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Old Feb 04, 2012, 02:40 AM
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Trevorh's Avatar
Hampshire, U.K.
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Nick, the motors you used in your DH88 should give you a benchmark. How would the wing area of the 90in Starship compare with the 80in DH88?

Trevor
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Old Feb 04, 2012, 06:41 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Hi Trevor!

The DH88 Comet is 750 sq", mine weighs 96oz. This model comes out at 1280 sq" when I put it's dimensions into the CoG calculator. The reason I think I can keep the weight so low is that my original depron Starship weighs 31 oz. This one is double the size, it'll use 4 times as much depron on the surfaces. The new depron is 20% lighter and there should be some economies of scale.

Looking at the spec for the motors from the Comet, I think they should be fine. I'd very much like to fit 5 blade props but, with 4" spinners they might be hard to find. I've sent an email to Charlie at Manzano Lazer who supplied the hubs and spinners for my original Starship.

Cheers

Nick
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Old Feb 04, 2012, 08:52 AM
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United States, OH, Bradford
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Nick, the flaw in that reasoning is the effect that size has on loads and structural weight.

The "law of squares and cubes" says that dimensions scale linearly, areas are squared, but volumes (and therefore weights) are cubed. Yes, you can try making some structures the same wall thickness as before, making it lighter (and weaker) than it would be otherwise. However, loads on the wing spars increase with the square of the span.

Yes, some things on your original model were probably beefier than necessary, with some potential for weight savings, and it should be possible to do better than the law of squares and cubes. However, just how much better is an open question.
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Old Feb 04, 2012, 09:34 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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Thanks Don

Advice is what I'm after, any improvement on my usual TLAR approach will be helpful. In particular, my planned spars are much stronger than before. I figure depron is pretty good in compression, not tension, so I've got 6mm cf strips on the top and bottom of each of those green spars shown on the diagram. At the root, the depth of the spars is 49mm, at the transition from inner panel to outer, it's 24mm and at the tip it's 11mm. Other bracing is shown in blue.

I don't plan to do slope soaring or aerobatics. But I'd like to avoid learning Reggie's lessons all over again.

Here's a link to a very successful C-47. and some pictures of it shown below.

Cheers

Nick
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Old Feb 04, 2012, 11:34 AM
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Nick, as you know from looking out the window on your transatlantic jaunts, wing flex is not in itself much of a problem - wing twist though is frequently terminal. Judging by the DH88 performance, the motors will cope with an AUW of 6.5 - 7lb and the wing loading would still be reasonable so you do have a bit of leeway.
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Old Feb 04, 2012, 12:41 PM
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Bud! You are a total hoot! I really like your enthusiasm in 'the search'! could you expound on your root joint/ structure? I'm a woody kind of guy- and usually stick with this type of setup! (pic)
Johnny
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Old Feb 04, 2012, 03:08 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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Thanks Johnny

This is what I did with the 46" depron Starship, with some notes on my next cunning plan... There will be some ribs this time, to preserve the shape, especially on each side of the cut where the outboard wings detach, next to the nacelles.

What could go wrong?

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Old Feb 04, 2012, 04:13 PM
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no balloon tires in the grass??!
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Old Feb 04, 2012, 04:58 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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I will try retracts this time. At least I'll see how the weight goes before I add them. And I'll keep the hook for my bungee launcher, just in case.

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Old Feb 04, 2012, 11:42 PM
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Your welcome Nick! looks pretty tough to me!
I'm in redraw for laser files myself!
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Old Feb 05, 2012, 09:16 AM
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The shear strength of the Depron is way below the loads the carbon spar caps can shrug off with ease. If you substitute vertical grain balsa for the Depron shear webs, you can pick up a lot of strength for very little additional weight.

There's a stress concentration at the ends of those spars. That's where it's most likely to break.
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Old Feb 05, 2012, 11:29 AM
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Market Harborough
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That's a useful suggestion Don, Thank you.

