Wing aspect ratio vs. airspeed ?

[[email protected]]

Today I did some volunteer work, screening potential entries into a
high school science fair. Lots of interesting experiments, but one in
particular caught my eye:

The student had created an RC plane out of styrofoam, balsa wood and an
electrical pusher prop. Now the experiment consisted of clocking the
time it takes for said plane to cover a distance of 100 yards, but with
two different sets of wings. Both wing sets are planar triangles (no
camber) and have the same surface area. One however had a wider
wingspan than the other. Thus one configuration looked more like a
delta wing than the other.

Interestingly enough the result was that the delta wing with shorter
wingspan travelled more slowly than the other wing. I thought D-wings
are generally less stable but faster, no !?

[Capt. Geoffrey Thorpe]

> wrote in message
oups.com...
> Today I did some volunteer work, screening potential entries into a
> high school science fair. Lots of interesting experiments, but one in
> particular caught my eye:
>
> The student had created an RC plane out of styrofoam, balsa wood and an
> electrical pusher prop. Now the experiment consisted of clocking the
> time it takes for said plane to cover a distance of 100 yards, but with
> two different sets of wings. Both wing sets are planar triangles (no
> camber) and have the same surface area. One however had a wider
> wingspan than the other. Thus one configuration looked more like a
> delta wing than the other.
>
> Interestingly enough the result was that the delta wing with shorter
> wingspan travelled more slowly than the other wing. I thought D-wings
> are generally less stable but faster, no !?

Shorter span would typically have more induced drag so, for the same power
would fly slower. Another difference would be exactly where the Mean
Aerodynamic Chord and Center of Gravity aligned with each wing - one might
be trimmed up a little different which will change the speed at which it
flys. A little warp or flex in the wings (I would expect the two sets to
flex differently) would also affect the speed that it will fly with no other
trim. If the chord of one wing is significantly shorter than the other, then
there are Reynolds number effects.

I'd say with a little effort, you could make either fly faster.

--
Geoff
the sea hawk at wow way d0t com
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[Morgans]

> wrote in message
oups.com...
> Today I did some volunteer work, screening potential entries into a
> high school science fair. Lots of interesting experiments, but one in
> particular caught my eye:
>
> The student had created an RC plane out of styrofoam, balsa wood and an
> electrical pusher prop. Now the experiment consisted of clocking the
> time it takes for said plane to cover a distance of 100 yards, but with
> two different sets of wings. Both wing sets are planar triangles (no
> camber) and have the same surface area. One however had a wider
> wingspan than the other. Thus one configuration looked more like a
> delta wing than the other.
>
> Interestingly enough the result was that the delta wing with shorter
> wingspan travelled more slowly than the other wing. I thought D-wings
> are generally less stable but faster, no !?
>

There are too many factors that have to be considered, before you could tell
what is happening here. There is the question of if the delta's wing area
is appropriate for the weight, and the thrust is what would be appropriate
for that. If there was little thrust available, for example, the high
aspect ratio would be more efficient, and go further.
--
Jim in NC

[Peter Duniho]

"Capt. Geoffrey Thorpe" <The Sea Hawk at wow way d0t com> wrote in message
...
> Shorter span would typically have more induced drag so, for the same power
> would fly slower.

But it will have less parasitic drag.

It sounds as though, at least in this case, the increase in induced drag is
greater than the decrease in parasitic drag. But the result is by no means
applicable to all wings.

As you point out, there are other differences that are relevant as well.
It's hard to say, knowing only that one wing has greater wingspan than the
other, what accounts for the actual difference in total drag. One hopes
that the student with the science fair project has accounted for those
differences, but you never know. :)

Pete

[Denny]

Where are the Wright brothers when you need them...

denny

[[email protected]]

Thanks for the ideas. The student in question only compared total
travel time between the two wings. I suggested that he try out
throttling the motor to check performance in different flight regimes.
My hypothesis is that the increased length of the leading edge
increases parasitic drag and that at the airspeed he was using this
rather than induced drag was the limiting factor.

[Peter Duniho]

> wrote in message
oups.com...
> Thanks for the ideas. The student in question only compared total
> travel time between the two wings. I suggested that he try out
> throttling the motor to check performance in different flight regimes.
> My hypothesis is that the increased length of the leading edge
> increases parasitic drag and that at the airspeed he was using this
> rather than induced drag was the limiting factor.

Parasitic drag should be related to wing surface area, not solely to frontal
cross-section (ie increased length of the leading edge). Of all the
possible explanations, that seems the least compelling to me.

I agree that throttling the motor is important. If the student did not
control for the motor's power output, then the two trials are not
comparable.

I still agree with the issues raised by Geoff, the CG in particular. A
stable airplane configuration results from CG forward of the center of lift,
and a down-force aft of the center of lift (usually from some sort of
horizontal stabilizer and/or elevator). It's not clear from the information
provided how this test aircraft was designed, but unless the airplane was
100% unstable (CG and center of lift at the same position), the CG needs to
be taken into account (I would do so simply by ensuring that the CG and
center of lift are the same distance apart in both trials, resulting in the
same total lift and stabilizing downforce for both trials).

It should go without saying that unless both flights have perfectly level
flight paths, then any climb or descent must be accounted for as well (and
preferably eliminated so that a true comparison can be made).

As far as wing flex goes, again without knowing the actual design of the
airplane, it's not possible to comment specifically. But I would think that
on an airplane of that scale, it should be possible to construct wings that
have effectively no flex at all, eliminating that source of error for all
practical purposes.

The Reynolds number issue sounds like a red herring to me, but I suppose a
drastic enough difference in the wings could affect the trial.

Pete