Optimal Plane Dimensions & Flight Times

[AdithA]

Hi all,

I'm currently competing in Division C in the SoCal Area. Based on the histograms of the BirdSO invitational at USC, I noticed that the winning flight time was 2:20. Is this a similar time that others are getting?

I've built a plane, but I am worried that I am losing out on flight time for having too big of a chord. Is there a certain point where it is possible to have too much chord? However, I did confirm with Coach Chuck that there is no point where you can have too much wing span.

Also, my planes are currently building out to 7g. I'm considering building a balsa propeller, but I need to build it before my first competition which is February 3rd. I'm questioning if it will be of much value for a division C flyer. If it will bring significant flight times, what design do you suggest that I use, and what material do you suggest I use to strengthen the spar as I suffered from that severely while experimenting with it last year?

Thank you,
Adith Alagiri

[coachchuckaahs]

Adith:

On the C div planes, the short tail moment means that a wider chord starts to eat into useable tail moment, and thus the plane may become harder to trim. Also, as chord increases, and the tips remain square, the tip drag may increase. Where does this become an issue? Hard to say, I think you would have to explore this experimentally. Clearly more chord means more wing area, which should improve the flight. However, secondary effects such as tip length and tail moment come into play with the box restrictions. This is a big part of the experimentation.

On B div, we tried going from 5" chord (last year) to 6" chord this year. While B div does not have the tail moment issues to the same extent, at this point we have not yet seen improvement in the duration, We are still exploring and comparing this design to last year's design. This is with tip dihedral, not tip plates. But, we could not tell this until we built the plane. Once we have it optimized, we may see some improvement, but it does not appear it will be dramatic, if at all.

We exclusively use balsa props, mainly because we can get a correct pitch angle progression compared to the Ikara props. I have heard of one team re-forming the Ikara prop blade on a prop block with heat to get a proper pitch angle progression. Is it worth it? The Ikara props are not bad at all. The wood props add more variables to worry about, track, and adjust, especially if flaring. The flaring wood props are probably easier to get the desired flaring amount than an Ikara prop. But that adds another 2-3 variables!

Getting a homemade prop perfected and matched to your plane and rubber may have advantages, but will take significant test time that may be better spent on your basic plane trim and rubber/prop matching. In past years in Albuquerque we used a basic prop at Regionals and States, but then spent hundreds of flights on prop development and testing before Nationals. Most years we built 25 or more props.

We generally use a scaled Ikara prop blade planform, just from the spar to leading edge, based on the flaring prop. I use the same for LPP. We use 1/16" square basswood for the spar, and sand one dimension to about 0.045" on each side to allow more twist. We tune this thickness to get the twist we need. For SO we use 1/32 balsa, 6-8 pound density, for the blades. Careful selection of grain and change the flexibility of the blades themselves, but I prefer C-grain (stiff) blades and allow the spar to do the flaring.

The wood spars are more susceptible to damage. However, if you are starting with decent trim the plane is flying slow enough that even girder hits should not break the spar. Definitely do not want rough handling, and you should have a spare identical prop, but we simply have not had prop damage issues. Last year our plane tangled in a hanging wire and hung from the prop. While the spar did not break, we discovered the spar to blade joint became partially loose. Inspection after potentially damaging incidents is critical.

Coach Chuck