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

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

Originally Posted by nerobro
...And that's why you make a SWAG. Then you try to make that math work. Then you revise your guess. Then you do that math. Wash, rinse, repeat, until you're close. once you're close, you start designing pretty good parts. Then you run the numbers again. Then you design real parts. And wonder where all your spare weight went. :-)
Exactly. Raymer's method is just an organized process of doing exactly that, with specific tools to help you "get into the ballpark" faster than mere guesswork.

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

Topaz, normal category (3.8 g) is considered inadequate for a relatively clean glider.
5.3g is standard, I think. Of course, you can do what you want.
If the intent is to enter strong conditions, I would consider a higher design load. Check Basic Glider Criteria.

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

Quit calling it a \$100 burger! With a cheaper to build and cheaper to operate plane that gets flown more often I think you/we should be striving to achieve a \$30 burger goal. At the very least a \$57 burger

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

Originally Posted by BBerson
Topaz, normal category (3.8 g) is considered inadequate for a relatively clean glider.
5.3g is standard, I think. Of course, you can do what you want.
If the intent is to enter strong conditions, I would consider a higher design load. Check Basic Glider Criteria.
Yep. Note I'm only talking about powered flight at the moment. You're absolutely right, and you're going to see the limit load jump up to JAR 22.337 standards almost the moment I start doing the "What" stuff for soaring flight. I'm copying this material over to the thread pretty much exactly how the development goes in my notebook, so you're seeing some "process" here, as well as simple results.

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

Originally Posted by Hot Wings
Quit calling it a \$100 burger! With a cheaper to build and cheaper to operate plane that gets flown more often I think you/we should be striving to achieve a \$30 burger goal. At the very least a \$57 burger
I'm amortizing in aircraft development cost.

I like the sound of \$30 burger better, too. With the price of avgas, I might have to shut off the engine and soar all the way there to accomplish it, though...

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

Originally Posted by nerobro
The cross dependencies in airplanes are a real pain in the rear. How do you know how much a tail section will weigh, if you don't know the forces it needs to support. You don't know the forces it needs to support until you design the tail empennage. The size of the tail feathers varies based on the weight of the rest of the plane. But the weight of the plane is affected by the material you use to make that tail, and tailplane...

And that's why you make a SWAG. Then you try to make that math work. Then you revise your guess. Then you do that math. Wash, rinse, repeat, until you're close. once you're close, you start designing pretty good parts. Then you run the numbers again. Then you design real parts. And wonder where all your spare weight went. :-)
Okay, I have to respectfully disagree that a SWAG is required. In his book, Light Aircraft Design, Ladislao Pazmany has an equation for wing weight that should get you within 10% of the actual weight, assuming an optimized design. The equation accounts for aspect ratio, load factor, airfoil thickness, and other variables. Likewise, he has nomographs for fuselage weight with one curve for "average" design and another for "optimized" designs. There are larger variables that affect fuselage weight, but you can get within 15% using this method. His methods for horizontal tail weight yield similar results. And there are equations for estimating landing gear weight also.

The bottom line is that once you determine the gross weight and certain loading variables, the weight fraction for various structures on modern aircraft are not that different. Many aircraft design books have equations or graphs for estimating the weight of aircraft structure. When they are based on historical databases, the results come pretty close to reality. Airplane PDQ software has 6 different methods for estimating weight, and except for a few outliers, the results are within 20% of each other (+/- 10% from average). With all these tools available, a SWAG should really be more of an "educated estimate".

That's not to say a flying wing or BWB won't throw things off a bit. Those configurations makes weight estimates a little more difficult, but there are proven methods for those structures too. None of these weight estimation methods provide "final" results, so your comments about the design, analyze, redesign cycle is right on. But we're talking about optimization here, not huge changes due to SWAG.

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

So... Would I be nutty to make a suggestion? I know your designing for your needs... But...

Taller and heavier is largely missing from the homebuilt market... I'm 6'2 250# (not round, I just work in construction - and am built like a Scottish lad should be ) - kinda tiresome seeing all these designs based around a 180# 6' pilot adjustable ballast beats airframe limitations don't you think?

Worst case its extra capacity for cargo/gear/too many 37\$ burgers

Looking forward to see where you go with this - hoping closer to carbon dragon instead of EMG/goat

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

You're not nutty. Well, not any nuttier than this descendant of good Welsh stock might be.

I hear you. There's a couple of things going on here. For one, as you note, this airplane is a one-off for me. Even if it turns out to be the greatest thing since haggis, this particular airplane will never be offered as plans or a kit. IF it turns out to be dripping with awesomeness, and IF I judge there to be a sufficient market to risk a business venture, and IF I feel like opening a second business to pursue that, I'd be doing a ground-up review and redesign of the aircraft in order to create a "production" version. Obviously, the review would include the range of pilot weights and sizes to be accommodated, and the baggage allotment. For a commercial product, determining the design payload range is a far larger, much more complex process than "how big am I and my friends who might fly this airplane?". I'm not putting that effort into this design study because it isn't relevant to my needs on this one. Simplicity in the design process means I get it done more quickly.

