Design

Less_Incidence
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Re: Design

Post by Less_Incidence »

To the question about thin cross sections and turns left - I use .081 rubber and usually land with about a full row of knots in the motor - that's 150ish turns. I wind between 1800 and 2050 depending on how broken-in the motor is.

However, my plane's best flight was on a piece of .077 rubber (Basically thick pennyplane rubber) that I wound to something like 2400 turns and landed with about 400 left. I've found that using a thin piece of rubber and landing with lots of turns left has given me my best times - I switched from .094 to .087 and instantly improved by 30 seconds, then from .087 to .081 and instantly improved another 25. Different people have had different results though.
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Re: Design

Post by bernard »

With thinner rubber, my plane doesn't climb as high. Do you have any suggestions for improving that? Should I play again with prop pitch and incidence when I switch to thinner rubber?
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Re: Design

Post by Chris_L »

To the question about thin cross sections and turns left - I use .081 rubber and usually land with about a full row of knots in the motor - that's 150ish turns. I wind between 1800 and 2050 depending on how broken-in the motor is.

However, my plane's best flight was on a piece of .077 rubber (Basically thick pennyplane rubber) that I wound to something like 2400 turns and landed with about 400 left. I've found that using a thin piece of rubber and landing with lots of turns left has given me my best times - I switched from .094 to .087 and instantly improved by 30 seconds, then from .087 to .081 and instantly improved another 25. Different people have had different results though.
What are your flight times? and at what height?
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Less_Incidence
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Re: Design

Post by Less_Incidence »

Bernard:

I've found that prop pitch is the biggest factor in climb, much more so than wing incidence. I fly a prop with a very, very low effective pitch - it's a flaring Ikara with the small bit of material on the back of the blades cut off, and also sanded on a flat surface rather than on a round soda can so that the blades were flattened slightly in the sanding process. I don't have a pitch gauge so I couldn't give you a number (and Ikara props don't follow helical pitch anyways), but generally a lower pitch prop will provide a better climb with thinner rubber. The trade off is that a low-pitch prop will run at a higher RPM than an equivalent high-pitch prop, but I believe that the large blade area of the Ikara combined with the flaring property and the thin, low-torque rubber makes up for this.

On another note, I think I mentioned this earlier in this thread, but I made a modification to the Freedom Flight plane that involved making the tail boom drop downward so that the rudder sat about 2 1/4" below the motor stick. After I did that, I stopped having problems with the plane stalling on descent and it gained that nice efficient nose-up attitude throughout the whole flight that probably helped conserve turns on the thin rubber as well. Probably more a function of the slight change in tail incidence than of the stab height relative to the wing, but whatever I did definitely worked.

Chris_L:

The best flight my plane has done with .081 Tan SS was a 3:07 in a 22' gym, and that was a no-touch (barely). I made a strip of February 1999 Tan II rubber in .077 and the plane did 3:26 in the same gym. When I go to USIC in June and fly in the 35' auditorium there, I think I can get near four minutes. I really wish I could fly in the 60' site at nationals... Unfortunately my school's team didn't qualify. We never do... *sigh*
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Re: Design

Post by DoctaDave »

Do you know why dropping the tail prevents stalls on descents? just curious how what affects it.
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Re: Design

Post by Less_Incidence »

The theory behind the dropped tail boom is that the low stab height keeps the stabilizer away from the turbulent air coming off of the wing, specifically the induced vortexes from the tips and dihedral joints. This allows the lifting tail to do its job a bit better. Also, I think that when I first built the plane, the tail incidence was slightly positive, which probably contributed to the stalling. When I dropped the tail I made sure to give the stab about 1 degree negative incidence, which stopped the stalling and fixed the attitude in flight.
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Re: Design

Post by bonysteak »

Hey, I'm doing Wright Stuff for nationals. It's my first year in Science Olympiad. Last week I bought a freedom flight plane for the first time, because i used Osprey for regionals (didn't go to state -kind of a long story). My partner already bought a kit but broke one of them, and the one he built was working well but not nearly good enough to be competitive (best time was like 2 min). So stupid me thought I could build a plane that went better.


Aaaaaaaaand...

Well I wasted like $60 and tens of hours of my time. I went and tested it in my school gym today (large, circular, very high ceiling - probably like 30' no ac) and...well please don't judge but the plane wouldn't even fly for more than 10 seconds. I tried with all three types of rubber from the kit, I moved both the wing posts and the wing itself (not both at the same time). So quite literally I tried everything. The best it would do is fly smoothly but with no climb and it wouldn't even stay at the same level. At some point it even just dived straight to the ground. This is really retarded because partner and I medaled at regionals (and he medaled at state) I really don't know what to do, and any help would be greatly appreciated.

If this is of any help: I built the plane as exactly and precisely as I possibly can, and since (gonna brag a bit) I'm kind of good at building things it's almost perfect in terms of looks. I don't have any model cement so I used superglue. It has a 7 cm wing chord, 50 cm wing span, 6 cm stabilizer chord, etc. etc. The colored wing panel is already on it, I smeared about .05 grams worth of pink paint on it. I think the biggest problem is the mylar is kind of loose. It's not hanging off the plane or anything, it's only loose enough so that you can see it tremble ever so slightly if you shake the plane. Is that the problem? I tried 20-40 winds with all three rubber widths provided in the kit, moved the wing posts up (did absolutely nothing until it went so high that it started stalling/diving) moved wings forward, but nothing changed the flight pattern. Oh and the plane is kind of heavy - it's 8.5 grams with the propeller. The motors are all 1.92-1.98 grams.

Think I can make use of this plane? Thanks in advance!

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Re: Design

Post by DoctaDave »

20-40 winds simply isn't enough. If you're using a 10:1 winder, the thickest rubber that comes with the kit, .094, should be able to take at least 1500 turns, or 150 winds. Try going for 100 winds and see how that goes, and after that start playing with the torque and dewinding etc.
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Re: Design

Post by bonysteak »

Sorry, I wasn't being specific. I was only trimming, tonight was my first time taking the plane out for a ride. I didn't dare go more than 40 on a 15:1 if it wouldn't even work properly at such a low level.
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Re: Design

Post by DoctaDave »

if your plane still goes straight into the ground, have you checked to see if your motorstick is bending? or if you have enough wing washin on the left wing? Without wing washin, the plane will probably bank too much during the beginning of its flight, and thus it won't get enough lift, so it could just go into the ground or not climb at all. If your motor stick is bending, it could give your propellor some downthrust, which would also send it to the ground.
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