Prop efficiency at high pitch (was: News from Romania)
On Sun, 18 Mar 2007, Fred Tellier wrote:
> One of the biggest problems in flying the mine is getting enough climb
> without burning off all the turns while getting to the top. A lot of
> flyers at the world champs got great climbs due to high climbing rpm but
> not real long times because they wasted turns getting there. The trick
> is to get the model to climb with a lower RPM so that there is turns
> remaining for a long cruise. I used a flaring prop for that purpose but
> did not quite get it right, I think a little more launch torque was
This is really interesting, and is related to a further question about
prop efficiency.
As a background, VP props have been the norm also in F1B for the past 10
years or so. The common understanding is that they add 10 to 20% to the
climb altitude. In recent years, however, the appearance of altitude
loggers have questioned this (while also showing how prone to error the
evaluation of model performance by the eye is). For example, in my own
tests, models with a fixed pitch prop climb about the same as ones with
VP. Some discussion about this thing has suggested, that even though the
models with VP make the initial climb at lower speed, thus wasting less
energy to drag, the increased pitch also reduces prop efficiency, which
counteraffects the benefit from a slower climb. Thus all the gain you get
from VP is possibly 5 to 10 seconds more time the climb lasts.
This seems to come quite close to Fred's observation, that in the salt
mine the question is not how to get to the ceiling, but how to get there
with the smallest number of turns used. Sounds to me that this is
precisely the same thing, how to keep the prop efficiency up at the high
pitch setting. Am I right if I speculate from these messages, that a
flaring prop might have a better efficiency at high pitch than a VP? If
so, why?
-Tapio-
Received on Mon Mar 19 2007 - 04:44:45 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:45 CET