Stolspeed VG’s And Their Effect On RV-6
The following is a pilot’s perspective of the
Stolspeed VG kit installed on my RV-6 airframe: O-320 engine, Bernie Warnke
performance prop, selective 90/60/30mm full span VG installation on the wings
and horizontal stabilizer.
I will also be conducting full span tests
on the vertical stabilizer to see if low speed rudder authority can be improved
on the RV-6 in the near future.
My recommendations are general in nature
due to the different engines, props, airframe builds, loadings and CG’s typical
in the RV fleet.
Pilots of RV-6 airframes should see similar
benefits configured in this fashion.
I consider my configuration to be a major alteration of the aircraft’s flying
and handling qualities which require new procedures to extract the benefits and
avoid potential hazards they can impose.
My Van's model RV-6 was completed in 1998.
I have 1,780 hours pic time and 716 hours in type.
First flight with VGs occurred on 4/30/2013
preceded by test flights using all flap settings in slow flight and stalls at
all flap settings at altitude. I also performed 60 high performance landings
without VGs to establish a base line of experience just prior to the
I have accumulated over 240 hours flying
the Stolspeed VGs, as they are currently configured, in benign as well as some
of the most challenging flying and atmospheric conditions most general aviation
pilots are likely to encounter.
A survey of RV pilots who have installed
the Stolspeed system was also completed. Their responses demonstrate the need
for individual flight testing to derive the maximum benefit VGs offer on this
The performance enhancements of this
installation make the aircraft safer to operate, with greater safety margins at
high and low speeds. Control harmony in pitch and roll are significantly
improved with much greater stability during landings in challenging atmospheric
conditions, altitudes and loadings.
These improvements can become liabilities during
takeoffs and landings if the aircraft and/or VGs are not configured and flown properly.
Their effect needs to be experienced in the
airframe you’re flying and test flown in a variety of conditions to establish
your individual requirements.
You may determine, for instance, that a
full span installation does not meet your needs and prefer
the aileron and horizontal stabilizer installation.
Experiencing the VG effect through test
flying is the only way to derive maximum benefit from their use.
The non-destructive 3M adhesive and
polycarbonate vanes Stolspeed uses make them the perfect system for customizing
an installation in conjunction with test flying.
Independent Test Data
I do not make any claims about reduction in
stall or top speed which requires specialized testing equipment.
In my research I discovered a pilot who
tested his airframe with VGs using instrumentation similar to that used during
certification testing. Here is his web link: www.iwantarocket.com/pitot/pitot.htm
The tests were performed on a Harmon Rocket
using a VG kit designed by Paul Robertson of Aeronautical Testing Services Inc. Note: the Harmon Rocket has a shorter wing
than the RV-6. It also has a turtle back and other airframe differences that
are worth considering when assessing his data.
“The 4G decrease in stall is 10 mph and the
1G decrease is 5 mph. Takeoff roll decreased by 150 feet, climb enhanced,
landing roll decreased and no effect on top speed or negligible.” One
interesting discovery was that stall angle of attack increased from 15 degrees
to over 20 degrees.
VG Installation N726RV
My installation has 90 mm spacing beginning
at the wing root, 60 mm spacing for the ailerons all the way out to the wing
tip and 30 mm spacing on the bottom of the horizontal stabilizer just ahead of
On the wings the tips of the VGs are placed
at 7% of chord which puts the apex of the radius on the VG at 9% chord. I did
this placement to begin energizing the boundary layer as early as possible and for
maximum effect at high angles of attack.
Take offs in my aircraft are performed without
flaps including high, hot and heavy conditions with or without crosswinds.
The plane generates lift much sooner with VGs
which is easily discernable when departing from rough surfaces.
I have not measured the takeoff distances
on calm days with and without flaps.
The no flap take off decision is based on
flight experience, testing and a variety of factors that pertain to my
I recommend that any pilot who installs VGs
do a variety of flight tests to determine procedures that work best with your aircraft.
The RV-6 I fly gets off the ground very
quickly with VGs and without flaps at max gross even with my fixed pitch cruise
prop producing a maximum static rpm of 2,300 on a cold day at sea level.
My goal is to attain maximum flying speed
as quickly as possible by rapidly spooling my cruise pitched propeller to a
higher rpm for maximum horsepower. This requires “parking” in ground effect and
shallow climb angles.
This take off procedure maximizes cylinder
cooling while inducing the least amount of stress on engine components at first
I have found that a single notch of flap
delays my takeoffs, slows rpm spool up, inhibits speed and induces instability
on high, hot, heavy take offs in crosswinds.
Higher horsepower equipped aircraft with constant
speed propellers or fixed pitch climb propellers, may be less affected by the
drag and surface area of single notch flap settings. Thorough flight testing is
Before VGs on rough field takeoffs I didn't
use flaps till the plane acquired enough forward speed to minimize debris
damage when the tail is low.
On my first rough field take off with VGs the
plane hadn't rolled more than 100 feet before I felt the wings generating lift.
I know this because the shaking and vibration began to dissipate almost
I didn't believe my own bottom and dialed
in the standard rough field single notch of flaps which caused the plane to
slow down delaying the takeoff.
I've confirmed this behavior on subsequent
takeoffs so no flaps are now the norm. VGs on my aircraft add more net
lift than single notch flap settings and the aircraft is “bullied” less by
crosswinds without the added surface area.
The best confirmation of this increase in
takeoff performance has been demonstrated during formation flying. None of the
other aircraft I fly with get off the ground as quickly as I can including the
ones equipped with larger engines and constant speed propellers...the
difference is significant.
Since two ship takeoffs require the wing
pilot to remain fixed in their relative position off the lead aircraft it
offers an excellent side by side comparison. After many two ship take offs,
including at maximum load, none of the other aircraft has even come close to
breaking ground as quickly.
Tests At Altitude
No flap, power off
stalls remain abrupt with minimal buffet. Aileron authority is good.
A single notch of flap
requires a steep deck angle to produce the stall. There is some buffet before a
similarly abrupt stall.
Full flaps require a
very steep deck angle to produce a stall. There is pronounced bucking and the
plane resists letting go. I did not force or whip stall the plane in this
This is where I ended the
altitude stall series.
The specter of a
departure in my aircraft which favors the aft end of the flight envelope (wood
prop), with full flaps, riding an untested VG configuration exceeded my requirements
for flight data specifically and my enthusiasm for test flying generally.
I also did not explore
the power on departure stall series which produces even steeper deck angles and
elevated cylinder head temperatures.
I can discern no
benefit that can be derived from full flap departure and whip stall testing.
What I can say based on
the plane’s deck angle and resistance
to stalling with VGs and full flaps is that any
pilot, in this aircraft, who intentionally flies the machine into a departure
stall at low altitude richly deserves whatever outcome they produce.
After hundreds of landings and stall tests
in all configurations I have adopted the following procedures that produce the
best outcomes with the VG installation I’ve employed:
Takeoffs in all conditions are performed
WITHOUT the use of flaps.
Landings in all conditions are performed
exclusively with FULL FLAPS.
If there is a reduction
in stall speed it’s not enough to alter my normal landing approach speed of 80
This was determined
after many landing tests at 75 and 70 mph using full flaps on cold, windless
The flight tests
performed with telemetry by Terry Jantzi demonstrated no discernable change in
full flap stall speed.
Since full flap
landings produce the shortest possible landings, that has always been my
preference, however, there are flying behavior changes that have made full flap
The vortices generated
with this installation are tenacious. They resist span wise gusts and hang on
deep into the stall progression. In addition, aileron and pitch authority are
greatly improved at slow speeds.
Aileron inputs are much
smaller and produce a more rapid response making the aircraft significantly more
manageable in roiling, gusty crosswinds.
Without VGs and using full flaps in these conditions the aircraft
slews in yaw and is easily upset in roll.
This yaw instability is
still evident with VGs
in the no flap and single notch configuration.
However, with full spanVGs and full flaps the aircraft
noticeably settles down demonstrating a marked improvement in stability.
With power set to 1,200
rpm I can “drive” to the numbers with excellent roll authority, smaller roll
inputs and significantly less “fishtailing.” There is also better input harmony
in pitch and roll.
In ground effect with
full flaps the plane remains light at the stall and lifts off easily in the
early stages of settling when gusts are encountered.
In the majority of
balloon events now I rarely add power. The VG’s on the horizontal stabilizer provide
enough authority to enable pitching at the higher angles of attack where
reserve lift is available for settling the aircraft.
I should mention that
this technique produces some attention getting deck angles and took some
getting used to.
Only rarely now do I add
power to arrest sink which further shortens my roll outs. The majority of power
applications I have resorted to are due primarily to concern with early tail
No Flap & Single Notch Landings
I do not perform single
notch or no flap landings with my full span VGs. At a target airspeed of 80 mph
with power off the aircraft resists settling in these configurations.
In ground effect the
plane remains unacceptably light at those settings right through touchdown and
If the plane encounters
gusts and roiling crosswinds in this condition the flying gets demanding and
power applications only aggravate the situation.
I consider this to be an
unsafe operating condition which requires precise handling and risks eating up
Without the added drag and
stability of full flaps the plane remains vulnerable longer during the
transition while placing unnecessary demands on the pilot.
The added roll
authority of the ailerons more than compensates for the increased surface area being
presented to the wind.
Therefore full flap
landings are the only configuration I use even in the worst roiling crosswinds.
I should mention that with
full flaps, the aircraft exhibits a “lightness” and tendency to lift in gusting
winds early in the roll on especially at lighter loadings.
I would not discourage
anyone from experimenting with the removal of some inboard VG sets to damp this
In fact, you may find
that you prefer no inboard VGs
at all and only apply them ahead of the ailerons.