Secondly, there's just physics. Because this is intended to be a relatively inexpensive experience-builder, cost-control is a big factor. If it costs "too much" (whatever I decide that threshold to be), bulding the thing just isn't worth it to me. The engine is one of the biggest cost drivers in any airplane, and more weight generally means more motor to get equal performance. More motor usually means more money, and the engine market is such that, sometimes, a small increase in power requires a lot more money. You large guys are at a distinct disadvantage in this regard. It sucks, but physics is physics. I feel for you on that.

One of the ways I'm keeping costs under control on this "demonstrator" airplane is to be rather severe about keeping weight down to a practical minimum. This should pay off (literally) when I run the sizing for this airplane and find out how much power it will need to meet the design specifications.

So yeah, I feel for you larger folk when it comes to finding airplanes. That has to be very frustrating.

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

I would like to say "GOOD. Go for it." and Thank you Topaz for putting yourself on the line. I am looking to design my own aircraft. I am still in the Look See phase of design. (Having a looksee to see if I can even get in the air.) Seeing the design process from this end instead of just finished models or just the test results, will be helpful to noobs. For my advice, paper is cheaper than build material. Draw it, math it. Worry it until it is highly polished. Then buy parts. As an industrial mechanic, the best advice I was given was measure twice, cut once. Enjoy the process.

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

Originally Posted by Devilkidd1979
For my advice, paper is cheaper than build material. Draw it, math it. Worry it until it is highly polished. Then buy parts. As an industrial mechanic, the best advice I was given was measure twice, cut once. Enjoy the process.
Dang, I cut it twice and it's still too short!

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

Originally Posted by Hephaestus

I'm 6'2 250# (snip)
hoping closer to carbon dragon instead of EMG/goat
I have some baaaad news for you. The two excerpts from your post above are mutually exclusive

The Carbon Dragon was not a heavy duty glider. If I remember his explanation correctly, when a 200 pound pilot flew it (Dan Armstrong), they had to recalculate the loads for his weight - and Dan's weight made it into just about a 2G glider with him in it.

The Carbon Dragon was famous for the "joke" that Maupin played with the brochure... a drawing of a petite girl carrying the glider on her back ready to launch. Maupin admitted it had been a joke, since the thing was obviously way too heavy for such a young waif to lift.

What Maupin didn't admit, is that the girl in the drawing was also just about the maximum pilot weight

I'm sure that a Carbon Dragon built today, with the pulltrusions and the current material capability, could be built much stronger than the original.

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

Originally Posted by Hephaestus
So... Would I be nutty to make a suggestion? I know your designing for your needs... But...

Taller and heavier is largely missing from the homebuilt market... I'm 6'2 250# (not round, I just work in construction - and am built like a Scottish lad should be ) - kinda tiresome seeing all these designs based around a 180# 6' pilot adjustable ballast beats airframe limitations don't you think?

Worst case its extra capacity for cargo/gear/too many 37\$ burgers

Looking forward to see where you go with this - hoping closer to carbon dragon instead of EMG/goat
That's something bothering me, too (not with Topaz's airplane, but in general). I see a lot of neat little glider and ultralight designs that seem built for some guy who's 5'5" and 130lb soaking wet. Being just a shade under 200lb myself, and the shortest of my siblings at 6' even, I want an airplane that can carry real people. I'm aiming for something like you are--something more Carbon Dragon-looking than Goat-ish--and sized for bigger people like me. When I eventually get around to doing like Topaz is and posting my preliminary process in a few months, that'll be in the criteria.

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

Originally Posted by Hephaestus
So... Would I be nutty to make a suggestion? I know your designing for your needs... But...

Taller and heavier is largely missing from the homebuilt market... I'm 6'2 250# (not round, I just work in construction - and am built like a Scottish lad should be ) - kinda tiresome seeing all these designs based around a 180# 6' pilot adjustable ballast beats airframe limitations don't you think?

Worst case its extra capacity for cargo/gear/too many 37\$ burgers

Looking forward to see where you go with this - hoping closer to carbon dragon instead of EMG/goat
I have 2 grandsons that are 6' 4"/ 6' 5" X 250 lbs. That is why I am designing and building the JMR Special. It will fit a person their size. Also trying to make it as easy to build as possible. Not much out here on the market.
http://www.homebuiltairplanes.com/fo...my-design.html

Dan

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

I'm a pretty big guy as well. Stuff like this makes me wish I was still fourteen.

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

Originally Posted by gtae07
That's something bothering me, too (not with Topaz's airplane, but in general). I see a lot of neat little glider and ultralight designs that seem built for some guy who's 5'5" and 130lb soaking wet. Being just a shade under 200lb myself, and the shortest of my siblings at 6' even, I want an airplane that can carry real people. I'm aiming for something like you are--something more Carbon Dragon-looking than Goat-ish--and sized for bigger people like me. When I eventually get around to doing like Topaz is and posting my preliminary process in a few months, that'll be in the criteria.
Lots of homebuilts have grown over the years to accomodate / be more comfortable to more "normal-sized" people.

Examples include the Pitts Specials, Van's RV series, Glasairs / Glastars / Sportsman, Lancairs, etc.

So if a design is intended for more than just one or two known people, why not make it comfortable for the masses?

BJC

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•