Do vortex generators really work.

[Oscar]

That's a bit misleading - the damn prototype is very much better than roughly built.. Somewhere in the vicinity of half-completed, perhaps.

 

 

[Dafydd Llewellyn]

Sounds like an interesting exercise fellas, but the small market may not warrant the development cost. Are Jabiru's days of innovation behind them? In recent years they seem to have concentrated on refining a few models with similar performance. Their 230 airframe could surely be adapted to suit.

It would have to offer things that the J230 does not provide. I'm not going to go into that at this stage; I don't believe in telegraphing my punches.

 

 

[Oscar]

Sounds like an interesting exercise fellas, but the small market may not warrant the development cost. Are Jabiru's days of innovation behind them? In recent years they seem to have concentrated on refining a few models with similar performance. Their 230 airframe could surely be adapted to suit.

A 230 (or even better, a 250) with a CAMit engine and some aerodynamic mods (effective vg's, for a start) would be a damn good general-purpose device with really good back-up for airframe and engine -as an 'investment' in a general workhorse, extremely hard to beat. However, you need to think of it as a Toyota Landcruiser of the air; if what you need is somewhat closer to a Unimog, it won't quite get there. Horses for courses.

 

 

[Yenn]

A friend of mine has the plane for the job, A c120 taildragger with an O-360 engine. Looks great, cruise 130kts tough as nails and it is one of the oldest cessna's flyinh.

 

 

[jetjr]

J200 is close to your spec except stall. Take off distance might be a bit long thouh

 

With ground adjustable prop will do 128kts TAS especially with a few stronger horses from camit engine or SDI injection.

 

Stall ranges from 45 to 48 dirty (600-700 kg)

 

Can even go to 750 kg with wing bolt upgrade Im told

 

 

[SDQDI]

It depends who is going to use it as something that cruises at 130 isn't going to interest everyone, hitc hit on a few good points most want something that can drop into most of their paddocks even if it is just to drop off smoko, they want something that can cruise around at 50 knots with ease and safety for mustering and stock checks. And on the certificated side of things I'd be careful that the cost doesn't get in the way as the dollars are watched a lot more carefully these days, which also is the same for running costs. The rotax line seem to be a good pick as they handle Mogas (which is convenient for most ag ops) well and don't chew through heaps of juice.

 

I see that as one of the biggest problems with going with a bigger plane, I budget for running costs including maintenance, fuel of $40 per hour if you step up the size much more you won't keep that below $100 which might not seem like a lot but if its getting used most days it adds up quickly, and as I alluded to earlier us farmers are getting shrewder by the day.

 

 

[Dafydd Llewellyn]

Thanks for that Dafydd. I'll do some more testing.My fences seem to be doing a great job of preventing a wing drop- but at the expense of about 4kt extra minimum speed.

In trying to understand why, I am questioning one interesting source:

 

"The short fence (skewed in) does not increase CLmax, but makes the stall behavior more gradual. The short fence (skewed out) actually worsens stall performance." (http://gtae6343.wikia.com/wiki/Stall_Fences)

 

I could not find more detail on what they mean by skewed in. I installed my fences on the second rib out, the rear of the fence is closer to the fuselage than the front- ie paralleling the airflow

 

Could they have meant the opposite?

OK, first question: How are you measuring your stall speed? If you are using the original aircraft's airspeed system, you are likely to be seeing an exaggerated effect; just as the marketing of VG kits tends to rely on the increasing error on the pitot head at higher angles of attack, giving an exaggerated reduction in stall speed, the same effect gives an exaggerated idea of the penalty for removing the VGs inboard of the fences. To get a more accurate idea of what is happening, I would normally use the sort of airspeed measuring system that is used for a certification exercise; i.e. at the very least, a swivelling-vane pitot, mounted one wing chord ahead of the wing leading edge, and a trailing-cone static. Accurate measurement of stall speed is one of the most difficult pieces of flight testing.

Re the wing fences, I simply aligned them parallel with the aircraft's fore & aft axis. The form I use is what is termed a "short fence" in that wikipaedia article. It will develop the chordwise vortex when the inboard portion of the wing is forced to stall by the stall strips. Skewing them I assume means setting them at an angle, as one does with VGs; but that generates a vortex at all angles of attack, which is not what you need.

 

 

[Dafydd Llewellyn]

In case you have trouble discovering what a "swivelling-vane" pitot is, the terminology is imprecise - I mean, a pitot head mounted on a weathercock device, so it always points directly into the local flow direction.

 

 

[SDQDI]

In case you have trouble discovering what a "swivelling-vane" pitot is, the terminology is imprecise - I mean, a pitot head mounted on a weathercock device, so it always points directly into the local flow direction.

And that would be mounted to swing to changes in vertical air direction? Or would it also do horizontal changes?

 

 

[Dafydd Llewellyn]

Mostly, vertical; however, I simply use a rod-end as my pivot, so it can accommodate either; it has about plus/minus 12 degrees of self-align capability, which is sufficient for most purposes. The version shown in the attachment is a bit more complex than a simple pitot, but the basic construction form is the same; it's mass-balanced about the pivot.

 

The example shown comprises a shrouded pitot tube (which has high angle capability) co-axial with a form of venturi that has large-angle capability; I call it a "2-Q" head because in principle it should read double the dynamic pressure (this one reads almost three times the dynamic pressure). It makes it easy to do really accurate stall-speed testing, because you do not need a trailing-cone static with this setup; but the calibration factor has to be measured, either in a wind tunnel or by cross-reference against a simple weathercock pitot plus a trailing-cone static.

 

A simple weathercock pitot plus a trailing-cone static is just about 100% accurate PROVIDED YOU ARE NEITHER CLIMBING NOR DESCENDING. If you are changing height with that sort of setup, there is an error due to the change in static pressure.

 

 

 

[Old Koreelah]

OK, first question: How are you measuring your stall speed? If you are using the original aircraft's airspeed system, you are likely to be seeing an exaggerated effect; just as the marketing of VG kits tends to rely on the increasing error on the pitot head at higher angles of attack, giving an exaggerated reduction in stall speed, the same effect gives an exaggerated idea of the penalty for removing the VGs inboard of the fences. To get a more accurate idea of what is happening, I would normally use the sort of airspeed measuring system that is used for a certification exercise; i.e. at the very least, a swivelling-vane pitot, mounted one wing chord ahead of the wing leading edge, and a trailing-cone static. Accurate measurement of stall speed is one of the most difficult pieces of flight testing.Re the wing fences, I simply aligned them parallel with the aircraft's fore & aft axis. The form I use is what is termed a "short fence" in that wikipaedia article. It will develop the chordwise vortex when the inboard portion of the wing is forced to stall by the stall strips. Skewing them I assume means setting them at an angle, as one does with VGs; but that generates a vortex at all angles of attack, which is not what you need.

Thanks for that, Dafydd.

I use the original fixed pitot, but have done quite a few tests by flying in squares and averaging the GPS reading.

 

Getting a totally accurate air speed has not been a high priority; comparing the IAS readings should give me a fair idea.

 

I would, however, like to set up the pitot as you suggest in order to get more reliable readings.

 

I was reviewing some old testing video tonight, and was dissappointed to realise that after an awful lot of thoughtful modifications, my stall speed has not changed very much.

 

My fences are only skewed in by a degree or so, and when I do the next series of tests with wool-tufts I expect they'll be pretty much in line with the airflow (with camera mounted on tailplane).

 

 

[rankamateur]

most want something that can drop into most of their paddocks even if it is just to drop off smoko, they want something that can cruise around at 50 knots with ease and safety for mustering and stock checks.

Isn't that why you fly a Hornet and I am building a Savannah, and others still use Foxbats for the same again or a Eurofox. We are spoilt for choice already. And importantly they are all well proven designs.

 

 

[David Isaac]

Isn't that why you fly a Hornet and I am building a Savannah, and others still use Foxbats for the same again or a Eurofox. We are spoilt for choice already. And importantly they are all well proven designs.

But ultimately not quite strong enough at the RAA MTOW limitations. You need around 700 to 750kg by my rough speculation.

 

 

[rankamateur]

600 is better than 544, baby steps!

 

 

[Dafydd Llewellyn]

Thanks for that, Dafydd.I use the original fixed pitot, but have done quite a few tests by flying in squares and averaging the GPS reading.

Getting a totally accurate air speed has not been a high priority; comparing the IAS readings should give me a fair idea.

 

I would, however, like to set up the pitot as you suggest in order to get more reliable readings.

 

I was reviewing some old testing video tonight, and was dissappointed to realise that after an awful lot of thoughtful modifications, my stall speed has not changed very much.

 

My fences are only skewed in by a degree or so, and when I do the next series of tests with wool-tufts I expect they'll be pretty much in line with the airflow (with camera mounted on tailplane).

Well, I did point out that the realistic maximum speed benefit is about 6% of your original stall speed; if that was, say, 45 kts CAS, the best you are likely to see would be about 42.3 knots CAS. Where you WILL make a gain, is in the ability to use the lower part of the aircraft's speed range without it being quite so liable to try to spin if you do more than a very gentle manoeuvre. If you are not seeing a couple of knots reduction, you probably have the stall strips set a trifle too high on the leading edge. Whether you will be able to lower them to get that small improvement, and still avoid the assymetric stalling problem, is something you will have to find by test.

Flying squares and averaging the GPS reading will give a rough idea of the airspeed system error, at cruise speed. It's useless around the stall speed.

 

Also, to get any sort of accurate comparison, your tests all need to be done at the same centre of gravity position.

 

The definition of Vso is, the speed in level flight at which the aircraft pitches uncontrollably nose-down, or the minimum steady flight speed with the stick on the up-elevator stop, in the landing configuration with zero power, at the maximum takeoff weight, and the most adverse centre of gravity position, when the speed is reducing at one knot per second during the approach to the stall. To measure this, in certification testing, it is normal practice to use a video camera to record both the test ASI instrument and a stopwatch and a light that shows when the stick is on the up-stop, and to perform perhaps a dozen stalls with differing rates of deceleration, (measured from 1.1 times the lowest speed recorded to the point at which the stick hits the stop). The results are then plotted on graph paper, to show the stall speed versus the deceleration rate, and a line of best fit drawn through the data, and where that line crosses one kt/sec is the Vso stall speed.

 

One knot per second is a very slow approach to the stall, and surprisingly difficult to achieve in some aircraft; but when you are getting down to one or two knots difference, this level of effort is necessary to get an accurate result.

 

 

[Dafydd Llewellyn]

But ultimately not quite strong enough at the RAA MTOW limitations. You need around 700 to 750kg by my rough speculation.

None of those designs get within a bull's roar of meeting FAR 23 certification criteria - and even those are minimal. David Isaac is correct, they need a larger weight limit in order to do so, and the large wing area necessary to get to the speeds SQUDI considers necessary*, limits the proportion of useful load to gross weight, so the MTOW has to increase disproportionately.

The whole business of using aircraft built to watered-down airworthiness standards, for what amounts to aerial work, via the loophole of private usage over your own land, is not something a manufacturer can countenance from a liability point of view; it can only be done by the relevant authorities "holding the telescope to a blind eye". How long that will continue to be tolerated is not something I'd gamble on.

 

* I wonder what the airspeed system error of these aircraft really is; putting leading-edge slats or VGs is likely to increase the available angle of attack sufficiently to cause considerable pitot error - a normal pitot is accurate only to about 12 degrees angle from directly into the airflow. Manufacturers have been taking advantage of that to get unrealistic indicated stall speeds, for at least the last seventy years; for example, an Auster ASI reads 13 knots low at stall speed. FAR 23 put something of a stop to that, in the late '60s - but none of the aircraft mentioned meet FAR 23, least of all the Hornet, so the ASI error can be anything the designer wants. Are you all fooling yourselves?

 

 

[Head in the clouds]

A friend of mine has the plane for the job, A c120 taildragger with an O-360 engine. Looks great, cruise 130kts tough as nails and it is one of the oldest cessna's flyinh.

Yes, with 180hp it would be quite a beast, better still if it had flaps ... they were the main difference between the 120 and 140 IIRC. I guess they'd have had to beef up the wing a fair bit to allow an increase in MTOW and Vne?

 

But ultimately not quite strong enough at the RAA MTOW limitations. You need around 700 to 750kg by my rough speculation.

It's perhaps arguable that there's never enough in the limitations but as Rank said 600 is better than 544 and as I see it, with a bit of innovation 600kg is not too bad where you're dealing with the average stockman who hasn't spent his life at McDonalds.

 

We quickly forget how times have changed, consider the original J3 Cub - Continental A65 engine of 64hp, empty weight 290kg, MTOW 499kg, i.e.209kg available for fuel, crew and baggage! And a Vne of just 92mph/80kt, and it served on the land for many years until the first Super Cubs came along. I spent many happy hours as station pilot flying a Super Cub, doing everything from boreman to fencer to mechanic to mustering, It was an exceptionally capable aircraft once you got to know it intimately and it gave lots of warning when you were pushing the edges of the envelope. I had it stacked high with heavy gear on many an occasion during the hectic times of BTEC and it never faltered. The empty weight was 450kg with the o-320 150hp and MTOW was 780kg IIRC, with max fuel it had a payload of just on 250kg and the best loaded climb rate I recall was about 8-900ft/min but more usually about 700ft/min. Vne was 110kts from memory and cruise was about 90kts.

 

Consider also that the Super Cub is really a bit bigger and a bit more expensive to buy intitially, and to own and operate, than the vast majority of stations today are willing to pay, and that's evidenced by the fact that they're available to buy but there are actually hardly any out there. So, the suggestion that 750kg to 800kg would be better puts the weight limit right on the specs of the most popular aircraft of all time, and they're there to be had but stations aren't buying them. That indicates to me that something cheaper to buy and run is needed these days.

 

Also - look again at those weights, speeds and climb rates. LSAs are beating every one of those, better payload, better performance (please don't compare 100hp with 150hp ..) and better climb rates. At present the penalty is that most of them are more fragile but is that just due to the weight limits or the design, construction materials and customer demand for creature comforts as well? We have to be careful to not be comparing apples with oranges. Compare a 912S 100hp Lightwing with the Super Cub and even giving the Super Cub a 50hp advantage the Lightwing still beats it. And is the Lightwing fragile? Having just read Dafydd's post while I'm composing this I'll continue the 'Lightwing thinking' in a following post.

 

I'd be careful that the cost doesn't get in the way as the dollars are watched a lot more carefully these days, which also is the same for running costs. The rotax line seem to be a good pick as they handle Mogas (which is convenient for most ag ops) well and don't chew through heaps of juice.I see that as one of the biggest problems with going with a bigger plane, I budget for running costs including maintenance, fuel of $40 per hour if you step up the size much more you won't keep that below $100 which might not seem like a lot but if its getting used most days it adds up quickly, and as I alluded to earlier us farmers are getting shrewder by the day.

The Rotax 912 is a very interesting engine, depending on who you're speaking with it's either far too expensive or fabulously affordable. The flying school or private LSA owner, and the 95.55 amateur builder will tell you they're ridiculously expensive but the cow cocky, used to paying double or triple for engine changes is dumbfounded at how cheap they are. The difference in peoples' viewpoints being caused by the intial cost compared to the number of years until it has to be rebuilt or replaced.

 

An amateur builder will pay his $20k and at average private usage that engine will still be only at half life fifteen years later. The average busy flying school will also have that engine for a long while, at least five years I would estimate. Whereas on a station the cocky would be limiting the use of the plane as hard as he can go and still re-build the engine every other year. When I delivered Lightwings to a station or two they laughed at the price of the engine and let the plane 'go for its life' because it's saving man-hours, and chuck another engine in each season. Not all stations are anywhere near that busy of course but I guess folks see the point?

 

 

[Dafydd Llewellyn]

Yes; and the airframe standards of recreational aircraft also reflect the expected usage of a private LSA owner or a flying school. They do NOT cater for the sort of usage that consumes a 912 every other year. Sooner of later, we'll see a repeat of what happened with agricultural usage of R22s; a few of these aircraft will lose wings, and the authorities will pull their heads out of the sand, and it will all change again.

 

Re the speed illusion thing - when I was towing gliders at Bathurst, in an Auster J1B, we thought nothing of avoiding the problem of taxiing it crosswind* in a westerly, to get to the fuel shed, by landing it alongside the shed - after all, with a 25 knot stall speed, you can do that kind of thing, right? It was some years later that I discovered that the full-flap stall speed is actually 39 knots CAS. Didn't stop us landing in what was essentially a parking lot, filling the tank, and flying out of there again.

 

*Austers are a complete pain to taxi crosswind with their original cable-operated drum brakes, which fade completely after about 100 yards, so the thing keeps rounding-up into wind. Most of them by now have Cleveland wheels and hydraulic disc brakes.

 

 

[facthunter]

And they end up on their noses (which neva used to happen) The heel operated cable brakes are pretty poor though. Nev

 

 

[Head in the clouds]

Yes; and the airframe standards of recreational aircraft also reflect the expected usage of a private LSA owner or a flying school. They do NOT cater for the sort of usage that consumes a 912 every other year. Sooner of later, we'll see a repeat of what happened with agricultural usage of R22s; a few of these aircraft will lose wings, and the authorities will pull their heads out of the sand, and it will all change again.

Erm - Dafydd, I'd know more than many people about the R22 'losing wings' thing because unknown to me at the time my first R22 was the one that had nearly lost a blade and the previous pilot spent time in gaol, and later I took over the heli base in Broome of the one that did lose a blade, and they aren't a fair comparison at all, both had been flown nearly 50% over the life of the blades.

 

Whilst I agree with you that some monitoring of critical structure on older or high-time 95.55s needs to be undertaken/introduced, we also need to keep in mind that those that are being used heavily are the most rugged of the genre and aren't being stressed to anywhere near their design loading. Cockies and similar folk who are using aircraft as tools-of-trade aren't naiive types at all. They're far more mechanically minded than the average Joe and they ask and listen to advice. In general they don't service their own aircraft and there are virtually no L2s out there so it's done by LAMEs or the distributor/manufacturer. If their Lame recommends a borescope inspection or a dye test or even a fabric strip and inspect they'll agree to it.

 

Like I said before, they're a conservative lot and I think you'll find that's why we haven't had instances of wings falling off and you'd be as aware as anyone that cracks will develop and spend a long while propagating before anything catastrophic happens. On helicopter blade root laminations, for example, we don't ground a machine just because cracks may appear, we mark their extent and date with a paint pen every so often and can soon determine the rate of propagation and hence the time when it will become u/s and need to go for repair.

 

Similarly I'm sure we'll eventually start to get reports of cracks in spars of planes used on the land and that will signal the time for inspections of all of that type, just as happened with the Blanik ...

