Head in the clouds

No, I didn't state it doesn't and then it does at all. You stated that the elevator in one instance doesn't apply enough force to cause a change in direction, I pointed out it never causes a change in direction, it causes a change in ATTITUDE and the WING causes the change in direction. I haven't changed my stance or wording on that, try re-reading what I wrote ... the second time I said the ROTATION caused by the elevator, which is nothing to do with a change in direction. Of course the elevator applies a force, I never said it did or didn't, there wouldn't be much point in having one if it didn't would there? But the force it produces doesn't EVER change the direction of the plane, just the attitude of it. If we can put that to bed and perhaps stop trying to ridicule me, you might get around to the correct answer about why Va gets lower as weight decreases. Hint - contrary to what you stated much earlier, it has a lot to do with the stall ... and your heavily loaded plane example is a really good way to envisage the varying conditions that affect the Va speed. You are right, by the way, that the rate at which the plane reacts is the issue, but its not anything to do with the plane attaining a new direction, if it could attain the new direction the load would have reduced again by that stage so you would have survived the incident. It has to do with the plane TRYING to attain a new direction but not actually changing direction at all, so the wings do then tear off. Which is why the issue is so confusing to so many because, since that is the case, then you would think that a plane with more momentum (call it inertia if you like) would resist the change more than a plane with less momentum (i.e. a lightly loaded one). But - another hint - the heavier plane does resist the 'acceleration' but the wings don't tear off - so - why don't they tear off? Just read FH's post, and yes, what he's said is true i.e. also contrary to what you said earlier the wings don't get ripped off by the acceleration(s), they get ripped off by the force trying to accelerate the craft but the craft actually not being accelerated. But FH's correct post still doesn't explain why the heavy plane which is "battering its way through the turbulence" and providing a smooth ride for the occupants, doesn't have its wings torn off during all that battering, the wings must still be copping all that load mustn't they? Or are they? Under what circumstances could those wings not reach the limit load? (another hint).

Granted - but I hope we can agree that in the above very heavy plane example the heavy cargo would normally be loaded close to, or on the CG, about which, or very close to which, the rotation caused by the elevator would occur, and so the elevator can still effect pitch/attitude changes.

OK, I like your good example - allow me to be the Devil's Advocate and play the part of the student just learning the subject, if you will? So - we have this massively heavy plane. And you said "the elevator will NOT apply enough force to cause a change in its direction", but the elevator never actually applies the force to change the plane's direction, the elevator applies a force behind the CG to change the attitude of the plane, and, if the elevator is deflected upward, the underside of the wing is then presented to the slipstream i.e. it has increased its AoA (angle of attack) and the wing then tries to cause a change in the direction of the plane. Regardless of how heavy the plane might be, the elevator will always be able to swing the plane's longitudinal axis around its lateral axis and thereby increase the AoA since it only has to cause a slight change in the longitudinal balance to achieve that. So - since the plane (load in the fuselage?) is massively heavy and won't respond to the wings' pulling, why doesn't the wing just get torn off the fuselage? And - if the plane was really light instead, and flying at the same speed, why does the "reduce Va speed if the plane is lighter rule" suggest that the wing would actually get torn off or fail, even though the plane is really light and could respond to the wing pulling it upward, when logic would then suggest that the upward movement would unload the wing, because the plane would then be travelling in the direction the wing is pointing and so the AoA (and hence the load) would be reduced again?

Hi MM. Thanks for that, not very helpful to a thread on Va but obviously important to you ... yes it's been a long while but nothing has changed much and as I said some posts ago in response to FH - I see what you're getting at. However, weathercocking, secondary effects of controls etc are still a trivial matter and more advantageously discussed elsewhere. I don't know of any cases of either which have caused inflight structural failure but I could be wrong about that too! On a matter that is far more critical to flight safety, and relevant to this thread, do you perhaps have anything that might be helpful to people wanting a better understanding about Va at varying weights?

Sorry, but I can't agree with you. Among a lot of other matters, I always brief pax about what my actions will be in event of EFATO because I need them to know what to do according to what action I take, and that varies according to the height that the EFATO might occur. I certainly won't have time to tell them what to do if I do get an EFATO, probably only enough time to check that they're doing it. I'd agree that it can be disconcerting but if it all ended up in court because they have a brain injury and you hadn't told them to protect their head and/or that we might end up inverted, for one of many possible scenario, I think it might go very heavily against you. Also don't forget that a pax briefing is mandatory. What do you brief your pax about?

