Helium bubbles used to show bird aerodynamics

On Thu, 19 Mar 2020 09:33:40 -0700, jjdk737 wrote:

"How do you determine the tail is lifting in gliding flight?"

Agree. Do we have wind tunnel testing to prove the theory that your
fixed stab is always providing upwards forces on the aft end of the
fuselage?

I don't have access to a wind tunnel, but Mark Drela does and may will
have done that, However, I have photos that have a bearing on it.

When I drew up my glider series I made calculations of where the wing
wake should be at the tail location and how thick the wake was at that
point. I wanted to be sure that the tailplane was outside the wing wake
because my flying mates and I had already discovered that an F1A isn't
trimmable if the tailplane is inside the wing wake at its normal flight
attitude.

Anyway, I did the calcs, drew the lines on the CAD screen and adjusted
the fuselage height and wing AOA to put the tailplane comfortably below
the wing wake - I like zero dihedral inner wing panels with all the
dihedral on the shorter tips, which occupy about 1/3 of the semispan, so
wing/tail wake interference would be a problem if it ocurred.

A long time later I checked this by making a rake to let me see the
airflow behind the wing and onto the tailplane. Here's a description and
pictures:

https://www.gregorie.org/freeflight/..._visualization

Notice in both pictures that the streamers are deflected up in front of
the tail - I think that's a fairly clear indication that lift is being
generated.

Here are some wind tunnel pics showing similar effects, but the flow
deflection is a lot less because flow velocities will much higher than a
FF models gliding speed. An F1A glider's gliding speed is quite low: in
flat calm its easy to run alongside and just behind one and then reach in
and grab the tail boom just behind the wing is it sinks past your waist.

In case it isn't obvious, these models weigh just over 410 grams. They
are generally 2.2 to 2.5 metres in span and fly are about 10 mph (16 kph,
4.5 m/s). Sinking speed is around 0.3 m/s, giving a glide ratio of about
1:16.


Anyway, here are the wind tunnel links:

https://www.aa.washington.edu/AERL/K...hguide/flowviz

https://www.youtube.com/watch?v=IIHFkp1aAOI




I used do some free flight modeling.
Mostly Dick Mathis designs.



--
Martin | martin at
Gregorie | gregorie dot org