 

 

[David Isaac]

......... It was some years later that I discovered that the full-flap stall speed is actually 39 knots CAS. Didn't stop us landing in what was essentially a parking lot, filling the tank, and flying out of there again. .......

Interesting Dafydd, the same speed as Cessna declare for the C150. I used to wonder at the accuracy of the ASI on the J1B, but distinctly remember the first time I flew one, how long it took in the hold off until she settled, it seemed inordinately slow compared to the Citabria that I had many hours in. I noted the ASI was waaaay slow when she sat down. I have always regarded the ASI readings at the hold off and settle to be suspect in all aircraftThe POH that I have sates 25 knots stall, I take it that is IAS, not CAS? The POH also states a full flap short field approach at 40 knots, I assume based on the 1.3 x VS0. I had assumed the apparent slow stall and settle speed was significantly due to the rather large 37' wing span, although at a MTOW of 916kg she is a lot heavier than the Citabria. So if the stated stall speed is so inaccurate, should I assume at 40knots IAS on short field approach that I am also traveling faster than the 40 Knots IAS? Again though, at a 40 knot approach it does 'appear' very slow and uses bugga all runway to stop.

 

 

[Dafydd Llewellyn]

I hope you're right - but the fact remains that these aircraft were not designed to meet any fatigue requirements whatever. The scenario you envisage requires what is referred to as "Damage-tolerant structure" - and the certification criteria for that are given in FAR 23.573. So what is needed is a jump from no fatigue considerations whatsoever, to something that is a step beyond what virtually ANY current GA aircraft has. You cannot count on slowly-propagating cracks in locations that can be readily inspected. The Blanik demonstrated that very clearly.

 

I take your point about farmers being much more mechanically savvy than your average city dweller; that's always been my experience. But I'd like to step over what I forsee as a painful learning phase.

 

 

[Dafydd Llewellyn]

And they end up on their noses (which neva used to happen) The heel operated cable brakes are pretty poor though. Nev

Don't kid yourself - the cable brakes were quite capable of grabbing the first time they were applied from dead cold.

 

 

[Dafydd Llewellyn]

Interesting Dafydd, the same speed as Cessna declare for the C150. I used to wonder at the accuracy of the ASI on the J1B, but distinctly remember the first time I flew one, how long it took in the hold off until she settled, it seemed inordinately slow compared to the Citabria that I had many hours in. I noted the ASI was waaaay slow when she sat down. I have always regarded the ASI readings at the hold off and settle to be suspect in all aircraftThe POH that I have sates 25 knots stall, I take it that is IAS, not CAS? The POH also states a full flap short field approach at 40 knots, I assume based on the 1.3 x VS0. I had assumed the apparent slow stall and settle speed was significantly due to the rather large 37' wing span, although at a MTOW of 916kg she is a lot heavier than the Citabria. So if the stated stall speed is so inaccurate, should I assume at 40knots IAS on short field approach that I am also traveling faster than the 40 Knots IAS? Again though, at a 40 knot approach it does 'appear' very slow and uses bugga all runway to stop.

All POH speeds are given in IAS - that's a requirement. We used to tow gliders in the Auster at 42 KIAS - which indicated 50 in the Blanik, which has a particularly accurate ASI system. My first landing in my Mk III Auster (using the placarded approach speed) floated the full length of the Bankstown -29 runway. That's what started me digging into the Position Error. The Mk III did not have a POH, but there was a military AOP that gave the figures. They all had the same pitot-static head, so they all have much the same ASI system error. 13 knots slow at stall, and about 8 knots high at cruise.

 

 

[facthunter]

Dafydd They probably had to be rusty to do that. In my books a "grabbing " brake is U/S. The cables are typical cables. Probably worked OK for a few weeks. Heel operation means you have a lot of force available , compared to TOE, but from memory It needed a lot of force to stop. The Clevelands are a horse of a different colour. The Citabria POH advises against using brakes for directional control and a mere touching of them will lock them easily if you are not careful. Nev