Installing VG's Did NOTHING? | Power-off Stalls | Aerodynamic Problems

[@kfatechnical]

On the Bushbaby the VG's must be installed 91mm from the LE. As you mentioned, lots of things changed from the one configuration to the next. As Mike said, too many things effect stalling speed and tests need to be consistently the same. The first action is to calibrate the pitot static system or at the least have a graph with the known error, then work form there. Lots of literature available on the techniques and data reduction methods. Data need to be reduced back to sealevel standard day in order to have accurate comparative numbers. Interesting nonetheless.

[@LetsGoAviate]

Appreciate the info. We'll move the VG's forward to the correct position, but I am fairly certain it will make no noticable difference on IJC until we get the bigger tail (horizontal stabiliser), which we will do after the upcoming annual/MPI. Getting that tail down with the heavier engine is the biggest problem, as you've also said from the start. Yeah just doing stalls is one thing, but doing it in a repeatable / comparable / consistent manner is something else. We're learning a lot.

[@mikevonbertouch]

Great intro video for comparative stalling. To perform the stall testing a little more rigorously the deceleration rate needs to be taken into account. The FAR 23 certification stall speed requires a 1kt/s deceleration and that deceleration can have a large impact on the speed. Plotting the stall speed vs. average deceleration rate over last 5 kts for all your tests should give you a quite straight line and you can then pluck off the value for the 1kt/s point which will be your stall speed. There are a number of other factors but that one is especially important and has a big impact of the final reference speed. If you decelerate faster then you get a lower speed. Pilot technique is also critical and being consistent to get that constant rate will mean you may have to disregard quite a lot of data along the way. A 2kt/s deceleration may see a stall speed 2 kts slower than a 1 kt/s rate. By varying the deceleration rate that regression of speed vs rate will provide an accurate number for your given weight. In a normal flight test programme you correct each point for weight change due to fuel burn and ensure the c.g. is in the most adverse position etc which will also have a significant effect. For example the more fwd c.g. requires greater tailplane down force as well which will tend to increase stall speed and may limit pitch control as demonstrated in this video.

[@LetsGoAviate]

Thanks for the comment. I've never really thought of it before, but the deceleration rate factor makes perfect sense. After a fast deceleration the indicated speed blasts past the actual real stall speed because of greater inertia, resulting in lower, but "false" stall speeds. Will take this into account for the next round of testing.

[@mikevonbertouch]

@@LetsGoAviate The reason for the reduced stall speed is a phenomenon known as dynamic stall where the stalling angle of an aerofoil is increased due to a higher pitch rate. If a wing would stall at a slow increase in angle of attack at say 14 deg, if the pitch rate is increased it may get to 16 deg before stalling. There are inertia factors as well but the aerodynamic pitch rate effect is significant. Well conducted stall tests give you a very straight line regression of deceleration vs speed.

[@TheAvGeek-kl3zf]

Hello, How did you attach VGs on the wing, (Adhesive, welding, riveting, etc)? 🛩

[@LetsGoAviate]

Hello. Adhesive. It comes included with 3M adhesive, already cut out in the shape of the VG's.

[@TheAvGeek-kl3zf]

@@LetsGoAviate 👍