Hmmm - I've never seen a text that describes it other than adverse yaw i.e. yaw in the opposite direction from the roll, but then I haven't read every text on the subject ... Anyway, secondary effects of controls, weathercocking and whatever really has nothing to do with Va (Design Manoeuvring Speed). I started this thread because a solid understanding of the reasons for the Va limitations at varying weights is critical to not overloading the airframe and failure to do so can be catastrophic. Therefore I consider this to be a critical subject for flight safety. Adverse yaw, weathercocking and the like I would have described as relatively trivial by comparison but might be interesting to some, so perhaps someone could start another thread to discuss that if they want to? djpacro must have missed this discussion and has kindly also started a thread on the Va subject with some very good input, his post is also embedded here - It's well worth taking the time to read the contents of the link as they are the first info we have had posted here which describes the formula for determining the Va at weights lower than MTOW. In effect the formula is that the speed should be reduced from Va to a new Va which is the old Va multiplied by the square root of the lesser (new) weight divided by the MTOW. That article also provides a passable description of the reason for the reduced Va at lower weight but might still prove a little hazy for some of us. At the beginning of the thread I asked what succinct descriptions people used for the reason for varying Va in layman's terms without graphs and formula but so far we've only had graphs, charts and mention of inertia and accelerations. Is this all that students are provided with to reach an understanding of the subject? I certainly wasn't given anything at all better when I did my GA exams and it was a long while before I came across a more understandable explanation. The charts do tell a student about airspeeds to avoid at varying weights but they don't actually provide any understanding of the reasons or the physics behind it, do they? A couple of typical questions that students used to ask - How does the inertia (momentum?) affect the airframe and help it to cope with the 'acceleration' at higher weight? Wouldn't one suppose that if the airframe is more heavily loaded and at, say 100kts, then when a gust (or control input) produces a force on the wing, and the wing tries to pull the airframe upward, surely the heavier the airframe, the more it will resist the wing, so the load must be higher? If not, what prevents it from being higher than the lighter airframe at the same speed, surely the lighter airframe will 'go with the wing' more easily and so unload/reduce the force? Like a yacht and a catamaran perhaps, the yacht gets a gust and heels over and unloads the sail, the catamaran with the same size rig gets dismasted because it can't unload the sail by heeling to the applied force ... It's been stated above that the stall of the wing doesn't come into it. Is that actually true? The stall certainly isn't mentioned as any part of any formula regarding Va, so does that actually mean that the stall isn't a consideration in determining the Va at varying weights?

Yes, but you don't need to be below the top of the ridge, or even on the windward face of the ridge to be slope soaring. There's plenty of lift above the ridge and we'll out from the face, as you'd see at any hanggliding site. World altitude records in gliders/sailplanes were set in USA at about 30,000ft flying mountain waves which is a form of slope soaring.

Weathercock the plane? That's a new one on me, I'd better go back to theory school ... I know what you're getting at but ... anyway let's stick to Va eh?

Yes, yaw certainly does have roll as a secondary effect but I think you miss the point about roll not causing yaw. Roll (or rolling) doesn't cause anything except a bank angle. The adverse yaw is an aileron effect, not a roll effect. I.e. ailerons cause roll, they also cause adverse yaw - but roll, itself, doesn't cause yaw.

I don't think yaw is a secondary effect of roll, but it certainly is an effect of increased induced drag from the downward deflected aileron, and reduced induced drag from the upward deflected aileron, in most aircraft. There are also some aircraft with ailerons that are designed to increase their drag when deflected up, by having the nose of the aileron protrude into the airstream below the wing. On a few types this is sufficient to equal the drag induced by the down deflected aileron, and hence cancel any adverse yaw effects. This means that the aircraft will roll around the point without any rudder input. I seem to recall that the Cassutt II is one such example, though it may have been another of the Formula One types, the designer decided that the intentional aileron drag was less than the rudder drag while rolling hard to turn around the pylons, so the aileron drag was the lesser of two evils for a race plane. On the Va subject - I think there is some confusion between inertia and momentum. Inertia is the resistance of a body at rest to move, and relates to its mass only. Momentum is the resistance of a moving body to change direction and relates to its mass and velocity (Mass x Velocity). But - are we sure that momentum actually has anything to do with it?

