That brings up another variation of the VP question. If you've established the best possible prop shape and pitch for the cruise portion of an indoor flight, I've wondered if using what you might call a "reverse VP" would be of benefit in high ceilings like Lakehurst. Or maybe a more simple method, a flaring prop that flares to a lower pitch during the power burst. I think most of us agree that you need to get as close to the ceiling as possible to do the best time. But if you have to use a heavier motor (less turns, faster run-down) or a lower prop pitch (higher than desired RPM for the bulk of the flight) then it seems you are losing turns at a faster rate than optimum. I realize that the trade-off is that with a lower initial pitch you'll burn more turns in the climb.
It seems like this line of reasoning is leading to the same set of problems as for the F1B flier. Maybe the best solution is what most everyone already does - use a fixed pitch symmetrical prop.
Last summer I was flying a normal flaring prop on my F1L at Lakehurst and trying to keep up with Dr. Larry. I was staying pretty close but ran out of motor stick strength (and launchability) at about 150 feet. Another 30' of climb would have made things a lot more interesting!
Local SciOly mentor Dave Ziegler has used reverse flaring props for years despite all my attempts to convince him it's a bad idea. Even with what seems like a backwards setup, his best kids are usually very competitive with my best kids.
----- Original Message -----
From: markdrela
To: Indoor_Construction_at_yahoogroups.com
Sent: Tuesday, August 29, 2006 4:01 PM
Subject: [Indoor_Construction] Re: Prop Efficiency
--- 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.
[Non-text portions of this message have been removed]
Received on Tue Aug 29 2006 - 15:00:23 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET