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Archive for June, 2021

# Reynolds numbers for models

Posted by phil alvirez | Jun 23, 2021 @ 10:22 PM | 17,592 Views
this is dedicated to those who are interested in improving their knowledge on aerodynamics. it is a science that has been applied also to model airplanes.
but is has been discovered that there are some laws that dont apply so well. and took some time and research to learn that indeed, there are laws that are specific for models.
and this is easier to understand by following the next:

Reynolds Number (Re)-A simplified approach.

There is a difference in performance of a wing, depending mainly of its size (chord) and speed, and Osborne Reynolds managed to put that together in an equation that lets us predict its behavior.
In charts that show the lift and drag, the curve changes depending of the Re. The stall, the max lift and drag happen at different angles of attack, and the efficiency changes, too.
There are several ways to determine the Reynolds number. A simplified way is if we multiply the wing chord (in feet) by the speed of the plane (in ft/sec), and then divide it by 0.000157
If we know the speed at which certain airplane is flying (and its chord), we can calculate its Re with this equation (and that speed can be measured with a device called ‘How Fast’).
Now we are ready for this:

Making sense of it all.

I am presenting this to be analyzed by everybody, and then each one to decide if it makes sense to him.
From the beginning of the 20th Century, there was great interest in flying, and in Germany there was a scientist who believed that aeromodeling...Continue Reading

# swifts

Posted by phil alvirez | Jun 08, 2021 @ 06:33 PM | 17,816 Views
i fly 2 meters sailplanes. climb with an electric motor to 200 meters in 30 seconds, stop the motor, the propeller folds and i search for thermals to soar.
i spend as much time as the weather conditions allow. 1 day with clear sky (no birds in sight) i went flying and , after the motor stopped i began to search for updrafts. but suddenly, about 50 meters above my plane, i noticed a huge swarm of tiny birds flying fast in all directions. looked as if they were going nuts! as i drifted searching for thermals, i noticed that they moved above my plane. then i turned back to where i was, and they followed my plane. tried every maneuver i could think of, but i realized that they were following me. it was not a coincidence ! as i couldnt find thermals, my plane began to loose height, but they followed me on the descent, until when i was about 60 meters high, they suddenly disappeared. i couldnt believe my eyes again ! how could they show up and vanish so fast? where were they before and after? i did 3 more flights and same story. as i recalled, they looked like swallows, but that was all that i could learn. took me some time searching to learn that they were swifts. what could they see in my plane to remain above it? and not so close, just above...
and this never happened again.