bands, finding an affordable engine of a certain weight and power is usually difficult. And just as importantly, it can be very expensive!

Of course, that is why many designers - once they have the rough numbers figured out - design around a certain engine. Such is the case for designing the Solo and Duet around the Generac and VW engines (respectively).

I know we all wish that we could go down to the local store and pick up an affordable engine in any power we like (within 10-20 hp steps), with each costing about the same per horsepower. Which is why we get excited every time a new engine development program gets started; hope springs eternal!

9. Jul 4, 2015

### Topaz

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Absolutely. What we're talking about here is the difference between a "fixed engine" sizing, and a "rubber engine" sizing. Raymer's terms.

Choosing a specific engine early in the design process is a perfectly valid process, where the performance of the aircraft becomes the fallout parameter to the sizing process. The first of the two conceptual design examples that Raymer works at the back of Aircraft Design: A Conceptual Approach - the single-seat aerobatic airplane - uses a fixed engine sizing. In that case, the aircraft as initially sized doesn't meet the range requirement set at the beginning of the process, and Raymer uses more advanced techniques later on to optimize the aircraft up to the desired range. It worked out in that case, but that's hardly guaranteed - he could easily have ended up unable to fully meet the range requirement with the chosen engine, and still meet all the other requirements for the design.

In my case, I have absolutely no preconceptions about what engine I want to use, I really want to hit all my design requirements, and in the size range into which my design will almost certainly fall, there actually is a fairly continuous range of engines available. Starting down with the 420cc Honda derivatives and extending up through the various displacements of 1/2 VW engines, up to the largest of those and V-twins such as the "Big Twin" from Valley Engineering, I'm fortunate to be able to pick and choose an engine nearly at-will. In terms of the desired total cost of the project, there's a relatively large range of prices on these motors. A 420cc derivative upgraded with racing components can be had for as little as $650, with another$400-500 needed to adapt it for airplane use, on up to, say, the Valley Engineering "Big Twin" which comes ready-to-fly (including prop) for $4,999. If I were working on a somewhat larger single-seat aircraft, such as your Solo, I agree that there aren't nearly as many available choices in powerplant and making a specific choice early on makes a lot of sense. It's a shame that what was once the most-populated band of power ranges for homebuilders is now full of gaps and holes. There are a couple of big V-twin models in the 40-55hp range, and the smaller 4-cylinder VW's fit in the higher end of the range, but all the options are fairly heavy for the power, and there are even more powerful options for little more price and weight. In the end, which sizing method one uses is a matter of design style and if there is a range of powerplants available in the likely range of needed choices. 10. Jul 4, 2015 ### Hole in the Ground ### Hole in the Ground #### Guest I presume that once you have nailed down your engine of choice you'll have to do a bit of rework for that power and weight? 11. Jul 5, 2015 ### Topaz ### Topaz #### Super ModeratorStaff Member Joined: Jul 30, 2005 Messages: 13,963 Likes Received: 5,577 Location: Orange County, California Short answer: Yes. Long answer: Aircraft design is always an iterative process. You never get the final answers in the first round. The initial sizing I'm about to do not only gets me in the ballpark, but onto the infield. It'll be fairly close to the final numbers. As I just discussed with Reink, in the horsepower range I expect this design to need - (20-40hp, by looking at comparable designs) - there is a pretty smooth range of engines available. I expect that, whatever the initial sizing says, I'll be able to choose an engine within a few horsepower of that figure. However, the initial sizing is just that - initial - and the second, final, sizing optimizes chosen characteristics of the airplane around more-fixed values, including a fixed engine choice. The final sizing/optimization will also tell me if any one requirement is unduly driving the design, and inform me about whether it's profitable to relax that particular requirement, particularly if it results in a significant cost savings. If that were to be the case, then I'd do another iteration of the design around the relaxed requirements set, possibly changing the engine choice in the process, if that seems like the right thing to do. ultralajt and mcrae0104 like this. 12. Jul 22, 2015 ### Manticore ### Manticore #### Guest The version I learned was a little longer - it goes "Think three times, measure twice, cut once." 13. Aug 2, 2015 ### Topaz ### Topaz #### Super ModeratorStaff Member Joined: Jul 30, 2005 Messages: 13,963 Likes Received: 5,577 Location: Orange County, California Is the link in the most-recent post (Stall and Takeoff wing loadings) not working? I'm seeing well over a hundred views, but not a single download of the linked spreadsheet. Or are you all waiting for the finished version in the next installment? :grin: Update: Whomever it was that downloaded the file this evening, thank you. Last edited: Aug 2, 2015 14. Aug 21, 2015 ### Topaz ### Topaz #### Super ModeratorStaff Member Joined: Jul 30, 2005 Messages: 13,963 Likes Received: 5,577 Location: Orange County, California Reiterating my call in the design thread, I'm looking at engine alternatives for the single-seat motorglider project. I've baselined the 35hp Hummel Engines 1/2 VW, but I'd like to see what alternatives are out there. I'm looking for an engine with the following characteristics: Power: 29-35 hp Brake Specific Fuel Consumption (BSFC): Approximately 0.43-0.45 Starter: In-flight restart possible, either electric or recoil. Weight: Less than 105 lbs. Cost: Less than$3,325, including everything but prop and exhaust system.

