Wright Stuff C

[coachchuckaahs]

Hello everyone,
I had a question about propellers. I know that the 8cm diameter is tiny, so I was thinking of modifying my propeller by adding more surface area to the propellers. However, I was wondering how much it would help my plane by increasing the surface area of the wing.

I think you mean “...increasing the surface area of the blade.”

Best way to determine if larger propeller blades helps is to build and try. That’s what my teams do. Have an idea? Build it and try it.

Brian T

I agree with coach Brian. We tested props Q, R, S, and T today. Every prop creates data. Not all of them are successful, but all data is good. We are starting to see trends in our data that will inform our next prop build session. While this year's plane is harder to trim, once it is trimmed it is all about optimizing the prop/rubber combination. With the small prop this year, there is little technical literature to guide the development, so the opportunities for empirical testing abound!

Coach Chuck

[lechassin]

One of the kits this year adds balsa sheet to a molded two-bladed plastic prop, and I respect that precedent for what it is... I also respect advice to maximize the scientific experience with empirical effort, but sometimes that advice predictably leads folks nowhere. Hopefully in the context of competition that is not the intention.

To the point: a two-bladed prop with huge blades is wasted effort.

Compare the long, thin wings of a sailplane to the short, wide wings of a fighter plane. The fighter's wing is maneuverable and strong but the airfoil has inherently more drag. The glider is not as maneuverable and is more frail, but drag is greatly reduced by the high aspect ratio. See here: https://en.wikipedia.org/wiki/Lift-indu ... duced_drag.

To summarize: for any airfoil, wing or prop, use the largest span and smallest chord you can. In the case of this year's props, do not enlarge the blade chord. If you want more surface area (which IMO is a great idea), keep the small chord and add a blade: https://www.youtube.com/watch?v=vmB4-4IBuXU&t=50s

[coachchuckaahs]

One of the kits this year adds balsa sheet to a molded two-bladed plastic prop, and I respect that precedent for what it is... I also respect advice to maximize the scientific experience with empirical effort, but sometimes that advice predictably leads folks nowhere. Hopefully in the context of competition that is not the intention.

To the point: a two-bladed prop with huge blades is wasted effort.

Compare the long, thin wings of a sailplane to the short, wide wings of a fighter plane. The fighter's wing is maneuverable and strong but the airfoil has inherently more drag. The glider is not as maneuverable and is more frail, but drag is greatly reduced by the high aspect ratio. See here: https://en.wikipedia.org/wiki/Lift-indu ... duced_drag.

To summarize: for any airfoil, wing or prop, use the largest span and smallest chord you can. In the case of this year's props, do not enlarge the blade chord. If you want more surface area (which IMO is a great idea), keep the small chord and add a blade: https://www.youtube.com/watch?v=vmB4-4IBuXU&t=50s

My point was not competition based, but rather that we are still early in our experimentation. We have tried a number of ideas, some subtle, some quite radical, and have not arrived at our approach yet.

The issue I see with a long span blade is that the small diameter means a huge difference in velocity over the blade from center to tip, something a high aspect glider wing does not see. Therefore, I think it unwise to limit the range of experiments at this early stage. Multiple props can be built in an hour, so don't limit yourself to a particular platform.

We cut down some props from several years ago. There was not much left of them. The results, though, were surprising. Not as good as our best, but not bad either.

My ornery point is that this is such a departure from normal that there is not a wide body of supporting knowledge. An empirical approach may still be best.

Coach Chuck

[lechassin]

One could argue for large chord blades based on the small scale of the props: https://en.wikipedia.org/wiki/Reynolds_ ... d_airfoils, but I don't think this could be used in any way to overcome the added drag.

We'll see!

[OpticsNerd]

One could argue for large chord blades based on the small scale of the props: https://en.wikipedia.org/wiki/Reynolds_ ... d_airfoils, but I don't think this could be used in any way to overcome the added drag.

We'll see!

Speaking of 2 bladed props this year with large chords:

I tried it last week and my times did improve. Still had a ton of rubber optimization to do so that I could actually wind to breaking torque without hitting the ceiling but it was flying 1:30+ in a ~21 ft gym.

[newflight]

percentage wise, how much did you improve with the bigger blade?

[lechassin]

At some point it might be helpful to define what we mean by "large chord". Even my blades have a relatively large chord, just not like this: https://www.youtube.com/watch?v=Ov7YEAPyefw (prop is at 31 minutes)

[OpticsNerd]

At some point it might be helpful to define what we mean by "large chord". Even my blades have a relatively large chord, just not like this: https://www.youtube.com/watch?v=Ov7YEAPyefw (prop is at 31 minutes)

"Large" in my case means bigger than the prop suggested in the Freedom Flight kit

[jsegal]

When making lechassin's tri blade propeller what angle is the triangular block that sets the angle of each blade. Is it 30 degrees?
Thanks

[lechassin]

Yes, 30 degrees. We used a drafting triangle to draw that shim you see on the jig.

I suggest a single serving Propel water bottle for the blades, it has just the right amount of flex (flare).