Stolspeed VG's

[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.

[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.

[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

[Mark Goldberg]

There is a Patrol owner who did very extensive testing on placement of VG's on his wings. My memory of his testing results is not 100% clear, but I believe the best position was more forward than the recommended placement from the manufacturers. I seem to remember it was pretty close to where Jonathan placed his - even though they are different airfoils. Mark

[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

[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.

[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.

[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.

[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.

[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...

[Utah-Jay]

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

[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.

[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.

[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

[Bissetg]

I’m really interested in where folks determine the best location for the VGs is. The figures I’ve heard range from 2.5% of chord to about 12%. Stolspeed I believe were recommending 7% but have revised that to 5%. Because my experience of VGs on wings is limited to a C210 and C172 which are covered by an STC that positions them at 2.5% I’ve gone for a forward position. I’m hoping my next engine lasts more than 18 hours so I can do some meaningful testing and report back but very interested to hear of others experience with a B model wing.