It is true...

[Guest Baphomet]

A couple of points.

 

1. It is a fantastic aircraft - and it's designer should get a lot of credit. (whatever the numbers)

 

2. Something IS happening, most likely a calibration issue in the IAS

 

3. If the data is correct, then Gary needs to be nominated for the Nobel prize in physics. You must understand, what has been claimed to have been achieved is the Holy Grail of light aircraft design, and the likes of Cessna, Piper etc have thrown millions of dollars of research at the problem over the years.

 

4. A 'seat of the pants' appraisal of its performance however well intensioned, is worthless, we need imperical data.

 

Baph

 

 

[Spin]

A couple of points.1. It is a fantastic aircraft - and it's designer should get a lot of credit. (whatever the numbers)

2. Something IS happening, most likely a calibration issue in the IAS

 

3. If the data is correct, then Gary needs to be nominated for the Nobel prize in physics. You must understand, what has been claimed to have been achieved is the Holy Grail of light aircraft design, and the likes of Cessna, Piper etc have thrown millions of dollars of research at the problem over the years.

 

4. A 'seat of the pants' appraisal of its performance however well intensioned, is worthless, we need imperical data.

 

Baph

Well put Baph,

 

I don't want to belabour the point, but this sort of argument has gone on around flying club bars since Pontius was a student pilate. I have heard grizzled old pilots who really should have know n better, talking about landing C206's with "nothing on the clock but the makers name, needle bouncing off the bottom stop" That may well be so, but even Mr Cessna says it actually stalls at 52kts. If I'm not mistaken it's referred to as position error and even wikipedia gives a fairly reasonable explanation of the phenomenon.

 

 

[Ballpoint 246niner]

No I won't change! After the Jab it's a Piper sport- STOP- you won't make me change.

 

Good one tomo, sorry I missed getting there.

 

Cheers.

 

 

[winsor68]

It is critical to stick to facts . Mere speculation can damage a reputation that may not deserve such denigration. It is also a recipe for legal action.

Bit late I think Blackrod... Speculation meter already off the scale... lol

 

 

[Tomo]

Clarification

 

Ok, so I've got some details now -

 

Stall as to spects - full 544kg mtow, 32kts with flap, 37kts clean, 22 kts pulling nose up at 32 kts you can still control the plane.

 

To ask or query about these aircraft - You can now go Here

 

 

[Spin]

Thanks Tomo, pity I didn't see your post earlier, I have just put the same questions to Garry. Really looking fwd to having a closer look at one of these beasts sometime.

 

 

[Tomo]

You've probably already seen it, but it's relevant to put it here I guess.

 

 

 

[Guest russ.mullins]

I am also a cheetah owner and whilst I don't have the technical knowledge to quote how the cheetah's manage a 20 kt stall, all I can say is that I have tried to stall mine quite a few times to simulate an emergency, and all I have been able to do is make it sink.. The plane will not do anything nasty like drop a wing or fall over on its back. At nil throttle and full back stick doing 20kts my cheetah just starts to sink and in a very gentle and manageble way. I've done GA training in Cessna's that have thrown themselves over on their backs and done all sorts of scarey things but these Cheetahs are just so stable is difficult to comprehend. I tried pretty hard to convince myself not to buy a cheetah but after trying a few other very nice planes (and if I'd had the money I would have gone for a Brumby) when it comes to bang for your buck the cheetah's are hard to beat, I can trim mine to cruise completely hands off at 110kts without any use of the rudders or stick and it just flys along brilliantly, and its definitely less effected by turbulence than the other small planes I've flown. Whilst I wish it had doors like the jabiru, and a wing over my head on a hot sunny day, I can't fault the flight characteristics of the cheetahs, by far and a way the easiest most predictable RA plane I've flown.

 

 

[Yenn]

They are a great plane and as Tomo must have seen they are easier to taxi than a Corby. I don't know the wing area, but it I did I would be computing the Coefficient of lift at the quoted stall speed to see if it was reasonable and anything over about 1.5 would be questionable. I can vouch for the good flying characteristics after having a flight in one. I nearly bought one and may still come to regret that I didn't.

