Weight restricted MTOW

[facthunter]

Probably the simplest way to modify for weight increase with same stall speed is to add wingspan. This requires full recalc of the wing structure but is pretty straightforward. It also ensures the plane will fly OK in a higher density altitude situation as well. I'm not sure I'm "easy" about upping stall speed. Landing OFF aerodrome is feasible with slow planes but becomes increasingly a problem when you are coming over the fence a fair bit faster.. Nev

 

 

[Bruce Tuncks]

Nev is right about the non-tightened sump-plug. I knew the pilot of a Cessna which came down near Parafield from this cause. He and his passenger crawled out through the shattered windscreen while soaked with avgas which didn't ignite. And that was a LAME service.

 

Mike Bush says every post-service flight is a test flight and should be treated as such.

 

 

[DonRamsay]

This assertion doesn't quite agree with physics.

The numbers surprised (and delighted) me. Too good to be true? They were a quote the factory POH for the Sling as a 600kg MTOW and as a 700kg MTOW. I'll definitely do a physical check one day when 700kg is allowed.

 

The Spec sheet on the Sling website:

 

Stall Speed - Full Flaps 600kg 39 KCAS 700kg 40 KCAS

 

I rechecked my POH at 600kg and it says 40KCAS and I've checked that in flight at altitude and it is close enough. So, the Sling LSA registered as VLA with 700kg MTOW with same wings should still be under the 45 KCAS.

 

Jab publish a stall of 45kts for the J430 in VH reg form for 700kg MTOW. A little extra weight might actually make it easier to land and stop it from floating as they are prone to do. The Sling and Tecnam Sierra are even more inclined to float if you don't manage speed well on final due to more ground effect with the low wing.

 

I think you'll find the difference is more like 3.5kts, but don't take my word for it.

For the sake of the argument, lets say it is 3.5 kts so at 600kg it stalls at 43 kts and at 700kg it stalls at 46.5 kts. Anyone here who can pick that difference without access to a super-accurate ASI? Would +3.5 kts really make it unsafe for your average RA Pilot?

 

OK, well based on the above spreadsheet the difference between you flying your Sling with 1 PoB and half fuel - or flying it with 2 PoB and enough fuel for MTOW at 600Kg would be about 3-4kts on the stall speed. I don't know about the Sling but I find there's a massively noticeable difference between half fuel and 1 PoB or 2 PoB and MTOW on any LSA types I've flown, and that's just 3-4kts difference at the stall.

I agree that there is a difference in feel with a light versus heavy payload as we all discovered on our first solo. But, isn't the point of RA pilots flying aircraft with a slow stall speed that touching down at a slow speed is easier to control than at a much faster speed. A good landing happens at the stall speed and landing at 43 or 46 kts would be, to most part time pilots, indistinguishable.

 

. . . My current 'DooMaw' build has much smaller wings than Don's Sling, and the same MTOW, but will stall about 7-8kts slower than his due to having various lift enhancing devices.

That's impressive! Is there a trade off against cruise speed with the "lift-enhancing" devices?

I agree that high stall speeds are undesirable. But I feel 50 kts would be manageable as demonstrated on the odd occasion when I come in hot.

 

 

[facthunter]

Don, most pilots say their "plane" has a stall speed of "X" knots without making any allowance for landing weight variations or Cof G position. There's a lot of difference in how a plane will handle with a light pilot and almost empty tanks and a rearward Cof G and when it's near max weight and a forward CofG. This probably explains why some landings are not as predictable as they should be. Gusts and hot air sitting over the sealed surface of a runway make a difference too. You might not fly to 3.5 knots exactly but "adding a bit" for a proper reason is the same principle. Some aircraft do require accuracy in airspeed near the flare to be achieved or it will be commented on unfavourably and will affect landings. IF it is decaying it will affect them also. Nev

 

 

[turboplanner]

I agree that high stall speeds are undesirable. But I feel 50 kts would be manageable as demonstrated on the odd occasion when I come in hot.

Judging by the amount of nose wheels snapping off, and the frequency of runway excursions and flip overs reported to RAA, existing landing speeds may be at the upper limit for frame constructions and steering geometries at this level aircraft.

