# P-51C at ~70% scale as ultralight?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by J.L. Frusha, Apr 30, 2019.

1. May 7, 2019

### pictsidhe

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This is why you have such pessimism directed at you. You said that you going to try and design an ultralight by using TLAR thicker aluminium to arbitrarily strengthen it instead of recalculating.

If you do the calculations as most of your skeptics have done on their own projects, you might appreciate why they are responding the way that they are.

If you don't do the math, the extra weight from the bits that you overdesign will further overstress the bits that you underdesign.

You think that you need a stronger tail, but you don't know that, you're just guessing. In my own design, the flight loads exceed the tailwheel loads. I only need to attach the tailwheel solidly enough to take landing loads to the fuselage that is strong enough to take the flight loads and I'm good. On to the next part.

Calculating the loads on the various parts is fairly easy. If you compare the loads of your project to the loads on part X of the donor project A, you can then make a judgement as to whether part X is too weak, about right, or too heavy. Perhaps part X on donor project B is about right. Perhaps you need to redesign it. That is where the real tedium starts

Or do you want something that is overweight and understrength?

2. May 8, 2019

### BJC

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You asked for opinions, and you received knowledgeable input.
I know of two airplanes that we’re originally designed that way. Both eventually abandoned it for fiberglass over foam.
Are any commercially available?
End plates were tested about 50 years ago. All they did was increase weight and drag.
Hovey sold lots of plans. From what I can find, the WD II is not capable of flight out of ground effect.
Lots of people here know what can be done, because they have done it. Likewise, they have seen lots of failures, and are willing to provide realistic information to you. What you choose to do with that information is up to you. If you do start a project, please share it with us via a build log.
Many of us took that statement to mean that you arbitrarily had decided to use thicker aluminum everywhere, not just in fuselage formers. BTW, the Watson Windwagon does not meet any of the Part 103 criteria.
Retractable her is not permitted in Part103. edit. This statement is wrong; Part 103 does not prohibit retractable LG.
I encourage you to seek to understand other’s comments before classifying them as ridiculous or baseless.

BJC

Last edited: May 8, 2019
3. May 8, 2019

### pictsidhe

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Whoah BJC, 103 doesn't permit retracts? Where does it say that?
Are you thinking of LSA?

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4. May 8, 2019

### BJC

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Thanks. You are correct about the retracts. Not sure what I was thinking of, but claiming that it was LSA seems a reasonable dodge.

BJC

5. May 8, 2019

### BBerson

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I was looking at the Hummel bird at wiki and it is 18 feet span and 300 pounds empty (apparently has lighter skins than the Windwagon).
The very similar Ultra Cruiser has 25 feet span yet somehow weighs only 249 pounds with the same engine options. Sure, no canopy. And ribs may be a lighter truss, but I still don't see how it can be 50 pounds lighter with the much larger span. What is different?

6. May 8, 2019

### 12notes

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I think the Ultracruiser uses thinner skins and bulkheads everywhere. The material list for the UC has 8 sheets of .016", the Hummelbird uses only 2 sheets @ .016", the UC uses a 1/4 sheet of .040", the Hummelbird calls for 1 full sheet of .040", but I've needed more than that. Also, in addition to the ribs, the Ultracruiser's spar is much lighter, I think there's only a single angle on top and bottom, and it's definitely smaller angle, as the UC uses 24' of 1x1x1/8 angle, 24' of .75x.75x1/8 angle and 52' of .75x.75x1/16 angle, while the Hummelbird uses 84' of 1.5x1.5x1/8 angle and 8' of 1.5x1.5x3/16 angle.
Ultracruiser:

Hummelbird, before much smaller lightening holes in outer spars only:

7. May 8, 2019

### 12notes

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The tricycle gear Windwagon and tailwheel Hummelbird do indeed have the same skin and bulkhead thicknesses. The Hummelbird has .75x.75x.040" angle stringers top and bottom between bulkheads C, D, and E.

8. May 8, 2019

### BBerson

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The Ultracruiser is 39% larger span and 20% longer and almost twice the wing area. The spar caps can be a bit smaller at the outer wing but not much if any at the root because of the greater moment.
The Hummel is listed at 6 g, however. I suppose the Ultracruiser is much less.