I think this is a subject well worth further discussion so I've taken the liberty of starting a new thread with some thoughts previously posted elsewhere. Manoeuvring speed calculation is certainly one of the matters that students seem to have some difficulty getting to grips with, and many old and bold pilots still don't seem to have a clue what it's really about so they just fly below the yellow arc if it gets altogether too bumpy. Some while ago, while researching the crash of American Airlines Flight 587 which showed that the pilot can, in fact, cause a structural overload with use of the controls whilst well within the placarded manoeuvring speed, I came across an excellent explanation of the subject, by a complete layman, which even I could understand easily. Being able to envisage the structural loading has allowed me to treat the airframe in a more kindly manner whether lightly or heavily loaded, because each of those conditions presents separate hazards. I'll write up that explanation in a following post but I'm sure others have good ones too ... we're looking for succinct wording that newbies can relate to rather than graphs and formulae. Structural failure while airborne is rare but does happen from time to time so this is a pretty serious issue worthy of better understanding by all. Here's Mac McClellan's Flying Mag article which discusses the general conditions which caused the vertical stabiliser failure of Flight 587 and Peter Garrison's as always excellent investigative article about the crash, its causes and the Perils of Flying by the Book where Va is concerned.

But then - if you have the flaps deployed, apply the brakes heavily and shove the stick full forward all at the same time it does make it difficult to prevent the aircraft from overturning ...

I quite see where you're coming from, Teck, and you make some very good points, as always. To answer your last question first, sure there is no issue if I'm willing to flout the regulations. On the basis that if nothing goes wrong where's the harm, after all I'm away from everyone else and who's going to know whether I have my engine running or not? Well, actually it doesn't work like that at all. As well as a rec flying certificate I also hold a commercial GA licence and if seen to be doing the wrong thing, whether in an ultralight or a Jetranger the consequences are the same, I still lose my Licence even though I might have been flying on my Certificate at the time. As a commercial pilot I'm expected to know better, be more disciplined than the average Sunday flyer, and all that. And as for not getting caught, well I wouldn't want to count on that. I can tell you for sure that every time someone sees a powered plane flying around with the engine stopped they start talking about it and it wouldn't be unlikely that at some stage someone might record the registration and report it in good faith, perhaps thinking that the plane is in strife. So - if the regulations say I can't switch my engine off then I won't be doing so and the regulator will be responsible for reducing my flying skills because I can't then practice. It'll also reduce my enjoyment of my sport - and for what reason? Solely because there are others who have joined the sport and aren't willing to, or capable of, becoming sufficiently proficient to save their own ass when the fan stops. To return to the earlier part of your post - you said, "The vast majority of people reckoned powered approaches were the way to go". This I find very concerning because it is the very thing we spent so much extra time in the earlier days, in re-training people who came from GA to stop doing these powered approaches because it was so damned dangerous particularly with our 2 strokes of the time and even more particularly with people coming from GA to use those 2Ts who weren't fully versed with their peculiarities. You, Teck, of course, know well what I mean, their tendencey to stop if allowed to idle for too long, cool and oil up the plugs, either they'd just cough and stop or they'd die when you opened the throttle if you happened to need an extra burst of power on final. So - we trained people to avoid the risk of being garroted when hitting the approach fence by having a bit more height and/or speed up their sleeve, and/or get used to aiming a bit further down the runway than the piano keys. And learn how to slip ...! I had a couple of thousand ultralight/hangie/gliding hours when I went for my GA licence and they tried valiantly to get me to adopt their ridiculous powered approaches and also aim for the piano keys and also not exceed 1.3Vs. What a load of cobblers, and I told them so in no uncertain terms. I'm eternally smug to report that I later took my GA instructors and CFI to our ultralight school and in a short while they had adopted our methods instead and one of them is still instructing and teaches glide approaches with excess height and slips in GA. He's actually well regarded for his 'superior handling skills'. Sadly though, I see that the GA invasion into our sport has now become complete and even the instructors no longer control the procedures that are taught. I suppose we are to imagine that our engines have now reached the same level of reliability that the GA engines have? Does this mean no-one is flying 2Ts any more? All our 4T engine types are known to be highly reliable and very unlikely to stop? I think not. I'm not surprised that some people are concerned about what might be expected of them during their BFR. Before ultralight BFRs only instructors used to check each other out and I was sometimes asked the same thing. I used to just answer "nothing to worry about, no-one's ever failed yet". That tended to start the thing on a relaxed note but it didn't mean it was going to be a walk in the park, we were all typically very tough on each other. But - no-one did ever fail, we just kept doing exercises until we got it right. When I went for a check-ride about six years ago I think the instructor felt a bit intimidated because although very competent he didn't have many hours and after about 20 mins he said I was right to go. He was a little surprised when I said I didn't think so, we hadn't yet done any engine failure practice nor been quizzed about current rules and regs, or inflight emergencies, fire etc. He said it was OK, he was sure I knew what I was doing. Now that kind of thing might come as a relief to some people who weren't confident about their ability and are afraid they might fail but it certainly doesn't help them with being safer in the air, and that reduced safety is where this is all leading. The irony of it all is that some folks seem to be so afraid of failing yet I've never heard of anyone actually failing - anyone know of someone who has? Come to that, does anyone not know of someone who they reckon should have failed, or at least been given a bit more helpful training rather than a rubber stamp job? I'm not pointing any fingers by the way, and mostly my examples refer to GA rather than Recflying.

Yup, I guess that's right, we must all be reduced, by legislation, to the level of the least competent of all - why not eh? That's a far better plan than legislating to raise the average standard of training ...

No, it's not really a good point in my view. If you're doing a normal landing with power available you may well be aiming for the piano keys and if you don't have excess height or speed available you may well need to use power to make your aiming point. That's not considered to be good airmanship in my book. If it's windy you should be allowing for wind gradient - which most people mistakenly call wind shear - and if the conditions are unstable you should be expecting wind shear. If either of those conditions are prevailing you should aim a little further down the runway than the threshold or alternatively have excess speed or height available and slip to your aiming point on the threshold. Any need to "instantaneously firewall the throttle" to avoid being 20ft below the threshold is simply poor planning and poor reading of the conditions - no offence meant, we all make mistakes and this is the very reason for practicing with engine off where you're forced to think more carefully. BUT - you don't have to pay any penalty for getting it wrong when you practice engine off forced landings, any more than than when you practice with the engine idling, because in any forced landing practice you aim 1/3 of the way down the runway and that gives plenty of margin for error or adjustment. AND - this whole engine off thing - who says that when you switch the engine off you always plan to be practicing forced landings? Much of the time when I go slope soaring or thermalling I restart the engine and continue the flight, to the next thermal perhaps, or to the destination, so why should that be outlawed? It doesn't involve anyone else, or the traffic circuit, it's just a part of the fun of flying. I have no plan to make a landing but I have selected a suitable outlanding in case I don't get a successful restart. As far as being hairy chested with the subject FH, no I don't think so. I don't see it as anything other than having more fun flying and developing more competence. The hairy chested ones are the ones who go flying even though they're still terrified of the day they will have an engine failure, that takes real guts!

https://www.youtube.com/watch?v=4_t5Z-YjwEs

Back to the 'whether we should, whether we shouldn't' debate - frankly I couldn't give a rats whether some people want to limit their skills development or not. From my point of view I want to be able to practice so as to retain my skills and improve them further. Since many people seem to feel that they don't want to have the liberty to stop their engine - don't forget no-one is forcing them to do so anyway, and no-one is saying it should become a mandatory part of training - then to protect the less experienced from themselves perhaps there should be an 'engine off' endorsement? This is recreational flying after all, as Ozzie pointed out so well. Judging from some of the comments though, the major problem might be the distance between instructors willing to stop the engine to issue the endorsement ...

I'm wondering if there is a typo in the above proposed rule because it doesn't make sense to me. As written it indicates that it'll be OK to shut your engine down if your engine doesn't have a starter, like some VW powered homebuilts, Tiger Moths and the like. So - you could shut the engine down if you have no means of restarting it, but you can't shut it down if you do have a starter, when all you would have to do is press the button to restart it. Does this make sense to anyone? And while we're on the subject - all these folks who are so much against shutting the engine down - are they aware that all it takes with the more modern 4 strokes that most planes have now, is to just press the button, or turn the key, to restart the engine - so what's the big issue?

I don't want to hijack this thread with irrelevant information but there's something interesting that some folks may not have seen before, so I've created a new thread with a link below. This was prompted by Deborah's comment above and MotzartMerv mentioned it earlier - that an F15 was flown having lost one wing. Not only was it flown but the Israeli Airforce pilot Zivi Nedivi had a midair collision during a dogfight, started spinning but recovered using afterburner and then flew the aircraft back to base. Although the handling was marginal for control he landed and didn't realise he only had one wing until he opened the canopy. This took place nearly 30yrs ago. Sounds like BS? Here's the link to the other thread where the History Channel video is posted.

