Stolspeed VG's

[Nev]

I've installed mine at 5%, but having given it some thought now I don't think we'll see a lot of variation in the results as long as they're not too far aft. I found it interesting that my power on stall speeds remained very similar with or without VG's, seeming to indicate that most of the results are there once the prop is firing that airflow over the wing. The noticeable change was in the handling, and the ability to descent in a controlled manner (fully stalled), and recover by applying power.

A789A002-AC52-491A-95E5-ECA6648B358A.jpg

[jaredyates]

I've installed mine at 5%, but having given it some thought now I don't think we'll see a lot of variation in the results as long as they're not too far aft. I found it interesting that my power on stall speeds remained very similar with or without VG's, seeming to indicate that most of the results are there once the prop is firing that airflow over the wing. The noticeable change was in the handling, and the ability to descent in a controlled manner (fully stalled), and recover by applying power.

A789A002-AC52-491A-95E5-ECA6648B358A.jpg

I think the propwash also makes a lot of induced airflow over the tail, which can keep the airflow from separating there. This also allows for slower stall speeds in certain configurations. If the flow separates at the elevator, the nose will drop.

[zkelley2]

Ya, I'm not really stalled at roughly 40kts power off. I'm just out of elevator.

Power on it actually stalls around 37-38.

[Utah-Jay]

No doubt I will be installing these, 5-6mph slower is a big difference

[nborer]

Great to see Nev's data on this. Nev, maybe you've posted it elsewhere but I haven't seen it... how is your airspeed calibration at these speeds? I'm not questioning the effectiveness of the VGs, just curious about the observed whole-aircraft lift coefficient at stall. For our club's 172N, indicated airspeed at stall with full flaps is 7 knots low, per the flight manual. Also, what was your CG? Stall behavior at forward vs. aft CG can be vastly different, and can be the difference between a tail stall vs. a wing stall. Can you comment on the differences in elevator? Are you full aft stick with one vs. finding you still have a little left over with another?

The difference in power-on vs. power-off could partially be related to differences in airspeed calibration at low speed (particularly if your static port may be effected by prop slipstream), but I wouldn't think that explains the bulk of it. A strong downward pitch at stall can be indicative of the tail letting go first before the wing stalls; a mush (particularly if you can still have some pitch authority) *can* (but not necessary *does*) indicate your tail is still working and your wing is indeed stalled. Power-on will probably have more effect on the tail. High-wing, low-tail airplanes have strong responses to change in downwash from the wing, and the power-on blowing effect could lead to locally higher wing lift and locally stronger downwash at the tail. There are probably some prop slipstream effects on the tail as well, but I'd think the wing and induced downwash would be a stronger effect. If only we could resurrect the NACA 30x60 tunnel...

[Nev]

These stalls were a basic look at mid weight, mid CG, and I've since done some at aft CG max weight to ensure they "pass" for the flight testing, but not enough to gather data as such. I've been pretty focused in other areas prior to finishing the test phase. My engineer used the word "pedantic" so I've probably done enough

In a couple of weeks I'll do a few hours testing the VG's in more detail to get some usable data. I suspect that at this stage what I was seeing was the elevators "letting go" without power on, and the elevators "holding on" with power. I didn't check for position error yet but there will be some for sure. Sorry I can't be more help yet but I'll report back once I've done more.

[Redneckmech]

Perhaps your engineer considers it pedantic but everyone here appreciates your methodical approach!
Thank you for taking the time and effort to gather data before and after the installation and share it with the community.

[nborer]

Agreed, I love seeing this data, and appreciate that you've taken the time to control the experiments, gather the data, and provide to the community.

[Bcone1381]

(I had to look it up.) "Pedantic is an insulting word used to describe someone who annoys others by correcting small errors, caring too much about minor details, or emphasizing their own expertise especially in some narrow or boring subject matter." HAHAHAHa

[Battson]

The shape of the aerofoil in a Bearhawk 4-place is different to a Bravo, because the curve of the wing is different - particularly in that area that VGs attach. This is a considerable variable, in terms of a wing - the shape is everything. I suggest people installing VGs consider which aerofoil they have, rather than going straight for a generic location - especially from the manufacturer, who offers one number to cater to all the common wing designs...

Aside from the above, I thought the location of the VGs generally was interesting to discuss. There has been a huge body of study done on this, and I am not an authority... maybe someone here is?

So far as I can tell, the best results are achieved in a relatively narrow range, a few percent of the chord. Lots of evidence to support that.