If more weight is built into the undercarriage and supporting fuselage structure and if slightly more length is available in the fuselage to allow a better castor angle, the higher landing speed forces could probably be overcome, but then you are on a circular path where you don't get all the payload/fuel you were looking for.

 

 

[Nobody]

Don,

 

I kind of disagree that stall speed change are not able to be felt by most pilots. They are able to perceive the changes but don't necessarily attribute that to increased stall speed. It may only be a few knots but the change is quite marked. An RV7 has a stall speed of 49 knots and it lands a lot faster than a jabiru.

 

 

[Head in the clouds]

The numbers surprised (and delighted) me. Too good to be true? They were a quote the factory POH for the Sling as a 600kg MTOW and as a 700kg MTOW. I'll definitely do a physical check one day when 700kg is allowed.The Spec sheet on the Sling website:

 

Stall Speed - Full Flaps 600kg 39 KCAS 700kg 40 KCAS

 

I rechecked my POH at 600kg and it says 40KCAS and I've checked that in flight at altitude and it is close enough. So, the Sling LSA registered as VLA with 700kg MTOW with same wings should still be under the 45 KCAS.

 

Jab publish a stall of 45kts for the J430 in VH reg form for 700kg MTOW. A little extra weight might actually make it easier to land and stop it from floating as they are prone to do. The Sling and Tecnam Sierra are even more inclined to float if you don't manage speed well on final due to more ground effect with the low wing.

 

For the sake of the argument, lets say it is 3.5 kts so at 600kg it stalls at 43 kts and at 700kg it stalls at 46.5 kts. Anyone here who can pick that difference without access to a super-accurate ASI? Would +3.5 kts really make it unsafe for your average RA Pilot?

 

I agree that there is a difference in feel with a light versus heavy payload as we all discovered on our first solo. But, isn't the point of RA pilots flying aircraft with a slow stall speed that touching down at a slow speed is easier to control than at a much faster speed. A good landing happens at the stall speed and landing at 43 or 46 kts would be, to most part time pilots, indistinguishable.

 

That's impressive! Is there a trade off against cruise speed with the "lift-enhancing" devices?

 

I agree that high stall speeds are undesirable. But I feel 50 kts would be manageable as demonstrated on the odd occasion when I come in hot.

It's all part of the 'inconvenient truth', the question is though - should we open the proverbial can of worms ...?

 

Some while ago there was a very informative thread called something like Do VGs really work and I think it was somewhere in there that Dafydd Llewellyn (Reg 35 approved aeronautical engineer) wrote up the story of the glider tug pilot who used to land his Auster tug at 32kts. If I recall the story correctly he'd been doing so for years and was quite certain of the numbers until Dafydd installed his calibrated ASI with swivelling pitot and trailing-cone static source test equipment. All of a sudden this Auster was landing at 48kts instead of 32kts. Now that's a massive and instant 50% increase, so what happened? The answer was nothing more than pitot error, and every single LSA out there suffers from it, and their unsuspecting owners are being duped daily ...

 

The first thing is that we're all too trusting, and consequently we tend to believe what people tell us, particularly if it's written in an important document like a POH. I recall all the way back at the Mangalore that AUF was established, 1983 IIRC, I was having a chat with my old mate Sander Veenstra. He was an avid builder/manufacturer of ultralights back then, and I was going on about the performance figures of some new plane of the time. Sander laughed and said "you don't believe what aircraft manufacturers tell you, do you? They're the world's biggest liars - even worse than fishermen". I didn't say much more on the subject, particularly since Sander was an aircraft manufacturer himself and I didn't want to offend him, but I took note of what he said and since then have always performed my own analysis of aircraft manufacturers' claims, and it's mighty rare that any of them even come close to the truth.

 

That particularly applies wherever performance limits are set by Regulation. One such limit we have in LSA/eLSA is the stall speed requirement. Very happily for the LSA manufacturers they get to self-approve their products under the ASTMs process, so no-one is out there actually checking the numbers they publish on their websites and POHs, it's all taken for granted under the ASTMs process, no-one will ever have to check it - unless, of course, someone decides to sue the manufacturer if something happens ... and good luck with that, I'd be pretty sure every manufacturer will be protected under several corporate layers much as the Robinson helicopter people proved to be when people had a go at them.