 

 

[sain]

Stall as to spects - full 544kg mtow, 32kts with flap, 37kts clean, 22 kts pulling nose up at 32 kts you can still control the plane.

Thats a little more reasonable and makes sense if you apply ye-olde 1.5*stall=landing speed to what people were reporting as the "over the fence" speed.

22kts pulling the nose up doesn't really mean squat - as others have mentioned the pitot tube is in turbulent airflow at that point and wont be giving an accurate reading.

 

Very nice aircraft design from what little i've seen of it.

 

 

[Guest Baphomet]

The truth is finally revealed, I'm not going to say "I told you so..." but if I've learned anything in 57 years, its that if it sounds too good to be true, it probably is. I'm not concerned about the 22knt figure on the web site, that could have been a typo. (I hope it was a typo, if not it was a deliberate attempt to mislead, and I don't want to think about what that may mean). What does concern me, is the near religous fervour some people have shown regarding this aircraft. So much so, that university educated individuals are prepared to suspend logic and healthy scepticism and use spurious arguments (you can't possibly know, you haven't flown it) in a headlong rush, to support it. It appears to be a good aircraft, but the numbers indicate its no better than a lot of other good aircraft. Value is another issue, and very subjective.

 

Baph

 

 

[Tomo]

I'm not going to say "I told you so..."

You sure?

 

What does concern me, is the near religous fervour some people have shown regarding this aircraft. So much so, that university educated individuals are prepared to suspend logic and healthy scepticism and use spurious arguments (you can't possibly know, you haven't flown it) in a headlong rush, to support it.

20kts or not, take your GPS, find some place where there is no wind - calibrate it with the ASI to ensure. And do your own checks. It'll blow the hat off your head, no joke. 22kts, or 32kts...

 

It appears to be a good aircraft, but the numbers indicate its no better than a lot of other good aircraft.

I say it is. At least it isn't out to kill.:thumb_up:

 

 

[Yenn]

Could someone who knows the numbers please work out the Coefficient of lift at the quoted stall speed, or give me the wing area, so that I can work it out.

 

 

[Guest Baphomet]

" Lack of credibility in the specifications works against the good design features and to reasonably educated aircraft purchasers, unrealistic numbers makes you walk away"

 

You've hit the nail on the head David, the design appears to be a good one, and shouldn't need 'shonky' data to support it, now I find myself wondering how the VNE figure was arrived at, and what about the load tests?

 

As it happens, a friend of mine is building a Cheetah, and I drop by every now and then to see what progress he's made. I'm not completely ignorant of the product.

 

Baph

 

 

[Guest Baphomet]

Could someone who knows the numbers please work out the Coefficient of lift at the quoted stall speed, or give me the wing area, so that I can work it out.

The wing area is in the specs on the web site Yenn, now you have to decide if it's accurate.;)

 

Baph

 

 

[Guest Baphomet]

Actually, this whole saga reminds me of an old joke which (slightly modified) goes..

 

Question: What's the difference between an aeroplane salesman and a used car salesman?

 

Answer: The used car salesman knows when he'e lying.

 

Baph

 

 

[Tomo]

Tomo physics is physics, how can you sit there and attempt to vindicate these very low figures when the wing has no special bits

Have you checked that? I've given the contact details - you can give him a call - email, whatever. Go for a fly... do what you have too.

 

But I don't really feel I need to argue this any longer, I'm not a salesperson for anyone. I just think it amusing how all those that haven't actually even touched one, or been in one... can sit here arguing. Go and do it! Then you can come back here and say... See I told you so!

 

Physics is physics exactly.... more flap more drag = STOL aircraft.... who said the Sierra was a STOL aircraft?

 

Remember there is a difference between STOL and Stall!!

 

Sorry, It's late and I'm not normally like this... but seriously talk to Garry if you want to argue about it, and if you are more than just an arguer, go for a fly and try it.

 

Ok, I'm going to bed now

 

 

[Tomo]

Just as a matter of interest, from memory the Puchacz glider has a stall around the 32-39kt mark I think. Hows it do that? It doesn't even have flaps!!

