Vortex Generators    Performance Enhancement for Light Aircraft
Home>Flight Testing

Flight Testing your Aircraft before and after

Vortex Generators

Every aircraft owner should do these flight tests, whether they are installing vortex generators or not. 
Every flyer should know the real life stall characteristics, best climb speed and best glide speed of his aircraft, as proven by his own flight testing.  Don’t just believe the manufacturer (salesman) - do it yourself!  There are far too many impossible claims flying around out there that won’t hold up when you really need them......... 
Just doing the procedure of the tests considerably improves your aircraft knowledge and skill level.  For safe flying you need to know how your aircraft will react at the slow boundary of the flight envelope, and more importantly how to recognise the ‘feel’ of nearing that boundary and reacting correctly immediately, so you can avoid crossing it. These are skills that you only get and keep by practice, so don’t just do the stall characteristics once and leave it at that – practice it every now and again to stay in touch. 
And do the practice with your head up and eyes ‘outside’ – pay attention to the attitude of the aircraft and the ‘feel’ of the controls – if you depend on the ASI you’ll be way too far behind the situation.....
If you haven’t actually done these tests then I would say that you don’t know your aircraft well enough to fly safely – wake up!!!
The following is a set of basic flight tests to give you a better ‘feel’ of your aircraft.  For more comprehensive flight testing see the FAA circular AC 90-89.
Flight Testing Conditions
For dependable results all flight testing must be done in calm, stable air – this means at first light in the morning before any thermal activity. 
For comparisons always flight test with same weight, at the same altitude, and close to the same temperature. 
Preferably select a time when the weather is looking stable for a couple of days  (ie- a large high pressure cell spreading over your area).
Do the tests without VGs early one morning, install the VGs during the day, then do the tests with VGs next morning in very similar conditions. 

First Calibrate your ASI
This is an essential test for every aircraft – the results might surprise you!
Select an early morning with no noticeable wind.
Fly a 4-way GPS course, with the legs 90º apart, long enough on each leg to get a good stable reading.
Maintain the same indicated airspeed for all four legs.
Carefully observe the GPS for an average and consistent ground speed on each leg.
Average the GPS readings from the four legs.
Repeat for other chosen airspeeds.
Write it all down!

Note - The ASI will read lower than the True Air Speed (TAS) as altitude increases.  A convenient approximate correction is 1.75% of the Indicated Air Speed (IAS) multiplied by the height in thousands of feet.  For example, at 80kts IAS the ASI will read about 3kts lower than TAS at 2000ft.


Climb and Glide tests
It’s quite astounding the number of fliers who haven’t even established the best climb and glide speeds for their aircraft by flight test.  When the engine stops for real some day, the overwhelming tendency will be to fly slower than needed – these flight tests will demonstrate how much is lost if you err that way.....

Essential to be conducted early, early morning before any thermal activity. 
Even slight turbulence will give false and inconsistent results!

Climb Test:
Select a BASE altitude (eg. 2000ft AGL) and TOP altitude 1000ft higher.
Begin a full throttle climb well below BASE altitude and stabilise climb speed to approx 15kts above predicted best rate of climb speed.
Time the climb from BASE to TOP altitude.
Repeat test at 5kt decreasing intervals to approx 10kts above stall. 
Glide Test: (Can be conducted at same time as climb test)
Begin an idle descent well above TOP altitude and stabilise.
Time the glide from TOP to BASE altitude.
Use the same speed intervals as used in climb tests, plus whatever slower speeds the aircraft will maintain with stability.
It’s important to maintain a steady airspeed for the duration of each run.
To maintain that steady speed learn to watch the aircraft attitude to the horizon rather than fixate on the ASI.
Ensure that the throttle remains firmly at the idle stop for glide runs. 
Record all the results.
Divide the recorded times (in seconds) into 60,000 to get feet/minute rates.
Plot the results on graphs.  These graphs will show you best climb speed and best glide speed, and dramatically show how much that performance really suffers if you get too slow!  Far better to be a bit fast than slow, eh...... 

Stall Characteristics and Stall Speeds
Stall characteristics are far more important than the actual stall speed.
Before starting stall tests check that CofG is in the range specified by the manufacturer.
Ensure that there are no loose objects in the aircraft.
At altitude, minimum 3000ft AGL, still air conditions:

Power Off Stall
Reduce power to idle and go into a glide at 1.3 x predicted stall speed. 
Slowly ease the stick back so that the aircraft slows at approx 1kt/sec.
Do not fixate on the ASI - watch outside the aircraft to observe the attitude, and note changes in the ‘feel’ of the aircraft (buffeting, control forces, control response.........)
When the aircraft feels like it’s about to stall, ease the stick forward to recover, then slowly ease back until another stall is imminent, and then ease it forward again.
Keep ‘tickling’ the stall like this, deeper and deeper as you feel your way into it.
Keep doing this until the feeling of imminant stall is really familiar.
Take careful note of the characteristics of the pre-stall, stall, and recovery.
When you’re thoroughly comfortable with the ‘feel’ of these stalls, watch the ASI just enough to determine the indicated stall speed.  
Do not fixate on the ASI, ignore GPS!
Repeat the test with flaps.
Write it all down!

Note: Indicated airspeeds at these low speeds are truly only ‘indicated’.  Inherent limitations in the ASI and the pitot at these low speeds won’t necessarily give a true airspeed, but is still adequate for comparison purposes.
Remember the ‘feeling’ of the controls and the aircraft, cause that’s close to how it’s gunna  feel when the engine really does stop some day!!!
Power On Stall
Set power and trim to fly straight and level at 1.5 x stall speed.
Slowly ease the stick back so that the aircraft slows at approx 1kt/sec.
Repeat the stall tests and note the ‘feel’ of the aircraft, and the indicated stall speed.
Be prepared for a more abrupt stall and possible wing drop due to the prop torque effect.
Repeat the test with flaps.
Write it all down!
Always be ready to promptly drop the nose of the aircraft to recover.
Use rudder to counter any wing drop. Keep your feet wide awake. 
Resist the impulse to use aileron input at stall, especially if a wing drops!
Resist the impulse to use full power to recover from a full stall – very unpredictable!
Do not pull the nose up suddenly into an accelerated stall, or stall at full power – those are tests that need far more caution – see FAA circular AC 90-89.
Cruise Speed
At 2000 ft AGL, set power to 'fast cruise' setting.
Use exactly this same RPM reading for each comparison test before and after VGs.
Establish steady straight and level flight for 4-way GPS runs as above.
Record the airspeed.
Landing Characteristics
These are the tests that you will use every time you go up flying,
because every take-off must lead to a landing! 
Once again in steady air conditions -
Practice holding-off touchdowns longer while remaining close to the ground.
Focus on staying right close to the ground as the speed bleeds off.
Don't let it touch until it's completely lost flying speed and must set down.
Keep your feet wide awake on the rudder pedals to balance the wings.
If it touches early and bounces or balloons then go around.
Don't fixate on the ASI - just focus on flying the aircraft close to the ground.
Note the characteristics of control and handling in this hold-off mode.
Note the 'abruptness' of the stall when it finally does set down.
The characteristics of these final seconds of the landing are the most important.

When it does finally touch down glance at the ASI.