The BSFC pretty much rules out two-strokes, but I'm open to one if its BSFC is down close to this range and the engine provides other advantages. I'm especially interested in engines that have a small installed envelope of space. The 1/2 VW is a very wide engine for a single-seater and, even if cowled, the protruding cylinder heads will be a big source of drag.

I'm also looking for a solution that's nearly turn-key: I don't really want to experiment with engines or developing a PSRU in a brand-new airframe. I'm open to V-Twin conversions, provided they're available as a completely converted engine.

Anyone have any thoughts?

15. Aug 22, 2015

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O100 from Pegasus?

Parazoom

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16. Aug 22, 2015

### Topaz

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Love the O-100. I've been following that effort fairly closely. It's an exciting development, but at 58hp it's too powerful for this particular application.

Parazoom looks really interesting. It's about exactly what I need, and is more compact than the 1/2 VW. Is there another, similar, V-twin conversion that's more economical to purchase? At €4,899, that converts to US\$5,579, the Parazoom is more than 60% more expensive than the Hummel 1/2 VW.

17. Aug 23, 2015

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I think the 30-35 hp you're aiming for is grossly underpowered, even in the flatlands.

I've flown a bunch of similar planes. RF5B @ 1400 lbs and 67 hp (variable pitch). Performs about how you imagine min performance at sea level and 70F. Pathetic to unflyable at the density altitudes you're talking about. I would certainly not go below that power loading and I fear it's grossly inadequate for your application (high DA's).
For me they're about the same price. Note that in Europe prices are advertized including VAT. That's 20% off. Shipping a half VW and handling all that plus paying VAT over it puts them in the same price category for me. The Luciole conversion is about the same price range.

For you the difference is smaller than that 60%, but with twice shipping substracted it's a big difference. Were I picking between those on this side of the pond it'd be a 50/50 chance. In your case the VW is the clear winner.

Personally I'd make sure you can accomodate a full VW. Same price as the high-powered half VW's (cheaper actually) and about 50 lbs heavier. But it doesn't stop there. More power will allow you to avoid a variable pitch prop which you NEED with such low power rating @ 35 hp. Running it at lower RPM, bigger prop so much improved low-speed performance.

Your numbers look ambitious, but realistic. The problem is that it's an avalanche effect. A variable pitch prop might just add 15 lbs, but without it, performance it unacceptable. The tiniest weight excursion makes performance, even with a variable prop unsafe. Going for a 50 lbs heavier engine however (don't know about the O100) opens up a whole range of engines. 60 HP, even if there's an additional 50 lbs of more weight creap will still give good performance and you can ditch some complexity like variable pitch.

18. Aug 23, 2015

### cluttonfred

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I agree with autoreply on this one, a cheaper full VW makes a lot more sense if inexpensive is part of the equation. Yes, you carry more weight, but you gain more power in proven, unstressed package for long life and reliability. The Fournier RF-3, which used a VW-derived 1200cc engine, would be a good baseline for comparison to check your numbers against reality. Even a bare bones, cheap as you can get 1600cc conversion would represent one-third again the displacement of the Fournier's engine for very little weight increase.

This Italian pilot sure loves his RF-3!

The other alternative would be to go with one of the more powerful modern two-stroke paramotor engines of around 35 hp, trading increased fuel consumption for lighter weight. For me, it would come down to whether you expect to use the plane more as a self-launching sailplane or as an efficient tourer, which would argue for the lightweight two-stroke in the first case and the more efficient and reliable four-stroke in the second case.

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19. Aug 24, 2015

### Topaz

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Ummmm... you do recall that I'm talking about a single-seater here, yes? Half the weight of a two-seat RF-5, and less even than the weight of the 39hp RF-4. With more span and (hopefully) lower drag.

My math in generating the performance numbers I have is out there in public for all the world to see. If there's a numerical mistake I've made, I'll be more than happy to have it pointed out. Otherwise, these are accepted methods from accepted authorities, and there are similar aircraft with similar power loadings already exant that are performing nearly the same mission. I've listed them. If their published performance numbers are lies, I need to know that, too. Nobody has said anything.

In that context, and with all due respect, "feeling" that it's underpowered isn't going to be enough to sway me. I'm going to need to see where I've done the math wrong, because the math I have says this thing will perform to the specifications I've written.

As for potential weight gain during development and such, I'm completely aware of that. It's one of the reasons I'm contemplating an engine at the high end of my stated power requirement, rather than one at the bottom. I've just finished (and was about to publish here yesterday, when the fan relay on my condo's A/C chose this moment to fail) some trade studies for parameters like range, engine BSFC, and so on, and the design is remarkably resilient to gains of up to 40 lbs or so with this 35hp motor.

So please, if I've done some math wrong, I'd love to have it pointed out. It's why I'm doing this in public.

20. Aug 24, 2015

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