Too far forward:
The available data seems to show that vortex is strongest immediately behind the VG and deteriorates as it passes over the wing, losing it's effectiveness. One could assume that placing the VG forward moves the effective separation point forward, away from the part of the wing which generates the lift and where the separation occurs. This has been modelled in 2D cad and tested on various aerofoils, but it's hard to draw a concrete conclusion for the Bearhawk - although you can make good assumptions.

Too far back:
VGs need attached airflow across them to generate an attached vortex (VGs behind the separation point actually reduced lift compared to a bare wing, in some experiments). If the VGs are masked by curve of the wing, as viewed by the on-coming free stream airflow, they are not going to work as effectively. I assume this is because the airflow begins to separate before it reaches the VG. Some designs prevent this by making the VG taller, but that's another thread.

Considering the latter point, to me it seems clear that the shape of the aerofoil is likely to be important when selecting a suitable VG location.

[Utah-Jay]

With any luck by the end of the year someone will have a definitive placement location on where to put the Stolspeed VG’s by the end of the year.

I am interested specifically for the Companion wing

[Bissetg]

I mentioned earlier that I’d post my findings with respect to Stolspeed VGs. The wing I am testing on is the 4B Riblett aerofoil. The first position I tried was 3.6% of chord. The result was disappointing, a loss of 4 odd kts top end and no reduction in the bottom end, there was an improvement in low speed handling but it came at a significant cost of cruise speed. I had an engine change during this process so removed the VGs and re established the base line clean wing performance figures. I then re fitted the VGs at 6% chord. This resulted in no reduction of cruise speed and 4 kts off the stall IAS in approach configuration of full flap and approach power. The aircraft is definitely better to fly slow and is performing very well. For my configuration and mission 6% seems to be the number.

JG had mentioned to me that he was considering changing his recommendation from 7% to 5%, 3.6% sent me aft hence I decided to try 6% and I’m very pleased with the result. Could I tell the difference between 5,6 and 7%? Who knows but I do know 6 produces a good result.

For what it’s worth I think the STOLSPEED VGs should be a standard fit given the cost benefit of them.

[Battson]

Totally agreed that STOLspeed VG should be standard on all Bearhawks. Huge safety and performance gains for basically an hour's work.
The 4 KIAS does not do it justice, the difference in handling and safety is hard to overstate.

[Bcone1381]

I understand the benefit you experienced....that it handles better at slow speeds, that it lowers stall speed, there is no loss of top end cruise speed.

I've heard (generally speaking and not Bearhawk/Riblet specific) that when a wing with vortex generators stalls, the stall event itself is more sudden and has less warning. I'm curious what your experience was with this event.

[Bissetg]

Hi Brooks, I too have heard that mentioned. It’s not my experience with the Bearhawk or the C172. My experience is that when you reach the critical angle of attack the nose pitches down but no more enthusiastically than without VGs. The difference is that you can hold it in a mushing descent with aileron control with out the pitching oscillations of bouncing in and out of the stall you get without VGs.

The Stolspeed set up requires VGs at 60mm spacing for the 900mm in from the wing tip, then 90mm for the remainder of the wing. My understanding is that this gives the outer 900mm of wing better boundary layer adhesion hence the aileron control.

What I’m going to say next is speculation on my part not based on research I’ve done. I imagine that different wing profiles will produce different results. A P51D has the point of max camber two thirds of the way back from the leading edge of the wing. That was to maximise the low drag laminar flow that will only exist in a decreasing pressure gradient, hence the Mustang went faster than the Spitfire with the same engine. However, I think that the Mustang wing would respond completely differently to VGs for that same reason, if you put them forward you would kill the speed and if you put them aft they wouldn’t help much because the wing area that benefitted from them would be small and likely be less effective. The reason I mention a Mustang wing is it’s the most different wing profile I know of to what we are flying.

What this means to me is that for our aircraft we need to fit and test ourselves. There maybe some wings that do break more positively and without warning but that’s not my experience with my aircraft. Bit long winded but hope that helps.

[Bcone1381]

Thanks Grant. Interesting about the spacing of the VG's!

[AKKen07]

My VG experience seconds Bissetg

[Nev]

I've heard (generally speaking and not Bearhawk/Riblet specific) that when a wing with vortex generators stalls, the stall event itself is more sudden and has less warning.

I haven't noticed this with mine Brooks. Perhaps it is more abrupt, but on my B wing before I installed the VG's it was a very docile stall, so maybe it's a very relative thing, I did manage to get the wing to drop a couple of times before putting on the VG's, and I managed to get one wing drop out of it since. The pitch attitude is very high before it breaks, and it dropped to about 10° below the horizon and turned about 15° laterally. It was then fully controllable in all axis immediately after the break. I thought it was unlikely to be placed in this attitude in slow flight ops because I needed a lot of power to achieve it and really had to try hard to get it to break. I'm able to fly around at about 40kts TAS in full control.

I have to mention that the audio AOA feature in the Dynon is an amazing feature, and I would rate it even above the VG's for an increase in safety, awareness, and performance. I'm just a beginner in the slow flight arena, but between the audio AOA and the VG's I certainly feel a lot more comfortable.

Edit to add that in early flight testing the horizontal stabilizer was stalling before the wing, leading to the impression of a docile stall. With power on, the main wing can be stalled at a higher pitch attitude, and will usually drop a wing in a pronounced manner. It is easily recoverable and has sound handling leading up to the stall, especially with VG's installed.

[Bcone1381]

Thanks Nev!!

[Bissetg]

After doing the previously mentioned testing and repositioning of the Stolspeed VGs I found a complete new set of cut adhesives in my mail today sent free of charge by JG at Stolspeed. Outstanding customer service for an outstanding product.

[AKKen07]

I put the VGs on my Bearhawk B model wings minus the 18” extended wingtips. Following Stolspeed’s guidance installation was a breeze. I used 5% chord to front of VG. Weather cooperated for a limited window today so I called my fellow test pilot (Dad) to give her a spin. We tested at 2250lbs with a CG slightly forward of center. All speeds are indicated and highly suspect due to high AoAs. Also, wight and CG might be off due to all the changes made since I last weighed her.
I started with two high speed taxi runs to make sure there were no major problems and got airborne briefly - but then the wings just held on! It floated nose up high well after it “should” have landed.
Power off stalls were unrecognizable. Instead of a clear stall at 56 mph (flaps up) as per previous tests at this weight/cg it slowed to 50 and just stayed there, controllably mushing its way to Earth at 1000 fpm. Flaps 2 of 4 notches went from 53 to 48 with the same controllability increase. Wild!
Full flaps now do nothing to the stall speed. It was minor before and now they are just drag.
Power on, flaps up, the plane now sustains a 33 degree nose up attitude at full power and 46 mph. Climbing at roughly 300fpm. In all cases the control stick is fully aft at the stop and the ailerons are effective through the stall.

[Bcone1381]

Is there any down side? Did you see any loss in efficient cruise performance?

[AKKen07]

No change in cruise whatsoever. I can’t think of a downside other than the obstacle they pose when washing the bugs off the wing. On second thought - previously I could carry a touch extra speed to land then chop the throttle and the plane would plop down and stay down. Now if I have any extra speed it bounces back up and tries to fly again… but that’s just a matter of adapting my landings to the new characteristics.

[500AGL]

I thought I would have responded to this long thread but perhaps I didn’t as so much info was already posted.

They work as described but my suggestion is to use the 5% of chord as the maximum you would want.

The whole purpose is to have them effective when your AOA starts increasing, and quite literally they work the best between 1.1 and 4% of the chord. Many think it’s too far forward but it’s not. Round it out and place them at 2-3% and they work great.

No effect on cruise speed.
As others mentioned there is no stall “break”, just a mush down like an elevator, in full control.

pb

jerry burr’s data on a cub wing IMG_9136.jpg

[AKKen07]

That is some interesting info. 5 percent is what the Stolspeed manufacturer (JG) recommends and is also the farthest forward I’ve seen until your post. He also mentions that they can’t be too far forward for stall performance but at some point they are more likely to cause cruise speed losses. Maybe less of an issue on a cub than a Bearhawk.

[whee]

Hmm, I might have to rethink my “I like it how it is” position. I have a box of StolSpeed VGs on the sheet for a while but haven’t been able to convince myself I need to spend the time to install them.

Thanks a lot Ken…

[AKKen07]

Happy to help :P wouldn’t want you to get bored!

[500AGL]

It’s just one man’s opinion and of course they work different on each aircraft and airfoil.

But, after a lot of time listening, reading and learning from wise old pilots, I installed the stolspeed VG’s on both cubs I had and the factory Patrol using the above chart as my reference. All three aircraft showed the same improvement characteristics over a stock wing.

With the VG’s and tail weight to get the near empty CG back close to the 25% of MAC, I had to use two additional seat cushions to get my vertically challenged torso high enough to see anything on Stol landings.