 

Two things we DO know - without a swivelling pitot an ASI will almost certainly suffer from pitot error. AND - pitot error assists manufacturers' claims of lower stall speeds, since the pitot error invariably makes the ASI read low at high AoA i.e. when approaching the stall. It's well documented that airflow off-axis to the pitot by around 6-7 degrees or more provides a reading error which increases substantially as the angle increases to more than that. The stall happens at around 13-15 degrees for most airfoils and by that time pitot error can be substantial. A good example was my C172 where the ASI would read between 0-15kts while I was still able to keep flying under full control with full 40 degrees flaps and a very high deck angle. I even had some pax believing that we were flying at around 10kts ....

 

There are a few ways to make the pitot error not so bad. Since there is very little effect on the reading accuracy below about 5 degrees off-axis, the manufacturer could point the pitot down by 5 degrees. That would mean the pitot error wouldn't start to become of great significance until 12 degrees AoA, so the reading at the stall would be a lot nearer the truth. So - do they do that? I wonder why not ...

 

OK, so let's just for the moment suggest that just maybe there's a mistake in the POH of some LSAs, and the stall speed written in there is perhaps not the real stall speed, but is the indicated stall speed ... Is that possible? Wouldn't that be a bit fraudulent? Well, the answer is no it wouldn't be. The numbers written in the POH are NOT the true stall speeds, true Vne, true Va etc etc, they are the indicated speeds. The reason is simple - indicated speeds are the only ones we can use because the Air Speed Indicator is the only reference we have! So it's quite legit for the manufacturers to bandy around indicated performance figures instead of true ones ... AND there's no requirement for them to go to any lengths to make the indications nearer to the true numbers AND it's not to their benefit to do so. In fact it's to their benefit to make them read low at low speeds and high at high speeds. But they wouldn't do that would they? Would they? Just ask yourself how competitive the LSA sales market is and then ask yourself again whether they would or not.

 

So - if that might just be the case, then what are the real numbers? There are two easy ways to find out. First is more fun than the second so we'll do that first. Load the aircraft to MTOW, strap in nice and tight, do all your checks etc, go to higher than 3000ft AGL. Determine the wind direction as accurately as possible by observation of drift, complete a clearance turn, broadcast intentions etc, cover the ASI so you don't cheat on yourself, point into wind, close the throttle and slowly bring the aircraft to a stall. Observe the GPS groundspeed. Do it again with half throttle, if the figure is lower do it again with more throttle until the stall (or apparent stall do to insufficient elevator effectiveness) speed doesn't get any lower.

 

Once you've discovered the power requirement for lowest speed before wing/nose drop, do it again pointing downwind. The average of the two groundspeeds is your stall speed at that air density, you can apply a correction for the ISA air density if you like.

 

Don't blame me that your plane doesn't stall at 40kts like the book said ... I'm just the messenger.

 

OK, so you didn't like those results so what's the second way? We'll calculate it instead, that should provide a more pleasing result shouldn't it?

 

Yesterday I provided a very handy spreadsheet, so lets fill in the boxes for a fairly typical LSA aircraft. You can all do this yourselves so no-one has to take my word for it. So let's put 600kg in the weight box, click anywhere else on the spreadsheet and see that the weight input then gives us 5886N (Newtons) for the mass. Leave the angle of bank empty since we want to know the level stall speed. Most LSAs have a wing area of 120-150sqft which is 12-15sqm so let's be generous and input 15sqm. A random click elsewhere shows us we have a wing-loading of 40kg/m^2, a quick mental check of 600/15=40 shows us the spreadsheet formula is working so far ...

 

Next we input the wing's CLmax, that's a number (co-efficient) describing the maximum amount of lift the wing shape can generate in perfect conditions i.e. smooth air, smooth and clean surface to the wing, accurately built wing. We get that co-efficient from a graph showing the Section's polars. There are numerous websites which will show the polars for any airfoil, here's an example showing the Clark Y section, but most of the sections used for LSAs have reasonably similar CLmax, at least not sufficiently different to make any significant difference to the stall speed, more so to the stall characteristics i.e. how kindly or horribly the plane behaves at the stall. The one we're interested in is the Cl/alpha graph, alpha being AoA. We can see that the max lift co-efficient is about 1.35, so we input that to the spreadsheet.

