Vortex Generators

[gasax]

It's worth remembering that the main purpose of these things is to keep the boundary layer energised. Now with a half decent design the boundary layer only starts to detach at high angles of attack.

So for an aircraft like the 701 which is designed to achieve a high angle of attack (using slats), there is an obvious corollary. For the ARV to get off the ground quicker it will have to fly at a higher angle of attack. So the elevator element is vital - but so is the ground angle you can achieve and more importantly what the drag of the airframe will be at that high angle of attack.

It will be a fascinating exercise probably best started somewhere with a really long runway!

[Trevor Lyons]

For the ARV to get off the ground quicker it will have to fly at a higher angle of attack.

Not necessarily! During take-off, the ARV (like most trike aircraft) proceeds down the runway with a fixed angle of attack until flying speed (say, 50 knots) is reached. One then rotates, gets airborne, picks up a bit of speed in ground effect and climbs away. Even if the stick is held back to keep the weight off the nosewheel, the aircraft will present this fixed AoA to the airflow. Until flying speed is reached, the wing is effectively stalled, or, in other words, insufficiently energised. I have reason to believe that, with VGs fitted, the wing will energise sooner at this fixed AoA and thus will be ready to fly earlier.

However, when testing begins, it would be unwise to presume that an improved takeoff must occur; and it is prudent to anticipate instead a slight worsening of performance. It follows that the first takeoff with VGs should be on a longer than usual runway, with a higher than usual rotate speed, a longer period spent in ground effect, and a gentler climb.

Having reached a safe height, one would carry out tests to discover whether the stall speeds and stall behaviour had changed. Only if these findings were beneficial, giving, say, a lower stall speed and a more benign stall, would one contemplate experimenting with lower takeoff and landing speeds.

[gasax]

During takeoff the wing is insufficiently energised until 'lift off' speed is achieved - certainly true.
Vortex generators however do not increase the lift coefficient of an aerofoil by any real amount which would be needed to lift off at the same AOA but lower speed, instead they extend the relationship between AOA and lift coefficient - thereby giving the reduced stall speeds and higher maximum lift coefficient - effectively these occur 'further along' the line of the graph of lift coefficient versus AOA.
Where vortex generators are used purely for drag reduction the typical results are less than 3% reduction - with the best will in the world that is about the maximum increase you might see which in the real world is probably barely observable.

Still hopefully you'll get them fitted and have some trails data for an article in Light Aircraft!

[Trevor Lyons]

Vortex generators however do not increase the lift coefficient of an aerofoil ...

I’m not an aerodynamicist, and I have no more knowledge than the average amateur; but this is how I understand a tricycle aircraft takes off:

A wing with a standard asymmetric aerofoil provides lift partly through the “negative” Bernoulli effect and partly through the “positive” Newton effect. Airflow over the top of the wing adheres to it, creating a low pressure “suction”. During the take-off run, the suction increases until it is sufficient to lift the aircraft. On rotating, the AoA is increased, and a fresh blast of air to the underside of the wing provides further lift. However, the increased AoA means that the airflow on the top of the wing is liable to break away and stall the wing, thereby sacrificing this suction. Because of this, one must take care not to rotate too early or too violently.

But with VGs fitted, when the AoA increases on rotation, the airflow over the wing should be sufficiently energized to resist the breakup of the boundary layer, with the result that the aircraft takes off sooner.

The “Landshorter” testimonial webpage seems to show that lots of flyers have found this to be true: http://www.landshorter.com/page3.html

(Yes, I know that some scientists frown at the word “suction”, but it is a useful everyday concept. It is more normal to tell a repair man that “My Hoover has lost suction” than to say “The atmosphere is failing to respond to the induced low pressure within my vacuum cleaner”!)

[gasax]

well best of luck arrivisto.

Once you have your test pilot lined up you can discuss this theory with him and create a test programme to prove it. I wish you luck.

[Pete]

The point of keeping the nosewheel on/close to the deck ( or the tail wheel up ) is to make sure that the aircraft does not become airborne before it reaches a sustainable flying speed plus a bit more to cover the effect of wind variation. So if stall speed is 40kt, you should use the old 1 1/3 rule and lift off at 55kt (ish). If your VGs lower the stall speed by 3kt, then I guess you can pull back gently on the stick 4kt slower (IF).

You could possibly shorten the take off roll by 50m or so.

Most aircraft best angle of climb is usually acheived at somewhat higher than stall speed, so I cannot see the benefit when you are in the climb.

BTW as you believe all those "testimonials", I have a deal for you, I am a friend of the ex president of Nigeria, he has asked me to help him .....