The root chord on my drawing is 719mm and the max thickness is 61mm, which is only about 8.5%. My other Starships have had that proportion whilst further outboard it's more like 12.5%. They both handle very well. I wonder if this suggests that I could get away with something more like 10% at the root in order to get a bit more "strength in depth".

A trick I have learned from Ivan Pettigrew is to distribute the transition outwards in steps from strong reinforcement at the root where the loads are high towards the tip where they are not.. In other words, it's necessary to have at least one intermediate stage midway.

Also, I don't plan to build with crashing in mind. Just some gentle loops and rolls. Maybe slow tight turns as well.

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Old Feb 05, 2012, 12:08 PM
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8.5% excellent for these Re's. 12.5% is way too thick. Assuming your structure can handle it, carrying the 8.5% all the way to the tip (or even a slight thinning to about 7.5%) is about ideal from an aerodynamic standpoint.

Also, good structural design isn't about crashing. It's about making the plane fly better because it isn't dragging around any excess weight.

On the Bucker Bu133 Jungmeister (famous for its extraordinary aerobatic capabilities, Jose L. Aresti, who wrote the Aresti Aerobatic Dictionary used as the basis for modern aerobatic competition, flew one) the fuselage longerons are not one single piece (as in most steel-tube fuselages), they are sleeved together from seven different sizes of tubing, to save weight in the tail. This weight savings in the extremities is part of the secret to the Jungmeister's amazing precision and consistency in things like snap maneuvers.

For those of you not familiar with it, the Jungmeister (with a modern engine) is the plane used for the 10-turn spin in the movie "The Great Waldo Pepper". The Jungmeister is probably the only plane that could safely do this maneuver as done in the movie (with a touch-and-go at the end). Just about anything else will come out of a snap maneuver precisely where and when you want it to 9 out of 10 times. Only the Jungmeister can be counted on to come out exactly when and where you want EVERY time.

Stress concentrations mean you need more structure to do the same job, because some of that structure isn't being used to its best advantage. Even if you aren't "designing to crash", the plane will still fly better if you use the absolute minimum amount of the very best material, to its maximum possible advantage.
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Old Feb 06, 2012, 04:41 PM
What could possibly go wrong?
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Market Harborough
Joined Apr 2006
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92" Starship - Now I'm committed. This is because I was foolish enough to believe I could take my box of depron home in my luggage. Delta charge $80 for an extra bag, or one as big as the box the depron came in. The post office charges $25, UPS $40. So the only solution was to start cutting it up, so I can fit it in my bag.

Here's the link to my build thread.

Thanks as always for the help.

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Old Feb 06, 2012, 08:01 PM
China, Guangdong, Shenzhen
Joined Nov 2010
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Tandem trainer . . . first plane.

I think I now understand why beginners are advised to buy a kit for their first plane However, all four wing cores are cut and the motor mount with its associated bulkhead is complete, so it is too late to back out now . . .

It is intended to be a low wing loading, low power (inexpensive) trainer. The power system is an EPS 400C with a 3:1 reduction gear driving a 10:4.5 slow fly prop. The battery is a 7 cell, 1100 mAh NiMh - I sacrificed flying time and power for lower weight, however, I didn't want the expenses / problems of LiPo for a first project.

Both wings have a span of 1m, wing loading at the design weight of 32 oz. will be 8 oz / ft^2, and the front wing is 67% of the area of the back wing. I expect that the downwash from the front wing will move the centre of lift forward by reducing the effective AoA of the main wing - but I don't know by how much. I am planning on calculating a centre of lift based upon the area weighted average of the distance between the MAC's of the two wings and then moving the battery so as to get the CG 5% forward of this position.
I expect that even with 0 deg decalage the front wing will stall first because the combination of downwash and difference in planform (the back wing has a much lower AR) should both act to decrease the slope of the Co. Lift / AoA function for the back wing. I might build in 1 or 2 deg. decalage anyway, because I am basically a coward
Please comment - I am a beginner and a check on my reasoning would be welcome
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