--- In Indoor_Construction_at_yahoogroups.com, Tapio Linkosalo
<tapio.linkosalo_at_...> wrote:
>
> Sorry to get off-topic, but this discussion is really
> getting intersting to me...
>
> Does the above apply also to outdoor rubber props,
> where the pitch change is typically around 12 degrees,
> and high pitch is similarly used to slow down the
> climb speed and slightly increase the climb duration?
I've never flown with a VP prop. But from my understanding, its main
function in indoor FF is to reduce the climb rate during the power
burst. This then allows you to launch with more energy in the rubber
without slamming into a low ceiling. Even though the prop is very
inefficient in the high-pitch mode, the extra stored energy more than
makes up for it.
In outdoor FF you always launch with the same maximum energy, so
controlling the climb rate makes less sense. The *only* reason for a
VP prop in this case is perhaps to reduce the airframe-drag energy
losses during the climb. But if slowing down the airplane during the
burst causes the blades to stall (I'll bet that they do stall), then
the net overall energy savings are unclear. Your observation that a
VP prop on an F1B does not improve duration does not surprise me.
It's useful to consider an idealized flight profile where the
propeller is always at or near its maximum efficiency point. The
following is then true throughout the torque curve:
1) Blade cl is constant
2) Diameter is constant
3) Advance pitch is constant (distance flown per revolution).
4) RPM is proportional to torque^0.5
5) Velocity is proportional to torque^0.5
6) Thrust is proportional to torque
From a propulsion viewpoint, this flight profile is optimal. Its
*only* drawback is that the velocity must be rather large during the
torque burst, which increases the airframe drag loss. This is made
worse by the fact that during the torque burst the wing's CL is nearly
zero, where the typical undercambered F1B section has rather high drag.
I don't know if people have looked into this, but using somewhat lower
camber airfoils on an F1B might make sense to reduce the burst drag
loss. This approach would also reduce any benefit of a VP prop (if
there is a benefit). Yes, the glide sink rate will increase a bit,
but you will certainly get a higher climb because the prop efficiency
during the burst will be better. As is clear with the lower-camber
F1C airfoils, there is clearly a tradeoff between climb and glide. An
optimum tradeoff point must exist for F1B, but it's not obvious where
this is. It's surely not at the high-camber extreme of min sink at
the expense of climb.
Received on Tue Aug 29 2006 - 13:03:41 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET