Light Aircraft Association

I have applied for LAA Modification Approval to fit plastic Vortex Generators to my ARV Super2. I propose to fit VGs from http://www.landshorter.com . On the top of the wing, VGs will be put 10% of the chord back from the leading edge. On the underside of the tailplane, VGs will go immediately in front of the elevator. The VGs are not expected to have any effect at maximum speed; but VGs should provide significant benefits at low speed and at the cruise speed of 90 knots, as follows:

Landing: The ARV's high wing loading dictates a large AoA, resulting in a fairly high stall speed. Once VGs are in place, the wing should provide more lift, so the AoA will be reduced. Even when a large AoA is reached (now at a lower landing speed), the VGs should operate to keep the wing flying, further reducing the stall speed. Since the elevators will be more effective with VGs, the flare should more positive. Aileron response should be improved, giving better control at low speeds and in cross-winds.

Take-off: The VGs should energize the wing earlier, increasing lift and allowing a shorter take-off run.

Cruise: the VGs should still be enhancing lift at cruise speeds, slightly reducing the AoA. This should yield less drag from both tailplane and wings; and may even reduce fuel-consumption in the cruise.

Here are two UK reports that I have personally received about VGs:

(i) A pilot fitted PFA-approved "LandShorter" VGs to his Kolb microlights. He reports that the VGs are "brilliant", reducing stall speed by 5kts and improved low-speed handling, without reducing top speed.

(ii) A friend has just fitted VGs to his Zenair 601, and he confirms that they actually work. Before the VG's were fitted, there was a mushy stall at 50 mph; but now there is a very positive and crisp stall at 45 mph with excellent stability and no wing drop. In short, a 10% reduction in stall speed with much better low speed stability.

It is sometimes claimed that if VGs reduce the stall speed by, say, 10%, the manoevering speed (Va) must similarly reduce. That may indeed be the case; but I simply can’t see why that should be! VGs work by increasing the lifting efficiency of the wing without a concomitant increase in drag; and the most useful effect of efficiency this is felt at low speed. But even at high speeds with a small AoA, the VGs will still encourage more efficient lift with minimal drag. So how can this lessen the safe manoevering speed?

I suspect this issue of Va reduction is an old wives’ tale based on the notion of a fixed ratio between Vs & Va; but I’m ready to be corrected on this. Any views?

Interesting about the VGs - I read about them on the net and seemed like a "good" thing. Be interested in the LAA engineering view and the eventual outcome/effectiveness.

The manoeuvre speed is the minimum speed you can pull the structural g-limit without stalling. If your VG mod keeps the airflow attached longer and reduces the Vs then, yes, the manoeuvre speed will also decrease since you can pull more g at a lower speed without stalling. Doesn't mean it's any less safe. It's not a limit as such - more an awareness issue.

I thought VA was the max speed at which you can abruptly apply full control deflection without over stressing the airframe. If the effectiveness of the controls increases due to VG's then you would expect a decrease in VA to avoid overstress conditions.

I have some suspicion regarding the claims of folks selling little bits of plastic and double sided tape. There seems something of an element of snake oil about them. I doubt their effectiveness on a an aircraft with a 45kt stall.

I have no doubt that they work when set in the correct positions as determined by wind tunnel and experimentation, I always suspect folks who spend lots of time effort and money on an idea will always claim it was a success, it's partly wishful thinking, and part pride.

It looks to me like the approach phase is not the big problem with the Super 2, it's marginal take off performance on long grass or muddy runways is the real worry.

Pete wrote:I have some suspicion regarding the claims of folks selling little bits of plastic and double sided tape. There seems something of an element of snake oil about them. I doubt their effectiveness on a an aircraft with a 45kt stall.

There's nothing wrong with requiring evidence before being convinced; but there are dozens of people who have tried them, and all the evidence suggests that stall speeds drop by about 10% and slow-speed handling and aileron-response are improved. I have yet to hear a negative report! For example, the latest 701-based Savannah has dispensed with leading edge slats in favour of a conventional aerofoil with VGs. Its stall speed is only marginally higher, with far less drag in the cruise. The proof of the pudding is in the eating!

I have no doubt that they work when set in the correct positions as determined by wind tunnel and experimentation.

As it happens, I have access to a wind-tunnel and will conduct a trial; but the experience of others is that the "sweet spot" is 10% of the chord aft of the leading edge.

I always suspect folks who spend lots of time effort and money on an idea will always claim it was a success, it's partly wishful thinking, and part pride.

A set of these VGs will cost around £150; and they take a only couple of hours to fit. That seems to me a very cost-effective solution!

It looks to me like the approach phase is not the big problem with the Super 2, it's marginal take off performance on long grass or muddy runways is the real worry.

The ARV's performance is already good, but if VGs can make it even better, it seems sensible to do so. Any Super2 will benefit from extra elevator authority in the flare; but aileron response is good enough already. Again, the ARV's take-off performance is sufficient, particularly if larger tyres are fitted (to reduce rolling resistance); but if one can shorten the take-off distance and improve the rate of climb with VGs, why not?! I appreciate this might seem a little breathless and excited, but I really feel that VGs might be the best thing to hit Light Aviation for a long time!

Vortex generators always strike me as something used to offset problems elsewhere in a design.

But then I'm well known for my cynicism!

I do appreciate that this is not the case with an ARV, so why turn a nice looking aircraft into a short spined porcupine.

As a matter of interest Mike Moulai of Silver Fern Microlights uses VGs on the Kolb Extra so he might be a (cheaper) source of little plastic fins.

Rob Swain wrote:Vortex generators always strike me as something used to offset problems elsewhere in a design.. ..so why turn a nice looking aircraft into a short spined porcupine?

Some aerodyamic afterthoughts, such as added-on ventral fins, reveal that things weren't right first time. But there is no reason to suggest that VGs are merely a cure for an imperfect design. (It is true that VGs aren't things of beauty; but then nor are the hanging hinges of ultra-effective Fowler flaps). I reckon that VGs are unfairly ignored, the Cinderella tool in the designer's toolbox. If VGs can improve lift by 10% or more, then the aircraft designer can specify a wing 10% smaller. If it's smaller, it's lighter; and that is the key to good aviation design. The virtuous circle: add lightness and simplicate!

Mike Moulai of Silver Fern Microlights uses VGs on the Kolb Extra.

The Silver Fern Kolbs use the same type of "landshorter" VGs that I propose to fit!

Jetstream aircraft that fly in and out of Wick have vortex generators inboard and outboard of the engines. They are placed forward on the wing chord.

'Any Super2 will benefit from extra elevator authority in the flare; but aileron response is good enough already.'

Mention this to Joa Harrison as fitting VG's to the wing might aggrivate the elevator issue? the solution of course is to fit VG's to the tailplane too..

If buying some you might try fitting them to the tail surfaces first to see if that improves elevator authority at high AoA's.

Gordon

NJ673 wrote:Mention this to Joa Harrison as fitting VG's to the wing might aggrivate the elevator issue? the solution of course is to fit VG's to the tailplane too..
If buying some you might try fitting them to the tail surfaces first to see if that improves elevator authority at high AoA's.

I indicated in my first post (above) that I proposed to put VGs under the tailplane. I have spoken to Joa about it; and he has given the project his blessing.

Oop's, so you did!

Gordon

Since reading about this subject on the Matronics Zenair forum, I have been very interested and am seriously contemplating fitting them to my CH701 as a substitute for the slats. I have read some very interesting information on an Australian Website: http://www.stolspeed.com who market their own VG's called feathers. They have done extensive trials on both the Savanna and the 701 with quite conclusive results, especially regarding drag and fuel consumtion I recommend reading it.

Michael

I've just received provisional approval from LAA Engineering to fit and test these VGs. The letter states:
"You will need to team up with a test pilot with an aeronautical engineering background able to put together, carry out and then write up a full test program on the aeroplane. The program will need to include a comparison of the aeroplane as standard and as modified. .... The next stage will be for the evaluating pilot to put forward a flight test proposal showing the proposed positioning of the VGs using a suitable drawing and setting out his proposals regarding the test schedule". Phew!!

I remember that Beech Barons had VGs on the rudder itself the purpose was to give greater authority to the rudder before it stalled when used in an engine out situation to reduce the yaw, hence the single engine out speed was dropped quite a lot.

Arrivisto
Sounds like you have a mountain to climb there, sticking on the VGs will be the easy bit.
Good Luck