9. May 8, 2019

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I built a Hummelbird years ago and if I'm not mistaken the weight came out right at 308 or 309 lbs.
I would have to dig up the paperwork to verify that exact number but I believe it is correct.
I could have saved a tiny bit of weight by not adding disc brakes.....5 or 6 lbs. maybe.

I've never had the chance to look over a set of Ultracruiser plans but I would be sceptical at its claimed dry weight.
I don't see how it can physically be a bigger aircraft than the Hummelbird and have shed a claimed 50 lbs.
The only logical way the weight reduction is possible is to have lost some wing ribs,bulkheads,canopy and skins being thinner.
But even if that little bit of metal and lexan are gone ....it just doesn't scream 50 lbs. to me.
Think about how much sheetmetal it would take to equal 50 lbs...........alot.

I'm hoping someone comes up with a logical and well thought out reply to your inquiry BBerson.....

Kevin

10. May 8, 2019

### BBerson

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11. May 8, 2019

### mcrae0104

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Don't forget the canopy.

12. May 8, 2019

### 12notes

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The canopy is 100% thinner, in that you can't have one to make weight. Pretty much everything is thinner, there's also lightening holes in the bulkheads and everywhere else they could fit. The smaller and thinner angle I listed in a previous post would also make a big difference.

EDIT: I just did the math, the difference in weights from the Hummelbird to Ultracruiser's material list is 23.5 lbs in angle and 9.25 lbs in sheet metal. That's 34.75 lbs out of the needed 45 lbs. of weight reduction, (there have been 300lbs. Hummelbirds built), If you assume similar amounts of waste material, and a 5 lbs canopy, that leaves about 5 lbs of weight reduction needed with lightening holes, other materials (I only calculated angles and sheet), etc.

Last edited: May 8, 2019
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13. May 8, 2019

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I included the canopy in my list......along with ribs,bulkheads and skins.

Kevin

14. May 8, 2019

### cluttonfred

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Here is an example of a low-wing Sky Pup that could serve as a realistic starting point for a Part 103 fighter pastiche.

Source and more pics: https://groups.yahoo.com/neo/groups/Skypup-club/photos/albums/1140149251

Personally, I think LSA or microlight category makes more sense for this, there are lots of proven little low wing designs (Taylor Mono, KR-1, etc.) that could be reworked cosmetically for a fighter look. The original single-seat Bushby Midget Mustang is an obvious option.

15. May 8, 2019

### BBerson

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The Ultracruiser might save some weight with the deeper spar. And no shock absorbers (from website note).

16. May 8, 2019

### pictsidhe

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I think that the UC is 4.4g

17. May 8, 2019

### BBerson

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The current website lists the Ultracruiser at 4g.
The Hummelbird at 6g.

18. May 8, 2019

### Norm Langlois

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With all that familiarity with the UC .Can you confirm that the wing area would perform to the 28 mph. power off stall and max speed level flight full throttle of 60 mph. ,that's required to make 103

I do understand That for the most part .If it looks like a 103 and has 1 seat and 5 gal. of fuel and makes weight or appears to. One could go about flying without the FAA breathing down your neck.
Are we all being a bit complacent . Flying anything that does not qualify can bring you a hefty 1K fine for each infraction.
Not to mention that a 103 is suppose to be proven before a board of 3 certified individuals like the EAA could provide . I don't know if you all have read the full version of 103 and 103-7 When we google Or search out FAR 103 you don't always get the whole document.
Or if any UL's ever where proven before a comity. but making bold claims will certainly stir things up.
I have seen where the use of other exemptions make total weight. Like carrying a chute.
Argued in other threads to not be acceptable .
The point I am trying to make is. It is still a grey area and there are many who still don't like Ultralights flying in there space.

Last edited: May 8, 2019
19. May 8, 2019

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Complacent...I don't think so. I would say "pragmatic". Are you aware of any enforcement actions ever taken against a plane that fits the description, "looks like a 103 and has 1 seat and 5 gal"? I would like to know of a documented case.

20. May 9, 2019

### pictsidhe

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Enforcement is by a suit telling someone to stop flying, or else. This is not recorded outside the FAA. When the potential fines are mentioned, people stop flying. The exception to that is Douglas Hughes, who was fined $11,000. Apparently, the maximum fine is$250,000 and prison.