 

It is possible folks. Just have to think "aerodynamics" ;)

 

Baph, regarding VNE, it's obtained just like any other aircraft you sit in.

 

 

[Ultralights]

you will really love the savannah then, stall comes at an indicated airspeed of just 1 kt!

 

 

[Tomo]

you will really love the savannah then, stall comes at an indicated airspeed of just 1 kt!

Good one Rob - :thumb_up::thumb_up::thumb_up:

 

 

[jordy]

Have you checked that? I've given the contact details - you can give him a call - email, whatever. Go for a fly... do what you have too.But I don't really feel I need to argue this any longer, I'm not a salesperson for anyone. I just think it amusing how all those that haven't actually even touched one, or been in one... can sit here arguing. Go and do it! Then you can come back here and say... See I told you so!

 

Physics is physics exactly.... more flap more drag = STOL aircraft.... who said the lightning was a STOL aircraft?

 

Remember there is a difference between STOL and Stall!!

 

Sorry, It's late and I'm not normally like this... but seriously talk to Garry if you want to argue about it, and if you are more than just an arguer, go for a fly and try it.

 

Ok, I'm going to bed now

 

 

[Guest Baphomet]

You are wasting your time David, Tomo (and others) are having a great deal of trouble understanding the concepts involved with this.

 

I refer all of you to Airspeed (xhtml w3c 12/09)

 

Pay particular attention to the section on claimed stall speeds. here it is

 

• Checking validity of claimed stall speeds. There is a simple method to check the validity of published stall speeds. Practically all very light aircraft (except those with single surface wings like the Wheeler Scout or weight-shift aircraft) use simple, long proven, standard camber aerofoils to form the wings. The lift coefficient attainable at maximum aoa with such wings without flaps is about 1.2 or 1.3 for faster-cruising aircraft, and 1.5 or 1.6 for the slower, higher-lift sections. If equipped with flaps over, say, half the trailing edge, then CLmax might be increased by 0.5 when the flaps are extended to at least 35°. When other high-lift devices (for example, full length leading edge slats/slots) are added to the wing, then CLmax might increase 0.6. Thus, a specialised short take-off and landing aircraft fitted with a high-lift aerofoil, full-length leading edge slats and large extended flaps would have a CLmax of (at least) 1.6 + 0.5 + 0.6 = 2.7.

 

The lift equation at normal stall speed is:

 

Lift = CLmax × ½rV² × S = weight

 

or re-arranged:

 

CLmax = weight / (½rV² × S)

 

We can use that equation to check the validity of stall speed claims if we know the maximum take-off weight [MTOW] and the wing area . Let's say a supplier claims that an aircraft, lacking any high-lift devices, has a stall speed of 30 knots. The MTOW is 450 kg and the wing area is 12 m².

 

In the equation, the weight must be expressed in newtons — so multiply kg × 10 = 4500 N; and the stall speed must be expressed in metres per second — so just halve the velocity in knots = 15 m/s: the air density used must be the ISA msl density = 1.225 kg/m³.

 

Thus CLmax = 4500 / (0.5 × 1.225 × 15 × 15 × 12) = 2.7

 

A lift coefficient of 2.7 is very much higher than that achievable without high-lift devices, so you would conclude that the claimed stall speed is nonsense; a figure of 38 knots is probably closer to the mark.

 

• Conversely you can do a rough approximation of stall speeds using the following simplified formulae if you (1) know the wing loading in kilograms per square metre or in pounds per square foot, and (2) can estimate CLmax with flaps stowed or fully extended.

 

Stall speed [knots] = 7.8 × square root (wing loading in kg/m² divided by CLmax)

 

(or)

 

Stall speed [knots] = 17.2 × square root (wing loading in lb/ft² divided by CLmax)

 

Tomo said "It is possible folks. Just have to think "aerodynamics"" this is a very telling statement Tomo, and makes it clear to me that you really don't understand what is going on. Aerodynamics (reducing drag) has a big effect on max speed but almost no effect on stall speed. