Here is a careful analysis, based on considerable experience,
by Brian Parris in Oregon, flying his RV6 with Stolspeed VGs.

 

 

EAA,

 

I’m giving a presentation to some Van’s Aircraft factory personnel and attendees at Independence Oregon this weekend for the Van’s Homecoming event.

 

I’ve been working with the developer in Australia of the Stolspeed Vortex Generator system for several years and this is the culmination of that work.

  

I’m submitting the attached document to EAA to determine if there’s an interest in an article or series of articles on the subject and the impact it may have on reducing stall spin fatalities in the experimental fleet.

 

The RV-6 is a fantastic performer for a variety of flying activities. It’s also intolerant of mishandling as are many experimental aircraft.

 

As our association is well aware, the experimental accident rate is more than double the certificated aircraft accident rate which is not helping our cause.

 

The benign stall behavior engineered into certified airframes hinders performance.

 

Experimenters are all about performance so they build machines that put greater demands on the pilot.

 

An acquaintance who kept his plane across from mine died recently in an RV-8 stall spin accident.

 

I also witnessed a landing accident at my home field a month ago in an LSA due to mishandling by an instructor giving a tail wheel endorsement ride…on a no wind, cool morning!

 

The accident plane is a lightweight STOL which also puts demands on the pilot so there’s two sides to this coin.

 

The FAA and advocacy organizations have been trying for years to solve this problem with little success.

 

I began researching Angle of Attack indicators and VG’s for my own mount and chose to change the aircrafts performance and handling rather than receive timely information about stall margin from yet another panel instrument that diverts attention from actual flying.

 

Angle of Attack indicators are becoming more common due to advocacy and the widespread adoption of glass panel instrumentation that easily accommodates the data and display requirements.

 

Will AOA slay the stall/spin demon?  Time will tell.

 

But the device may not save a pilot who is cross controlling their machine while pulling on the control column or stick at low altitude and low airspeed.

 

I’m not convinced the “Chevrons” will save you on a turning approach In roiling crosswinds or wind shear, while flying in a cross controlled condition, then pulling on the yoke or stick in response to a sudden loss of lift.

 

Stall/Spins are completely avoidable with simple flying discipline that I outline at the end of the study.

 

However, In critical situations where distractions, poor technique or panic rear up, Vortex Generators may provide the added stall margin and controllability that can save lives.

 

I’m convinced that if the current fleet of higher performance experimental aircraft were to adopt VGs in combination with reinforcing specific stick and rudder recommendations for turning approaches, Stall/Spin accident statistics will improve.

 

Maybe this study and a different approach to the problem can help.

 

Brian Parris

EAA# 636323

 

 

 

 

 

Stolspeed VG’s And Their Effect On RV-6 Performance              8/8/15

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.

Note: 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.

Background

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 installation.

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 airframe.

General Information

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 the elevator.

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.

Takeoffs

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 installation.

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 start up.

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 recommended.

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 immediately.

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 configuration.

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.

Landing Tests

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 mph.

This was determined after many landing tests at 75 and 70 mph using full flaps on cold, windless days.

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 landings mandatory.

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 wheel contact.

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 early rollout.

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 available runway.

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 behavior.

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.

Cont…

Summary

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 airspeeds.

If your approach gets so out of shape that you find yourself cross controlling…power up, reconfigure and go around.

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 ground.

On rare occasions, that technique requires finding another airport.

Fly well, fly safe.

Brian Parris

 

 

 

Appendix

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 controllable.

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)