Propellers

jcollier
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Re: Propellers

Postby jcollier » February 25th, 2010, 5:27 am

Can anyone help with pitch? I don't know how what to change it to in relation to the number of turns and the rubber band size. Is it a big factor on how the plane flies?
My first ? for you is "How many props do you have to play with?" I have a half dozen props that our High School coach gave us that HS students attempted to repitch. They are all out of balance and basically worthless. You CAN repitch props if you make a gauge and are very, very careful. You have to be sure you are only twisting the base of the prop and not pulling it forward or backward, then get both sides EXACTLY the same pitch. IMHO, it is much, much easier to match the rubber size to a prop that has not been modified, especially for a 12-15yr. old builder/flyer. I don't doubt that repitching a prop can be a small advantage at times, but I'd be very surprised if it was significant, or could not be almost duplicated by doing something else.

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eta150
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Re: Propellers

Postby eta150 » March 4th, 2010, 1:01 pm

Is it possible to change the camber of ikara prop plastic
(Note, this is for the Phys. science lab event, and we are using the sheets with toothpicks that you buy off of the freedom flight website.)
Thanks!
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Re: Propellers

Postby StampingKid » March 4th, 2010, 1:25 pm

Thought about that for PSL but did not know whether it would "fly" . Found in sanding Ikaras that to much pressure deformed the plastic. Don't know about camber. But let us know how it goes. What type of #'s are you getting. I had a blade that I got 1800 Fast and 1200 Slow on a Walmart box paying attention to the earlier caveats on that thread but it broke and I could not get it to repeat .
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eta150
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Re: Propellers

Postby eta150 » March 4th, 2010, 1:44 pm

The blades are doing pretty well (not quite sure of the efficiency, I'm doing this for a teammate), but we figured we should try Ikara plastic. We bought it, and there was WAY to much camber, even for experimenting with free flight blades (which is what they're for). I still see more merit in a balsa design, but I think this could be worth a shot if we can get it right.
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Re: Propellers

Postby StampingKid » March 18th, 2010, 7:01 pm

What is a good angle to depitch the ikara broad blade to? I need to take care not to break the spar.
I WILL RETURN TO PHILMONT IN JULY!
07 Reg 1st BLG, 3rd WV.
08 Reg 1st Twr, 2nd BLG
State 1st Twr
09 Reg 1st WS, PSL and Crave the Wave, 2nd Robo-X, EB
State 1st EB, 3rd WS
10 Reg 1st EB, PSL, 2nd WS, Disease Det., 3rd Traj.
State 1st EB, PSL, 2nd WS, 3rd Disease Det.

erbach
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Re: Propellers

Postby erbach » March 19th, 2010, 9:11 am

Is there a certain material that is lighter to use for the propellers? Is it important to get that wait down so it doesn't nose dive? Our plane has been very inconsistent, it has nose dived then the next flight got stuck it the rafters. The only thing different was the number of turns on the rubber band.
This is likely to be the phenomenon called "going over the hump." The usual source is a motor stick which is a bit too light. When a tightly wound motor bends it, the difference in angle between the wing and stabilizer is reduced, resulting in a "dive" profile. This is easy to check. If it's happening, build the next motor stick a little stiffer.

But solutions on a completed model aren't always easy. You can back off a few turns to reduce the initial torque, but that can lose a bit of time. You can point it up or bank left a bit more at launch. If this works, it will be fine, but the process is delicate and inconsistent. You can replace down thrust with left thrust.

But these are all attacking symptoms instead of source problems. The source is a design which doesn't have a good balance between the center-of-gravity location and stabilizer size. If these are kept to standard ranges (stabilizer about 30%-40% of wing area, and c.g. around 50% of wing chord), the problem isn't likely to be severe. Check your model to see if it's in these ranges.

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Re: Propellers

Postby jander14indoor » March 19th, 2010, 12:18 pm

Correct on the problem of bending motor sticks (or possibly the Ikara prop hanger), but not on cg location or tail size.

For Wright stuff planes, more common cg location is at rear post of the large chord wings, behind the rear post for small chord wings. Stabs more often approach 50% of wing area. Planes tend to be very long.

