Propellers

erbach
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Re: Propellers

Postby erbach » Fri Mar 19, 2010 4:11 pm

duckiegirl2 wrote: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 » Fri Mar 19, 2010 7:18 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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calgoddard
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Re-pitching Propellers

Postby calgoddard » Sat Mar 20, 2010 12:27 am

StampingKid wrote: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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blue cobra
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Re: Re-pitching Propellers

Postby blue cobra » Sat Mar 20, 2010 1:25 am

calgoddard wrote:<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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Re: Propellers

Postby Greg Doe » Sat Mar 20, 2010 6:24 am

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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Re: Propellers

Postby StampingKid » Sat Mar 20, 2010 6:00 pm

What I learned from that post was the 1.8 ratio. I don't think I had heard that before. I can get that with the std ikara. I though have been looking at the broad blade as a way to get full motor flights in a small gym. I am worried about beating my plane to death in the rafters or worse bouncing down wall. I did though get some good flights in last night with the std blade winding to 80 percent. But I did have the weirdest accident-winding ring broke and motor snapped back and sheared prop spar. Oops. Need to make sure now I have two props of same weight and pitch.
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Re: Propellors

Postby illusionist » Sat Mar 20, 2010 11:17 pm

gyourkoshaven wrote:Do propellors have to be tilted upwards for the plane to get lift?

not nessacarily, thats the job of your wings
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Re: Propellers

Postby calgoddard » Sun Mar 21, 2010 5:17 pm

Blue Cobra:

Please re-read my post. I said:

"Some suppliers and kits will give you a range of rubber widths you can try."

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Re: Propellers

Postby illusionist » Mon Aug 30, 2010 10:25 pm

Jeff, how do you actually make your own thrust bearings (like a Harlan Penny Plane thrust bearing)?
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