To avoid hijacking another thread here is the video of Israeli Airforce pilot Zivi Nedivi flying and landing an F15 with one wing after a collision during a dogfight -

Jeez Teck, talk about people niggling ... maybe it was 2 litres, it was just an example that's all. I think an 0-200 or whatever they have uses about 20lt/hr in the cruise, say 30lt/hr in climb? With one PoB and half fuel a Pacer/tripacer would have a climb rate of about 1000ft/min I would guess, so that's 2 mins to 2000ft at 30lt/hr = 1lt. Maybe the climb rate is 700ft/min and it's burning 36lt/hr at WOT - that's 3 mins to 2000ft and therefore uses 1.8 lts. With an A65 engine or whatever I think the consumption would still be about the same. My point was it's still a miniscule cost to go and fly for an hour or two isn't it? And - working a thermal occasionally is not only skill-building because you're flying at best L/D close to the stall in turbulent air, but it's also more fun than just trawling around the usual local triangle just for somewhere to go. I don't think it's wrong 80kts. I say more people should enjoy it and get familiar with their craft's gliding characteristics. Well done Tom, that's one I was referring to. He's made a number of similar videos. Note the 'bowl' on the panel, he puts a tennis ball in it in other vids and flies around balancing the ball up on the edge of the rim, one side then the other - as well as keeping it dead centre when he wants to of course. It's a long time since I saw it so I hope I got some of the details right. There's a very good reason for tapping the altimeter. It's because it won't move smoothly when you're gliding so you won't know if you're gaining or losing height. Just like Grandma's wall barometer which she tapped every morning to see what the weather will be doing. When the plane's engine is running the engine vibration serves as a constant 'tapper'. Glider pilots use a visual and/or audio variometer to know about instantaneous lift or sink but many of us are still altimeter tappers too ... EDIT - just watched his great video again, his strip's at 6000ft, he switched off at 8500 and climbed above 10,000ft, so 1500ft height gain. Note how comfortable he is flying without power and no problem with steep turning to position for a spot landing - and no likelihood of a stall/spin final turn. If only more folks practiced ... IMHO

Yes ... This engine off thing is clearly a very emotive subject, every time it comes up in discussion, regardless of the pretext some people get a bit heated about it. The thing is - some people want to be able to practice their engine failures in a fully representative manner i.e. with the prop stopped and the 'eerie' silence and less responsive tailfeathers and all that, and some people don't want what they perceive to be 'added risk'. The argument is a little similar to the spin training thing - some want to be able to get out of any situation and others would rather just learn how not to get into the situation in the first place. Unfortunately though, we can't guarantee that we won't have a real engine failure sometime or other. There is another aspect though - some of us came from a gliding background, hang-gliders and/or gliders/sailplanes and I, for one, just enjoy switching off and gliding around for a while. When I want to fly I don't always have somewhere I want to go and visit and am very happy stooging around the slopes with engine off and trying to make a little height gain, or travel a distance across country 'free' or getting out on the flatlands and working a thermal. I once had a height gain of around 6000 ft in a Drifter and at a rate of over 2000ft/min. It was a hell of an exciting and bumpy ride too, in among the hawks, dust, leaves and insects. A good incentive to keep your mouth shut or get bugs in your teeth. I can't find it right now but I'll keep looking, there is (or was) a video on Youtube of a wily old fella in the USA midwest and he loved to take his Piper Pacer/Tripacer (I think I recall that's what it was) up on a hot afternoon. He'd use the engine up to about 2000ft AGL and then switch off and IIRC the video showed him thermalling up to about 14,000ft and then gliding back down to a landing. It was a really inspiring video, he worked and worked really hard for every foot of height gain and his pleasure at every gain on the altimeter was so evident. He'd have an hour or two of flight and use only a litre of fuel. I'll keep looking but if anyone has a link to that video please post it here.