 

Good flaps will give you an extra 0.2 or so, so let's input 0.25 extra i.e 1.6 in the flaps box. Most flaps cover less than 1/2 the wingspan, but let's input 0.5 ( it doesn't actually improve the whole wing's CLmax as much as you thought does it? But it does significantly improve the deck angle and provides very handy drag as well as making it 'safer' because the inboard will stall much sooner than outboard, so less problems near the ground with wing-drops).

 

Input .89 for the Aspect ratio factor, as our planes do fall in the 6-8:1 aspect ration bracket ... click anywhere and you have a stall speed of 44.5kts. Hang on, I hear you say, the POH says 40kts. But don't forget the 40kts is indicated, not actual stall speed.

 

Well, at least our LSA is legal. Or is it ...?

 

There are few things we left out unfortunately ... first there's washout. Many, probably most, LSAs have about 3 degrees of washout to help with the stall characteristics, so at the 'stall' only the first part of our wing is stalling, the rest hasn't reached the critical angle. The average is 1.5 degrees less than the stall angle, so let's have a look at that graph again and input the figures for 1.5 degrees before the fully stalled wing. On the graph it all gets a bit messy near the stall but in practical terms you'd drop about 0.1 to 0.15, so let's call it 1.25 instead of 1.35. The flaps setting won't change but irregularities on the wing's surface and build inaccuracies, and less than perfectly smooth air will bring the actual stall onset a little earlier, so for those factors, and just for the exercise, let's input 0.15 less on the CLmax, making it 1.1 instead.

 

Oops, now our real stall speed has gone up to 48.6kts. well, that can't be true or the plane wouldn't be legal, so we'll forget about it because there must be some error in the software. I think we all get the point though?

 

Ah, before we close the spreadsheet, just change the top number to 700kgs and then 750kgs and observe ... 52.5kts and 54.3kts. Another variable we can play with is the wing area, so let's input 12m^2 instead of 15, to see what some of the smaller winged LSAs do - yikes 60.7kts, we'd better not go there ...

 

Back to your post Don - first, at all times let's keep in mind that we're not talking real numbers when we look at the ASI, it's just a reference that we can use in the cockpit, and it's way off anywhere near the stall. And we don't know how much it's off in any particular condition i.e. lightly loaded or heavily loaded, and we absolutely MUST NOT assume that the error is linear. If the POH is showing one knot indicated difference for 700kg, compared with 600kg, and we already know from the spreadsheet that the truth is 52.5-48.6=3.9kts, then it follows that the pitot error is exponential (actually more than that) at these high alpha, and that tells you that the sink rate near the stall is accelerating as the weight goes up, and that's a horrible recipe for disaster.

 

OK, now we know we have a problem with performance that's getting exponentially worse as the weight increases. You've flown your plane at 500kg and, as you mentioned with the 'first solo' analogy, the difference between that and 600kg is remarkable, surprising, quite amazing in fact. Now, given that things are getting exponentially worse, would you really want to fly it at 700kg. Do you think that the performance would only be 700/600=1.167 i.e. 16% worse than at 600kg? I hope the examples above amply demonstrate that it's a lot worse than that - very much worse.

 

The scary part is that, as you say, the increased speed for landing and take-off is fairly minor and on a cool day you might not notice all that much change, little enough to provide a rather unhealthy false sense of security in fact. The plane will actually cruise much nicer than usual as it doesn't react to the turbulence nearly as much as when it's light. More sense of security ...

 

You've mentioned the landing speed being the main noticeable difference and perhaps that's just what you personally tend to notice more than other things. For me it would be much more about the take-off. And take-offs, statistically, are much more dangerous than landings. I'd notice the much greater use of runway to get airborne at all, the much longer time and distance required in ground-effect to reach a safe climb speed, the very much poorer climb rate and angle, the lack of runway left ahead to land on if the engine quits early, the proximity of trees and buildings on climb that previously were way below me, the long time it takes to reach a safe height for turning back if I needed to. The long time it takes to reach cruise altitude, the high fuel burn rate causing me to unexpectedly use some of my reserves before the destination, the much higher speed I need in the larger circuit I fly at the destination, the stall warning going off at the base/final turn ... glad to be back on terra firma, I'll fly lighter next time.

