Wright Stuff C

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Re: Wright Stuff C

Post by bjt4888 » July 8th, 2020, 9:28 am

I completely agree with Xiangyu, Crayola and Jasp, reading several years of these forums will only take a few hours and will give a solid base of understanding of the event.

This should make for a good “Pandemic Project”. Also agree that it’s helpful and fun to build something to experiment with while reading (maybe, possibly a “Scraps” kit from J&H Aerospace that can be flown 1:30 - 2:30+ in your living room).

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AkshayB (July 8th, 2020, 1:56 pm)

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Re: Wright Stuff C

Post by AkshayB » July 9th, 2020, 11:46 am

Ok, looks like I'm going to have a lot of questions :)

First off, is the propeller pitch essentially the angle of attack of the propeller blades? So on a Wright Stuff plane, you would have a fixed propeller pitch?

Also what would be benefits/consequences of increasing/decreasing your propeller length. I understand how the tips of a propeller rotate at a faster rpm than at the hub. Would that mean longer propellers would require a lower propeller pitch because the tips would be spinning faster?
Last edited by AkshayB on July 9th, 2020, 11:50 am, edited 1 time in total.

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Re: Wright Stuff C

Post by bjt4888 » July 9th, 2020, 5:43 pm

Good to have a lot of questions. Angle of attack is relative to airflow.

A good primer on aerodynamics from NASA is https://www.grc.nasa.gov/www/k-12/airplane/bga.html

Explains concepts like angle of attack and includes some helpful graphics.

Larger diameter propeller primary benefit is efficiency. See affect of aspect ratio on wing efficiency. And, no, larger diameter does not necessarily mean pitch should be less. Study designs like AMA Limited Penny Plane (12” diameter and 20” pitch is typical; many other examples). Optimal pitch and pitch distribution for a particular design is determined by experimentation.

Good job. Study, study, study and keep asking questions.

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Re: Wright Stuff C

Post by CrayolaCrayon » July 10th, 2020, 10:40 am

You'll definitely want to max your diameter to the 8 cm limit :P It's tiny enough as it is :(
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Re: Wright Stuff C

Post by xiangyu » July 11th, 2020, 7:54 am

AkshayB wrote:
July 9th, 2020, 11:46 am
Ok, looks like I'm going to have a lot of questions :)

First off, is the propeller pitch essentially the angle of attack of the propeller blades? So on a Wright Stuff plane, you would have a fixed propeller pitch?

Also what would be benefits/consequences of increasing/decreasing your propeller length. I understand how the tips of a propeller rotate at a faster rpm than at the hub. Would that mean longer propellers would require a lower propeller pitch because the tips would be spinning faster?
Hello.

Propellers are essentially rotating wings. When a wing moves forward it generates lift, right? Propellers generate "horizontal lift" by rotating. In theory, a variable pitch propeller would be the most efficient, but the WS rules prohibit the use of variable pitch propellers so you don't have to worry about that part.

Like others have said, you want to maximize your propeller diameter as it will be more efficient. As for longer propellers & lower pitch, if you look at propeller designs, you can see that most have a "twist" and the tips will be lower pitch naturally compared to the hub. This is to address that. The pitch your propeller needs will be determined by testing and data. (I suggest start by using Ikara propellers and playing around with the pitch, ultimately you want to match the pitch to rubber to get the most efficiency)

Xiangyu
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Re: Wright Stuff C

Post by AkshayB » July 13th, 2020, 6:59 am

So what exactly is torque and how does the number of winds/the width of the rubber band affect the torque?

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Re: Wright Stuff C

Post by CrayolaCrayon » July 14th, 2020, 6:18 am

Torque is the rotational energy exerted by the rubber band, a metric to measure the power the rubber band can exert; More winds = Higher torque. Thicker rubber can take more torque than thinner rubber.

There is a graph called a torque curve; beyond a certain point, for every wind you add, the torque will start to jump up tremendously- that is how you know you are close to the breaking point.
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Re: Wright Stuff C

Post by coachchuckaahs » July 20th, 2020, 7:27 pm

I'll get a little more detailed than Andrew (Crayola) did, and correct a few terms.

Torque is the "rotational force" provided by the rubber band, to the propeller. You can also measure this rotational force while winding. It is how hard you twist the rubber, or how hard it twists back.

Power is a measure of how this torque is released to produce a positive effect, in this case propelling the plane. Power released by the rubber band is torque times the rotational velocity (RPM). Power to fly the airplane is linear (force to push the plane forward, times velocity of the plane).

Energy is the integrated amount of power stored in the rubber band, or released to move the plane. Conversely, power is how quickly the energy is released or used.

The total energy stored has a direct relationship to the duration of the flight. The power, or rate at which the energy is released, will determine whether the plane climbs or descends, given a reasonable trim condition of the airplane. Torque is an intermediate static value that we can measure, involved in the conversion from stored energy to power to the propeller.

In terms of a rubber powered plane, the Energy stored in the rubber is proportional to the torque and the number of winds. So, more winds means more energy. Higher torque, for a given propeller, means more power because the prop will turn faster (and power is torque times RPM, so power goes up quickly with torque and a fixed pitch prop).

Wider rubber will provide higher torque for a given number of winds, but also will not take as many turns. The maximum energy stored in the rubber will be roughly proportional to the rubber mass. But too wide a piece of rubber will release the energy quickly (more power), climbing quickly and shortening the flight. Too thin, and you will not be able to generate enough power to climb, but the prop will turn much longer (assuming rubber is wound to its full potential). More energy storage requires more rubber mass, which means more lifting force is needed, which also (for a given plane increases drag), and thus increases the power needed to climb.

So, for a given plane, trim condition, ceiling height, etc., there will be a particular optimal rubber mass, width, and launch torque to maximize the flight time.

Hope this helps! Glad to see you asking questions in the summer. Enjoy learning the nuances of indoor freeflight modeling!

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xiangyu (July 21st, 2020, 12:34 pm)
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