That's a super cool fan Nuzza! Just a word of caution though, the blades appear to have a lot of pitch. An acquaintance of mine in Darwin thought he'd make a ceiling fan blow more air so he twisted the metal fan blades to give them more pitch. It resulted in the fan running slower than it should and as a result it overheated and caused an electrical fire ... I'm not an electrician but it has something to do with the AC frequency and the number of 'poles' of the motor determining the speed of the fan. I think ceiling fans should run at 1440rpm on the fast setting with no load so with the blades on providing a load and a bit of 'phase slip' it should be around 1400rpm I think. Much slower than that and the heat builds up. As I said I'm not an electrician and have probably used the wrong terms but it might be wise to check, it'd be a mongrel to burn your plane down. Also - on second look, did you intend for the fan to blow air upwards rather than downwards? Or is that the winter setting?

Hang on Matt and Ozzie, I've got the greatest respect for both of you, so don't want to see you at each others' throats ... Matt, actually I feel that Ozzie has a point, I don't ever recall a time when SAAA didn't want anything to do with ultralights, in fact they were exceptionally supportive and increasingly shared their Easter Mangalore event with us as the ultralight movement grew. By 1983 it was becoming eminently clear that we weren't going to be able to continue to operate under the exemption of 95.10 issue 1 without some kind of self-administered regulatory body to ensure that people were actually sticking to the requirements, which, to be fair, virtually no-one was! We were all building planes that were either overweight or exceeded the permitted wing-loading, or both. You pretty well had to, to make the structures safe. There were some nice planes but ... SAAA and GFA were both approached about helping us with this new 'requirement' and GFA didn't want to get involved, they (correctly in my mind) were very puristic about competition gliding, regularly hosting and winning the world championships, and didn't want any hiccups or headaches. SAAA gave us a lot of assistance in the form of advice, introductions to people who were knowledgeable about the required processes to set up a new organisation, and support in dealings with the DoA, as CASA was then. SAAA wasn't in a position to take us under their wing because they were, as has been mentioned, an organisation solely oriented toward Amateur Building of aircraft and had nothing to do with flight training or licencing or overseeing regulatory matters as all their aircraft were VH registered and required a GA licence, as is still the case. In 1983 a large part of the SAAA's Gala Dinner was set aside for the inauguration ceremony of AUF complete with then Minister for Aviation Kim Beasley and that year and subsequent years the prize-giving ceremony was extended to include the AUF's participants for longest flights to attend and other competitions. Because of the fundamental differences between the RAAus and SAAA I agree with Matt and can't see that there would be any practical aspect of merging the two organisations but I could imagine that the two could work together much more closely. It's tragically evident that Amateur Building is very much on the decline in SAAA, our local chapter closed a couple of years ago due to lack of interest/members. Even so there is a strong demand for cheaper flying and, like it or not, to do that you either buy second-hand or build it yourself. Self built is certain to work out cheaper in the long run if the L1 maintenance thing is scrapped, as seems likely. Sharing a National fly-in event again would seem to me to be the best way to kick off a new era of mutual support between the two organisations and could well lead to a sharing of facilities, office staff and aviation-related promotions that could benefit both Associations as well as sport aviation in general.

Mine is a Shorai LiFePO4 Lithium Iron Phosphate battery model LFX18L1-BS12. This is a Lithium Iron battery not a Lithium Ion or Lithium Polymer battery. Lithium Iron batteries are non-toxic and non-flammable and therefore safe to use in a plane. I wouldn't suggest using a Lithium Ion or Lipo in a plane as they can catch fire if charged too fast, discharged too fast or are short circuited. They're not cheap but have a few advantages over lead-acid, gel or chloride batteries. Mine weighs just 1kg instead of the usual 7kg, it's 18AH with 270CCA, has a carbon fibre MilSpec casing and is very small. The very low weight is an advantage in a plane but only if it doesn't upset your W&B, great if you're building a new plane and can move things around to re-balance everything and get a weight saving at the same time. If you have a large alternator they can also be charged at a much higher rate than other batteries, in batteries up to 30AH the charge rate is the same as the AH rating i.e. an 18AH battery can be re-charged at 18 Amps. Disadvantage is the cost, at $250. The Rotax manual says to use a battery of at least 16AH and you mention yours is 20AH but only 165CCA. That seems a very low capacity for 20AH, most 18AH batteries seem to be about 300CCA and even the Yuasa 14AH is 230CCA so I think perhaps the problem is the quality of the battery you have rather than the AH size. In general thicker plates means more CCA, and a heavier battery. Mine is a new installation so I can't say how well it cranks the engine but others who are using them seem to be very happy and say they provide much livelier cranking and starting than their previous batteries.