I personally prefer the
added lift for the extra margin it provides in aerobatics, aggressive flying
and high performance landings.
I consider the
non-destructive ability to change the configuration of your VG set up to dial
in the performance you prefer a salient advantage of the Stolspeed system.
The advantages of this
installation derive from improvements in controllability, stability, stall behavior
in ground effect and added margin at all airspeeds, weights and loadings.
I’m convinced that the
safety record of the RV fleet could be improved with the widespread adoption
and proper application of VGs.
In my research of wing
design and aerodynamics I came across the following pie chart depicting pilot
induced fatal accidents…
One comment made
by a pilot who installed VGs (different make) on his Pitts was that they made it impossible for him to
snap roll his aircraft.
A snap roll is
essentially a horizontal stall spin. If you look at the chart above you will
see that nearly a third of fatalities come under this category.
My first recommendation
to avoid the stall spin scenario is to religiously keep the ball centered in
turning approaches and avoid pulling back on the stick at low altitude and low
If your approach gets so
out of shape that you find yourself cross controlling…power up, reconfigure and
VGs may add just
enough margin to keep the snakes in the bag if you do ham fist an approach. It’s
an open question but one worth considering.
Put another way…would
you rather be flying a Piper Aerostar or a Piper Cub if the engine quits or you
experience an “inadvertent” stall at low altitude?
While landing mishaps in
ground effect have the potential to be induced by a “floating” airframe, they are
generally not fatal, whereas approach to landing stalls usually are.
The one possible
exception to this may be the landing over run which further reinforces the generous
use of flaps and proper landing discipline.
That said, you always
have the option of test flying wider spacing, fewer VG’s, further aft placement
and/or only applying them where they offer the best compromise of enhanced
performance for your intended mission.
In closing I’d like to address some of the
push back I’ve received from some RV drivers regarding VGs.
Is the RV-6 safe to land in challenging
conditions without VGs? Sure. I've been doing it for years. The RV is a
fine aircraft and with proper pilot technique will always get you safely on the
On rare occasions, that technique requires
finding another airport.
Fly well, fly safe.
JG Gilpin on Stolspeed’s VG Design
The streamlining is important, even the
base, when you consider that VGs are mounted in a position where the flow is
laminar, even on wings that go turbulent farther back. The air spilling over
the front ramp generates the vortex. The cleaner and less disturbed that
airflow the cleaner and tighter the vortex. This design generates very
slim, tidy vortices that cling close to the wing surface and persist right to
the trailing edge.
The longer blade types with blunt ramps
generate unnecessary turbulence. We found a drag penalty of 5kts on a 206 with
blade-type VGs compared to the Stolspeed profile.
Paired VGs generating counter-rotating
vortices are theoretically supposed to combine and enhance one another when in
fact they interfere with each other causing more parasite drag while lifting
away from the wing as they stream aft. The longer blade-type VGs do the same.
All my testing shows that there isn't a
specific 'sweet spot' as some suppliers claim. There is a rearward limit that
is critical. VGs only need to be in a position where they can get a good
'bite' on clean airflow.
The RV6 does need VGs under the horizon
stabilizer to give the elevator enough authority to pull higher angles of
attack at slower speeds. The RV6 is designed for the elevator to give up
before the main wing as a safety factor and therefore can't pull the higher
angles of attack VGs on the wings provide.
Steve Pankonin Regarding VG’s on his Piper
Pacer and Piper Super Cub
Our Pacer has much better roll control at
slow speeds than before which is a plus on those airplanes. The Super Cub, on
the other hand, will not stall like it used to without them. I prefer STOL
airplanes that stall right at touchdown on short strips.
One of my customers put them on his old
Cessna 170 and liked them so well it was the first mod he did on his new 170
with the 0-360 conversion.
What we have seen more than anything with VGs
is that they make the airplane more stable and the control inputs are cleaner
when the aircraft is slow. Not so much on making them any slower, just more
Stolspeed Survey Respondent: Name Withheld
RV4 - 150 hours TT on aircraft with VGs
since new. Originally 10% now at 7% of chord. Not installed on the tail, VGs on
the wings only. At 7% saw an increase of 1-2% additional power required at same
airspeed using Dynon flight deck.
160hp 0-320 - Static 2270 rpm
- Cato fixed pitch wood core, fiberglass wrap. Can exceed red line - 2500
fpm up to 3000ft - 120kts indicated cruise 2600 rpm 8000ft - 10,000ft 2700 rpm
120kts indicated. No wheel pants.
Light Sport Compliant, 180pnd pilot, 115pnd
copilot, 7 gallons per hour - with 20 gallons on board - bought the VGs for a
45kt stall to be in compliance.
Because this machine is so light, with VGs
it will not stall if you slow down gradually with or without flaps stick in your
lap with the ball centered. Observed the no flap phenomenon of plane resisting settling.
Lands short with power, dragging in at 45kts
(52 mph) 896 empty weight (150pnds lighter than my 6)