Contrails ! Airflow visualization
How to visualize ?
Much is being said on airflow in light aircraft, but we rarely have the opportunity to verify.
We availed ourselves of our recent stays at Darois to perform a few visualization flights with the help of some MCR boys.
See also Michel Bosek's tuft experiment at the bottom of this page.
First tests
Wool tuft
During our first weekend in Dijon-Darois (october 2005), the arrival of JJH prompted the quick setup of our first experiment. Thanks to Jean-Jacques !
Within a few minutes, the MCR 4S is equipped with tufts at strategical places, air inlets, fin to stabilator junction, rear of wing-root, wing tips (winglet).
The flight
After a quick briefing, the 4S took to the air, followed by JJH with this writer and Dyn Aero's camera.
JJH comes into close formation with IF. First observation : the 4S is no sluggard !
Visual tuft observation
- At the fin/stabilator intersection, the tufts are wriggling fiercely aft of the fin line of maximum thickness.
On the ground, the whirling tufts were found to be frayed. - Aft of the wing-root, the tufts appear perfectly fixed and straight.
One can see a contraction of the streamlines, probably denoting an acceleration. - Below the cowling, the tufts are fixed and well lined up at the air-inlet lip, and around the radiator-duct entrance.
After a few photo attempts, DA's camera battery quit altogether ;-(
Viewed from the MCR 4S
During this time, Igor in IF (with a working camera ;-) takes pictures of the photographer's airplane, and of the tufts at the winglets
As one can see, wingtip vortices concern only a small portion of the winglet. The intensity of the vortices gradually increases towards the tip.
Airspeed indicator comparison
The formation flight ends with a brief run at 100% power, "just to see". Measurements are taken on the fly and weights are not accurately known, but airspeed calibration is obviously not the same in the two aircraft.
Further tests
Oil visualization
Somewhat disappointed for having been unable to take useful pictures, we took the opportunity of a familiarization flight with Nicolas next day, to try visualizing the airflow with used motor oil.
"Ingredients" were kindly supplied by the local FBO, Aero Restauration Service ;-)
DA's camera had not fully recovered from its battery replacement. But looking closely....
Streamlines can be made out on the fuselage side, with the contraction observed on the previous day.
On the corner and the bottom of the fuselage, the lines are straight. We still have to determine the flow behaviour at the change of direction : boundary layer separation ?
Flaps
With close attention, one can discern the marked curve of the streamlines on the flap.
Starting chordwise, they then turn towards the fuselage.
Please note we used the flaps on landing, which of course must have influenced the oil pattern.
Second test session
Formation with F-PRTR
When we came back to Darois, we had the opportunity to fly formation with F-PRTR, flown by Philippe.
Our aircraft configuration has changed somewhat since the previous tests : the new fuel tank access covers are recessed a few millimeters, with sharp edges. This modification on the 10 covers will definitely affect performance, but will not change the airflow we intend to check.
The airplane is tufted only in the left wing-root area.
The flight
Prolonged formation flight with F-PRTR at 220 km/h IAS.
Observation
- Just like in the first tests, the tufts appear perfectly straight and fixed.
- Taking a closer look at the photos : at the fuselage corner, one can make out the shadow of certain tufts, which seem to be detached from the skin.
- Further tests with tufts on a larger portion of the fuselage will be required before concluding.
End of the afternoon
Like the first time, two runs at 75% then 100% power, in order to quickly compare the performance of the two aircraft, in spite of the different loads (two in TR, with not much fuel, three plus 120 liter avgas in IF).
On board TR, I carefully make sure the cruise revolutions stay at 5000 RPM, and not 5200 as usual, due to a loose pitch lever.
An observer from DA does the same on board IF, but our levers do stay where you set them ;-)
After a small trip in formation, we land at Til Chatel to say hello to the club, then Igor takes everybody back to Darois in TR, while I'm flying back direct to Vienne with our bird.
Michel Bosek's tests
Tuft tests on MCR 01
Michel Bosek owns an MCR 01 Club. He made a detailed video tuft study, and kindly allowed Contrails ! to publish some screenshots.
Observation
- The airflow over the empennage is studied first. Note that the tufts are carefully staggered.
- The detached zone under the stabilator is clearly apparent. The other tufts are perfectly fixed.
The separation affects a significant portion of the trim-tab. - We are now observing the influence of the separation on the flow over the the fin and rudder. The flow over the fin seems to be "sucked" by the turbulence zone.
- The airflow seems to be disturbed over about 1/4 to 1/3 of the rudder area.
- We are visualizing the airflow on the fuselage. No apparent separation aft of the canopy.
The flow leak at the flap-root, seems to disturb the airflow on part of the fuselage.
Note that the low pressure tends to suck open the canopy. Retightening the locks would slightly decrease drag.
- The state of the tufts at the end of the flight is a clear indication of the fierceness of the turbulence in the detached airflow.
Tuft test video
MCR 01 empennage tuft test video
Perturbation at the intersection of tailplane and vertical fin is clearly visible, as well as the portion of the tailplane - and trim tab - affected.
Video courtesy of Michel Bosek.
To be continued...
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20-04-2020 @ 13:28:23