 

Lastly - yes there's a huge trade-off with lift-enhancing devices (and even more so with a whole airframe designed for slow-flight capability) and cruise speed. You will probably get across country at 100+kts (130 indicated ;-) ) and I'l be doing so at 65 or so. My compensation is that I get to see more, land and take-off on 100m clear patch and I'll be having barra and oysters for tea, though I'll be late home if I get a few knots headwind ... In reality the (fixed) slats give about 6-7% increase of induced drag in cruise for the wing and that's all the wing drag increase when the flaps are retracted. The big drag increase is the large wheels (my tailwheel is about the size of your mains ...) and very long gear legs with long-travel suspension struts.

 

Hope all this was helpful to some. Oh, and apologies to Jj for the topic drift but at least it's keeping the thread 'live'.

 

 

[Guest Crezzi]

So, the Sling LSA registered as VLA with 700kg MTOW with same wings should still be under the 45 KCAS.

AFAIK the Sling isn't able to be registered as VLA as its not certificated to that standard. If it were, factory-built Sling on the VH register would already be able to fly at 700kg MTOW

 

Cheers

 

John

 

 

[coljones]

Don,I kind of disagree that stall speed change are not able to be felt by most pilots. They are able to perceive the changes but don't necessarily attribute that to increased stall speed. It may only be a few knots but the change is quite marked. An RV7 has a stall speed of 49 knots and it lands a lot faster than a jabiru.

wonderful language, this english. when you say "faster" do you mean "it lands at a greater velocity" or "lands sooner"?

 

 

[Head in the clouds]

wonderful language, this english. when you say "faster" do you mean "it lands at a greater velocity" or "lands sooner"?

I think he means it has less tendency to bounce i.e. becomes fast (fastened) to the ground better than a Jab does, doesn't he?

 

 

[facthunter]

I would think he means it is going at a faster speed when it lands. Common usage. Higher velocity if you want to be more scientific but it considers direction. I try to use the simplest explanation where possible. Less people do Science these days. Nev

 

 

[gandalph]

Hope all this was helpful to some.

Thanks HITC for your clearly explained exposition on the compromises and inconvenient truths we face when we fly. It was a delight to read and has been saved in my "useful flying stuff" folder.

 

 

[Nobody]

wonderful language, this english. when you say "faster" do you mean "it lands at a greater velocity" or "lands sooner"?

Sorry col, my grasp of the english language isnt complete despite my pilot licence professing me to be an expert in it. Perhaps thats why I ended up an engineer....

 

What I am trying to imply is that the difference in stall speed between a jabiru and an rv7 means that the approach speed is faster making them generally a little bit harder to handle. You have to be that little bit more precise to do a successful landing.

 

 

[turboplanner]

I don't know whether is just plain speed doing that; I make a standard approach at 70 kts, flapless at 75 in a Warrior, and get a much more precise result than I do in a J170.

 

 

[facthunter]

Extra speed will make you need more runway, so if the runway isn't more than adequate you will have to fly a more precise approach to stop comfortably. People who aren't used to short runways have difficulty flying precisely enough, if they have allowed themselves the comfort of flying the approach with excess speed all the time. This also results in the nosewheel loading wheel barrowing incidents that we see occasionally, due to touching down too fast. Nev

 

 

[turboplanner]

Agree if you are talking about different speeds in the same aircraft, and just coming in hot.

 

However, speed alone is not an issue; I'm landing at the start of the piano keys, and stopping just past the end if I use the brakes in the Warrior.

 

 

[SDQDI]

If you aren't stopping until just after the end of the runway Turbs you might have to push those brakes a bit harder, or close the throttle a bit more:whistling:

 

 

[DonRamsay]

Doesn't pay to leave the keyboard for too long as the conversation rushes along . . .

 

I appreciate the science from HITC as in the end only the science matters.

 

As IAS is all we can know, knowing at what IAS your aircraft at MTOW stalls is pretty useful info. Variations to approach speed to account for less take-off weight, hotter day, wind gusts etc need to be made to be safe.

 

The stall speeds for the Sling on the spec sheet are quoted as KCAS and, as they are for advertising purposes, I guess we should not expect too much science. However, the POH is a pretty serious document and if a manufacturer falsified that document they would be taking a risk of killing their customers. After all, almost the only person to read a POH is the owner/pilot.

 

But, not being a person of faith, I was keen to take my aircraft out with the local CFI and discover the stall characteristics at MTOW on an average sort of day - in the real world not just on paper. It was a fascinating day and the IAS numbers were reasonable compared with the POH. More interesting was what happened early in a stall and what happens at the extreme. I would never have attempted this stuff solo but with a CFI who was a very experienced and cautious test pilot it was a lot of fun.

 

In a two seat, side-by-side aircraft with Pilot, Pax and fuel sitting along the CofG and freight limited and close to CofG, balance is not really an issue. On a calculation of W&B, if I keep within the weight limit it can't go outside of balance specs either forward or aft.

 

 

[DonRamsay]

Probably the simplest way to modify for weight increase with same stall speed is to add wingspan. This requires full recalc of the wing structure but is pretty straightforward. It also ensures the plane will fly OK in a higher density altitude situation as well. I'm not sure I'm "easy" about upping stall speed. Landing OFF aerodrome is feasible with slow planes but becomes increasingly a problem when you are coming over the fence a fair bit faster.. Nev

When the Arion Lighting was being sold as experimental it had an MTOW of 650 kg. When they wanted to market a factory built LSA version they simply added wingtips with a more prominent upturned winglet. That got the stall within the 45kts Clean at 600kg MTOW that the USA LSA regs requires.

 

 

[DonRamsay]

AFAIK the Sling isn't able to be registered as VLA as its not certificated to that standard. If it were, factory-built Sling on the VH register would already be able to fly at 700kg MTOW

John, have a look at the Airplane Factory website and you will see their claim that they have done the testing to satisfy the Euro VLA requirements (which include 45kts stall in the landing config).

I know of at least one that is VH reg and has 700 kg to play with - but it was a kit build.

 

If the VLA standard was allowed in Australia and available to RAAus pilots my Sling could be reclassified with the manufacturer's agreement to VLA and be flown by me legally at 700kg

 

 

[jetjr]

OK, is there another method of representing the performance feature we are looking to limit?

 

HITC has provided excellent info on the topic and there are solutions, but complex and lots of variables. Which kind of says its not a great method for measuring a limitation across 3500 existing aircraft.

 

Using Jabiru 2/4 as an example again, many are taught to come over the fence at 70kts, stall warning is set nearer 55 or more - oh arent they all calibrated.... Id suggest very few get down to 50-55 kts in any case.

 

Bit faster makes for more repeatable landings, at slower speeds you quickly run out of normal controls and bad stuff happens fast.

 

Jab's arent a short strip performer but neither is their requirement very long. A poor approach and winds is going to make a much larger difference than 3kts capability.

 

Point being many are maybe going to exceed their personal capabilities before approaching real aircraft stall if attempting short field ops.

 

 

[Guest Crezzi]

John, have a look at the Airplane Factory website and you will see their claim that they have done the testing to satisfy the Euro VLA requirements (which include 45kts stall in the landing config).

I'm aware that the manufacturer has done "in-house" testing and are confident that the Sling should pass proper (i.e. independent") certification for VLA. That isn't the same as actually having VLA certification and a type certificate for 700kg.

The only category for which the manufacturer are allowed to self-certify factory built aircraft is LSA which are limited to 600kg.

 

I know of at least one that is VH reg and has 700 kg to play with - but it was a kit build.

The MTOW of kit-built aircraft is down to the manufacturer i.e. the individual builder so not relevant.

 

If the VLA standard was allowed in Australia and available to RAAus pilots my Sling could be reclassified with the manufacturer's agreement to VLA and be flown by me legally at 700kg

The VLA standard IS accepted in Australia but GA factory built Sling are still limited to 600kg because they are only certified as LSA not VLA.

The manufacturer can't just "declare" that an aircraft is now a VLA without an appropriate type certificate which the Sling doesn't have AFAIK.

 

On that basis, an increase in the weight limit for RAAus aircraft won't allow you to legally fly your aircraft at more than 600kg.

 

John

 

 

[Oscar]

Don, the figures for KCAS are required for certification / certifying purposes, to be accurate and validated - and the flight testing has to be carried out by an approved pilot to a a specific regime. I believe that you can - normally - trust them.

 

The POH figures will be for IAS: so they are what you fly to, off the instruments. However, there is (or should be) an IAS/KCAS error chart in the POH, so if working out e.g. adequate strip length for a hot-and-high-and heavy takeoff, you need to calculate from the KCAS figures and THEN convert them to the IAS corrected figures, for the ASI to actually tell you the correct information.

 

HITC's enormously useful information should be digested by all: but there is one additional factor that has not been covered.

 

That factor is, the actual elevator power available in the effect of wing downwash on the tailplane. That effect is not weight-dependent, but attitude-dependent. Speed makes a small difference to the impact of wing downwash on the elevator power, but in a flat, gentle approach stall, very little. ( In a dynamic stall - could be a VERY different reaction).

 

If you read the information supplied by Dafydd Llewellyn ( http://www.recreationalflying.com/threads/do-vortex-generators-really-work.117300/ - post #6) you will find that 'stall-speed' is often limited not by the airflow separation speed/AoA, but by the elevator power available in the wing downwash at a specific speed/AoA. In other words, if the elevator power is reduced by the wing downwash, you may not get to the the ultimate lift potential of the wing ( and hence the lowest possible stall speed) because the elevator cannot drive the aircraft to the peak airflow separation point AoA.

 

The Sunbird Seeker was required to be able to fly at marginally above the stall speed, held in a flat yaw of (I think) 25 degrees so the camera could be pointed at the powerlines. You can pull the Seeker back to just above the stall- using power to maintain height - and fly it around like a dodgem car: full rudder one way going to full rudder the other way- and it just bloody sits there and points itself at where you intended. I've done it.

 

That handling result was the end product of several years of work with VG's in particular, to balance the aerodynamic interactions. FWIW, I think it was early last year that the chief FAA 'Light Aircraft' directorate test pilot flew the Seeker and stated that it had 'perhaps the best stall characteristics of any aircraft I have ever flown' - and recommended to the FAA that they contact Dafydd Llewellyn so they could understand the principles involved. That's a pretty convincing endorsement of Australian expertise, IMHO.

 

And the point is: it is possible that the KCAS stall speed for your aircraft at the lower weight, is limited by elevator authority - NOT weight. The same elevator may have sufficient power ('authority') to pull the thing back to the same (or very slightly more) KCAS at higher weight for a very similar speed. In other words: you have an aircraft with a capabilty of achieving a KCAS at stall at 700 kgs, that could well do considerably better at a lower weight - but the elevator can't exert enough authority to exploit the lower weight, due to the downwash.

 

 

[facthunter]

The stall may be limited by elevator authority but such a plane is not pleasant to land. It is pretty safe however. Nothing I say here should make people fly slower approaches than they are comfortable with. A margin of speed is needed for safety. If you are consistently floating a long way or tending to land on the nosewheel, do some revision with an instructor and see if you can get a bit slower especially later in the approach . Speed control should be consistent. You can't really be looking at the A/S much under 100', but if power and attitude don't change nothing bad will happen. If you are unsure do it dual. Nev

 

 

[Oscar]

With reference to HITC's marvelous post: do manufacturers ( or their agents) 'not 'Lie, but just BS a bit' - to quote Elwood Blues.

 

A couple of years ago, one distributor of a particular aircraft that has sold some numbers here, had banner quotes on his site that listed the 'Stall speed' figure ( for the aircraft at 450 kgs. and the 'Cruise speed' (for the LSA version at 600 kgs). That 'Cruise Speed' was about 95% of VNE, and absolutely took NO reference to Australian gust conditions for Vra - and was, by owner report, about 20 knots over anything that was even remotely comfortable...

 

The brochure is intended to make you buy. The POH is what you should fly to.