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scaling down a defiant. What am I missing?


mcjon77

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Hi guys,

 

I have heard it mentioned before about building a smaller scale defiant, with Rotax or Jabiru engines. I am considering building a .91 scale defiant (.91Width*.91Length*.91Height = .75 total size and weight). Right now it is just in the pipe dream stage (still working on building my Sonex). The idea keeps sticking in my mind, and I want to know if there is something I am missing that would make this project less doable than I think. Essentially, I am wondering what the gotchas are of my plan.

 

Before starting I would tranfer all of the plans into CAD to scale them to the proper sizing. The deminsions (Length, Width, Height) of the plane would be 91% of the original defiant. This would create a plane that is .75% of the original defiant's size and weight. I would use Jabiru 3300 (120-127 hp) engines with electric constant speed props. Even with the props, the engines will be 75% or less of the weight of the original 0-320s .

 

In the end, the plane would have the same or better wing loading and power loading as the original defiant. Also all of the scaling down would be in equal proportions, so I cannot see how it would significantly change the aerodynamics of the plane. The biggest gotcha I see is that the defiant was designed for the lycoming engine, and I would have to not only design my own mounts, I probably also have to modify the baffling system for the engine. That doesn't seem so insurmountable.

 

Am I missing something as to why this would not be doable?

 

Jon McDonald

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> .91Width*.91Length*.91Height = .75 total size and weight

 

Weight doesn't scale unless you use glass and foam and other building materials that are only 91% the weight of the original. Which you can't, because the strenght requirements for many parts will still be the same as the original.

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Many of Burt's prototypes were 75% models. The original defiant was slightly smaller than the plans built. It still required power, two 150 hp engines to fly and several modifications in engines and props were made over the years it flew. They resulted in better climb performance but not much in speed as I recall. S

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One thing would be to ask whether you should scale the weight by volume or by surface area. Surface area should be squared, and volume cubed (to see why this should be so, imagine a 1cm cube, and a 2cm cube. The first will have an area of 6cm^2 and a volume of 1cm^3. The second will have an area of 24cm^2 and 8cm^3).

 

Some components of the aircraft will scale with volume - such as the foam cores - but others will scale with the surface area. This is likely to be the case for fiberglass because you won't generally be able to reduce the thickness of (e.g.) 3 ply by 10%, and the same will probably be true for foam sheets and plywood used to make bulkheads. Likewise, the instruments and some other components (seatbelts) etc. will probably weigh the same as for the full-size aircraft.

 

What this means for you is that although you are reducing the volume by 25% you are only reducing the area by 18% and you may end up weighing considerably more than your calculations would suggest. You might be able to ameliorate this with different materials - thinner fiberglass etc.

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Here is a quick drawing I did, in another forum section. It is effectively a Cozy with twin Jabiru 3300s. It's just a quick drawing with no engineering thought at all.

 

http://www.canardzone.com/forum/attachment.php?attachmentid=2331&d=1232196898

 

Personally, I think with reduced scale, Jabirus, fixed gear and some other lightening mods you should easily get to 75% weight of the Defiant. When you consider the Defiant empty weights around 1600lb, Cozy around 1200lb that 75%. And the two Jabiru engines weigh about as much as one lyco 360 in the Cozy. If you can't get your empty weight around 1300lb I'd be very suprised.

 

Again, no real engineering calcs done but you cant be far off beat. Because of the much reduced gross (less fuel and lower empty weigh), you could theoretically reduce structural members to values like the Cozy.

 

http://www.canardzone.com/forum/attachment.php?attachmentid=2331&d=1232196898

Adrian Smart

Cozy IV #1453

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Adrian,

 

I remember your post from before, but I couldn't find it. As you did, I also compared the numbers of a 75% defiant with that of a cozy and came to the same conclusion. The wing loading seams lower than the cozy as well. Also, the new Jabirus are actually producing closer to 127hp. In addition, 2 Jabiru 3300s with constant speed props still weigh the same or less than 1 IO-540 with prop that have been put on several cozys already.

 

I remember a Long EZ with twin Jabiru 2200s on the back (Mike Bowden's?) that was supposedly an underperformer. It seemed to suffer from the same problems that Chris Heintz's Zeniar Gemini did. In the case of the Zenair Gemini, Hientz found that he could not maintain enough altitude on a single engine. However, back when both of these twins were designed, they were using fixed pitch props, which could not be feathered. Furthermore, the early 2200s are suspected (at least among the Sonex guys who have flown them) of not producing the rated 80hp.

 

Now there are at least 3 companies that make featherable propellers for the Jabiru and rotax line of engines. The adjustable props weigh 15-25lbs. Combine that with the 120-127hp Jabiru 3300 and I think we have a winner.:)

 

The most obvious question is why go for a twin. I understand the pitfalls to a twin vs. single. I spend a lot of time in Central America, mostly Costa Rica. A twin would allow me to confidently fly over water and turn a 2-3 day trip into a 1 day trip. The overland route is 50% longer than the over water route. The only singles that I know which could make the trip in 1 day are the Glasair III, the Lancair Legacy, and the Lancair IV. However, if I can get even close to the reported performance numbers of the defiant, I can make the trip in 1 day. Also, the plane would be cool.:D

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One word....fuel.

Can you be a little more vague please?:D

 

If you are talking about fuel capacity, my estimate is that a twin of this size would be able to carry between 82-88 gal. fuel burn should be around 13-13.5 gph at 75% power for the two engines. Even taking the low end of the fuel capacity with the high end of the fuel burn, we're still looking at 6 hours. Minus 1 hour for reserves, leaves us with 5 hours of fuel. What was the point you were trying to make about fuel?

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