Jeff Anderson
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Re-pitching Propellers

Postby calgoddard » March 19th, 2010, 5:27 pm

What is a good angle to depitch the ikara broad blade to? I need to take care not to break the spar.
The optimum pitch of the propeller for an indoor endurance rubber powered airplane depends on many factors.

Indoor flyers usually talk about pitch-to-diameter (P/D) ratios for propellers. For Wright Stuff airplanes a P/D ratio of about 1.8 will yield excellent results.

If you keep the width of the rubber motor constant, increasing the pitch will cause it to spin at a slower rate and use up the turns more slowly. Decreasing the pitch will cause the propeller to spin faster and use up the turns quicker.

Re-pitching an Ikara prop is a difficult process, often leading to broken prop spars and unbalanced pitch.

It is far easier to vary the width of the rubber than to try to opimize your flight by re-pitching an Ikara propeller.

If your plane lands with too many unused turns, increase the width of the rubber.

If your plane lands dead stick, i.e. the turns are all used while the plane is still in the air, decrease the width of the rubber.

But changing rubber widths is not easy to do unless you have a stripper or a friend that will do the stripping. Proper use of a rubber stripper is an acquired skill, and of course, depends on the accurate and repeatable use of a thickness gauge, which itself is a tricky business because it is difficult to precisely measure the dimension of a soft deformable material like Tan Super Sport rubber.

Some suppliers and kits will give you a range of rubber widths you can try. But for Wright Stuff the longest duration in the air can usually only be achieved with the ability to vary rubber widths down to thousandths of an inch. For example going from .095 inches to .097 inches in the width of the rubber could produce an extra fifteen seconds in the air.

The best Wright Stuff planes have props that are balanced. They don't shudder and vibrate the tail and waste energy. Balance is achieved on several levels. The weight of each blade should be equal. If one blade spins down to the bottom when there is no motor attached and no wind, you know it is heavier than the other blade. People then trim the blade. This is a bad idea. Now the blades may have unequal area leading to assymetric thrust. Blades may have unequal pitch due to manufacturing tolerances. Checking the pitch of each blade requires a pitch gauge. Again if the pitch is unequal there will be an imbalance in the thrust and resulting vibration in flight. Then there is the problem of wobble in the prop bearing.

Experts can detect and fix all these problems in the propereller and its bearing but the fixes are complex and delicate. They can provide the winning edge. For example the weight of the blades can be equalized by adhering a suitable length strip of 1/2" wide 3M Scotch tape on the rear surface of the underweight blade.

My advice to novices is to switch Ikara props to one that exhibits the least vibration and then optimize the rubber width for that prop.

Good luck to all at your upcoming regional and state competitions.

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Re: Re-pitching Propellers

Postby blue cobra » March 19th, 2010, 6:25 pm

<SNIP>But changing rubber widths is not easy to do unless you have a stripper or a friend that will do the stripping.<SNIP>
Here I must disagree. Indoor Model Supply sells TSS stripped to sizes suitable for Wright Stuff every .005". You may not be able to get that last 15 seconds, but if you're not willing to master re-pitching a prop, I'd assume you're not quite going for that last few moments.

Everything else is spot on.

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Greg Doe
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Re: Propellers

Postby Greg Doe » March 19th, 2010, 11:24 pm

Excellent post by calgoddard. Some of the best advice to appear hear in a while! Too many people
think there is a magic bullet out there that if they just have the right prop, airfoil, rubber or some
other "secret" ingredient they will all of a sudden have a 4 minute WS airplane. There are many
concepts that need to be mastered before someone needs to start experimenting with the wide blade Ikara. That's not to say that it might not provide an edge. It could. But the standard equipment
can win. When I included rubber in my list of secret ingredients, I meant batches, NOT thickness.
One of the best ways to "tune" your airplane is with different size rubber, which either has to be
custom cut, or ordered, but even standard size rubber supplied in kits like Freedom Flights can
be VERY competitive. Again good advice. Read it over and over, and believe it.
Greg Doe,
Smyrna, TN


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