Trying to investigate the influence of stabiliser vs. elevators thickness on altitude holding ability in level and inverted flight, turbulator strips of different thickness have been installed at the hingeline of the stabiliser. Repeated test flights have been conducted at governed constant RPM in calm conditions.
Test configuration:
Crossfire, electric, 64 oz.
Lines 0.4 mm x 18.0 m (0.014 x 59 ft)
RPM 9’240
Speed 5.0 sec/lap
A: No turbulators added
B: 4 ea. 254 mm (10 in long) 4 mm (0.16 in) thick and 4 mm wide triangular turbulator strips added
C: 4 ea. 254 mm (10 in long).1.5 mm (1/16”) thick and 10 mm wide triangular turbulator strips added
Findings
A: With stabiliser TE and elevators LE being equally thick, the airplane feels a bit touchy. Maintaining altitude in level and inverted flight requires constant attention. This is well manageable in level flight, but critical during the inverted laps. With crisp input, corners are snappy with clean exits.
B: With 4 mm thick turbulator strips added to stab at the hingeline, the airplane became extremely sluggish and tight corners could not be flown. While holding altitude in level and inverted was easy, this configuration would not allow competitive flying. Added drag reduced speed by 0.3 sec/lap.
C: With 1.5 mm thick turbulator strips added to stab at the hingeline, the airplane felt solid in level and inverted. Round manoeuvres looked fine, displaying kind of a nice flow. Holding altitude in level and inverted was manageable. Corners were a bit round. Moving back the by C.G. 3 mm (1/8”) brought back most (not all) of the cornering ability. Added drag reduced speed by 0.1 sec/lap.
Résumé
Equally thick stab TE and elev. LE may:
• Compromise altitude holding ability in level and (more so) inverted flight
• Reduce solid look & feel in round manoeuvres
• Improve cornering ability.
At hingeline thicker stabilisers may:
• Improve altitude holding ability in level and inverted flight
• Add solid look & feel in round manoeuvres
• (Critically) Decrease cornering ability
The above are strictly personal findings and must not be relevant to other airplanes and/or flyers. For the time being, I will continue flying as per configuration C and report here.
Update Oct. 1st 2916
When flying the 1.5 mm turbulator in 5m/sec (11 mph)wind, I found it critical to pull out accurately and low from square or triangular maneouvres. It seems that the reduced efficiency of the elevators was no longer sufficient to rotate the airplane and when I removed the turbulators on site, this brought back the original manoeuverability immediately.
While a thinner than 1.5 mm turbulator (or thickness difference) might be a valid compromise, I now do not believe that the stab-elev thickness difference, or the lack of it, is the key to explain or cure the level flight altitude holding problem (not hunting) of some electric stunt airplanes. With this conclusion, I will now terminate the above tests.