Aspect ratio for performance with certain limitations

[HenryS]

This isn't a beginner question as much as a miscellaneous question, as it applies to free flight which I couldn't find a section for and FT forum is the only slightly-aeronautical forum I know.

I'm not a complete beginner to flying planes, but I'm not experienced by any means. I can probably fly a trainer, but anything outside the realm of "well tuned semi-hands off slow-medium plane" is out of my grasp

So basically, I work on a little bit of indoor rubber free flight in a school competition club, and I was wondering how my aspect ratio would affect my performance. Rules limit my wingspan and my thrust(rubber), so I was wondering if I increase my wing chord will I be better or worse. Basically, The rubber motor basically provides limited total impulse, and I can control my wing dimensions. I am probably not gonna mess with airfoil, but it is a single layer of film in an undercambered shape over a balsa frame. When I increase wing chord, there will be more drag and wing vortexes, but also I will have more wing area. Based on my limited thrust, should I try for as narrow a wing possible or as wide a wing as possible? By the way, the goal is to get as much airtime as possible.

Thanks in advance!

 

[perhapsleiana]

Wingtip vortices can be controlled with wingtip devices, especially in the trailing half of the wing, and overall, increasing the chord will increase wing area, which will let the plane stay aloft at slower speeds, and in turn reducing the amount of power needed to keep it aloft (same amount of force but less power).

 

[telnar1236]

This isn't a beginner question as much as a miscellaneous question, as it applies to free flight which I couldn't find a section for and FT forum is the only slightly-aeronautical forum I know.

I'm not a complete beginner to flying planes, but I'm not experienced by any means. I can probably fly a trainer, but anything outside the realm of "well tuned semi-hands off slow-medium plane" is out of my grasp

So basically, I work on a little bit of indoor rubber free flight in a school competition club, and I was wondering how my aspect ratio would affect my performance. Rules limit my wingspan and my thrust(rubber), so I was wondering if I increase my wing chord will I be better or worse. Basically, The rubber motor basically provides limited total impulse, and I can control my wing dimensions. I am probably not gonna mess with airfoil, but it is a single layer of film in an undercambered shape over a balsa frame. When I increase wing chord, there will be more drag and wing vortexes, but also I will have more wing area. Based on my limited thrust, should I try for as narrow a wing possible or as wide a wing as possible? By the way, the goal is to get as much airtime as possible.

Thanks in advance!

So that I'm sure I understand, you're looking for the longest flight times and not the longest range, right? In that case, you're going to want to minimize sink rate. A higher aspect ratio wing will indeed have less drag, but like you're thinking it will have less lift too, so fly faster. While you will likely have a less steep glide slope, at a certain point, your aircraft's speed will cancel that out, so you will want to balance between area and drag reduction.
For aircraft with a high L/D sink rate is pretty closely approximated by:
Usink = U * 1/(L/D)
The relationship between aspect ratio and L/D is way harder to figure out but will depend on the shape of the fuselage and the taper of the wing, as well as any wingtip devices as Perhapsleiana mentions. However, because of parasite drag from the fuselage and any other flying surfaces, there will be diminishing returns beyond a mid-aspect ratio wing when it comes to L/D anyway, so I wouldn't recommend going too high unless you have very carefully removed every bit of drag you can everywhere else. A wing aspect ratio in the region of 7 to 10 will be plenty for most rubber band gliders and any higher is likely to lose lift with little benefit, but to narrow it down more would require a lot more work in terms of analysis. If you have the time and budget to build several hand launch gliders, careful testing is honestly the best approach.

 

[HenryS]

Wow, Thanks!
I will probably do some tests to find out what works best.

 

[HenryS]

increasing the chord will increase wing area, which will let the plane stay aloft at slower speeds, and in turn reducing the amount of power needed to keep it aloft (same amount of force but less power).

But doesn't increasing wing area mean increasing drag, which means less thrust won't be able to pull the aircraft along?

 

[Piotrsko]

But doesn't increasing wing area mean increasing drag, which means less thrust won't be able to pull the aircraft along?

Like @telnar1236 indicated: lots of related factors for increased drag some of which will be induced drag controlled by airspeed. More or less rule of thumb: increasing wing area allows  slower flight without stalling which reduces overall drag but only for higher aspect ratios. Short fat wings are less efficient even though the wing loading per square is the same...
Not going to get much vortices flying at walking speed with an indoor plane, structure drag and bracing will add more drag

 

[HenryS]

Ok thanks!