Flying in turbulent conditions?

From: Tapio Linkosalo <tapio.linkosalo_at_iki.fi>
Date: Tue, 06 Oct 2009 12:46:50 +0300

We had our Finnish Championships last weekend in a 18m football hall. 4
fliers from Finland, and two visitors from Lithuania. The results were:

1. Jonas Deveikis (LIT, jun)
2. Leif Englund
3. Rimas Steponenas (LIT)
4. Tapio Linkosalo
5. Reijo Liljedahl
6. Juha Heikkinen

I do not know of all the times yet. My best was 18´40", a big-lucky
ceiling scraper, the rest of the flights did tail-slides and never got
away to the altitude. Leif did 18´49" on his last flight, passing me by
some 1 1/2 minutes, as his second was way better (15+ against 13+). In
the overall scores the top three were quite close to each other.

Anyway, I have two questions related to the turbulence.

First, this was the first time that this hall suffered from turbulence.
Typically it has been very calm and stable. I have been wondering, what
made the difference. It is unlikely that it is the outside temperature
(even though we flew late in the night and early in the morning, the
skies were clear, so convective radiation may have been quite high. But,
  on the other hand, we have flown in the call when a winter storm has
been outside, and the conditions were perfect.

So my guess is: as there were no users of the hall before us, the hall
was rather cool inside, and bringing the people inside provided a
temperature source on the floor, heating the air on floor level and
causing updrafts. On all previous conditions the hall has been in use
before, with air conditioning bringing in warm air. When the AC has been
turned off for the contest, the overall air temperature has been
reducing, and therefore providing more stable conditions? Does it make
any sense? A couple of hours later we flew a contest with F1M, at that
time the hall was packed with people, the other end full of RC, and
despite the RC's buzzing in the air, the conditions were calm and
smooth. Just as they ordinary are in that hall.

Second, about trimming for such turbulent conditions. It seemed to be
the major problem for me and many others, to climb to the roof. A couple
of tailslides/harriers, and you lost most of you climbing potential. On
the other hand, if you packed the motor with turns, and then did not
encounter turbulence, you ended up hanging in the ceiling structures. I
first reduced the decalage a bit (about 1.5mm = 0.5 degrees), this
helped a bit, but not totally. I did not want to reduce decalege
further, as the turbulence disturbed the model every now and then, and I
was afraid that making the model dive all the flight would have ruined
the performance. Instead I concluded that the problem if tailslips is
mostly the prop stalling, so I tried to reduce the prop pitch. This also
helped a little, but did not totally solve the problem. I was even
considering switching to a fixed prop, but did not get that thing going
right. So, what is the trick of flying in turbulence? How much does
reducing decalage hurt performance? Is a special, stall-resistant prop
needed for those conditions?




-Tapio-
Received on Tue Oct 06 2009 - 02:46:41 CEST