# On the importance of weight.

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by bifft, Jul 14, 2019.

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1. Jul 16, 2019

### pictsidhe

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Part 103 does not demand low power and high drag. It does have weight and speed restrictions. You could have a slippery 1000hp monster as long as it stalls power off below 24kts, maxes out at 55kts and is below 254lb. A airspeed sensitive governer is an acceptable alternative to massive drag with a powerful motor. My appendix permitted power will be around 15hp. It'll have around 40hp and a governer. Great climb, slippery, decent range but still legal. i haven't looked to see if it will pul a loop, I'm just trying to make it plenty of fun.

2. Jul 16, 2019

### rdj

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For Part 103 AND aerobatic, it seems to me something absolutely minimal but hi-tech is about the only option. Something like a Breezy made from carbon fiber or rods. Aluminum and steel weigh too much.

3. Jul 16, 2019

### pictsidhe

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My monoplane will have a 21% 13" thick root. 7075 spar caps save several lbs over 6061 and not too much extra \$. I pondered carbon, but I want folding wings and all the hardpoints make it complex. The depth helps a lot with strength. I am optimistically designing to 6g. If it looks like that can't meet weight, then I'll scowl and prune my targets down... A moderately tapered wing with a very thick root airfoil seems the way to go for 103 aerobatics. Perhaps a 24% root. have a look at TOWS, the drag penalty is not too bad. I'm probably going to use Zero style spar caps, no skin on top of them. Every little extra spar depth helps! I'm not using fabric though, which is so thin that it would be silly not to just stick it to the top of the caps.

4. Jul 17, 2019

### bifft

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Just for grins I tried the biplane as if I could put the O-100 from that other thread in it. With 58hp it lifts me just fine. Doesn't leave much weight for the structure tho.

I also used the same fuselage and made it one big 30x4 wing instead of spliting it into two. Then it easily lifts me on 40hp. Halves the (probably overestimated as noted upthread) roll rate. Loops became easy.

But with twice the span it would make the structure much heavier. How much? That will take learning how to do that math. Got the books, need the time.

Also loses the cute as a bug look (at least to me).

I'll have to run the math on a part 103 70% Ki-84 next, while I'm at it.

5. Jul 17, 2019

### flyboy2160

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Are you going to get custom extrusions or machine down stock angles? Did you get quotes for the extrusions? I've gone back and forth on this on my plane.

6. Jul 17, 2019

### pictsidhe

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I'll machine rectangular stock down, it's a wee bit cheaper. I've had good luck machining aluminium with woodwork routers in the past...

7. Jul 17, 2019

### BJC

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It looks like you are using a symmetric airfoil. If that is the case, you can change to a cambered airfoil and have a better chance of meeting the part 103 stall speed criteria, and improve your targeted aerobatic capability too.

BJC

8. Jul 17, 2019

### pictsidhe

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An aerobatic Ki-84? Please do, I look forward to our dogfight!

9. Jul 17, 2019

### TFF

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The Polecat is supposed to be 103 compliant but I don’t know if it was. The info from the Biplane Forum. Built in ‘84. It was flying at 550 gross and added BRS for the legal weight boost. One engine it had was a Yamaha 750 turbo air cooled motorcycle engine. 100” prop 5-1 gearbox. They were trying to get to get to one to one thrust weight. I think they were flying Intermediate schedule with it. Pitch authority was so fast that they shattered a prop practicing snaps. Bonded aluminum wing parts.

10. Jul 23, 2019

### bifft

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Had a busy week, so only now getting back to this. To get my "Ki-84" 103-7 compliant I had to put a "drag strut" between the wheels. Also moved the cockpit forward to be closer to the CG. (I expect that will be needed for when the pilot weighs the same as the plane). Makes it look more like a Ki-43 than an 84. Close enough? (still Nakajima and IJAAF) (more of a temporal match for a Hurricane). Also flies good with 255 lb pilot and full fuel on 40hp. 27 ft span. Doesn't climb quite as well as the 30 foot span parasol. Could have "on the importance of span" as another title for this thread. Interesting that the biplane has the same span loading, but I guess the extra pair of wingtip vortexes makes a difference?

On the symmetrical airfoil, I actually put it in as a 4415, but x-plane displays it this way.

In my calcs I've been using 12.5% thick for the biplane (longest span between bracing 5'4"), 15% thick for the parasol (10' to the strut) and 18% thick for the cantilever wing (12.5' semi-span) .

If I were to actually build one of these I'm torn between the parasol and the "warbird". High wing would be better if you wanted to go into the back country, low wing is better visibility for acro. Would also be easier to get into the cockpit if needs to be right on the 25% chord position. Hard to squeeze in between those struts. Could go high-wing instead of parasol.

11. Jul 23, 2019

### bifft

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Did just read up on that Polecat. More or less exactly what I was thinking of. Great to know it isn't impossible.

12. Jul 23, 2019

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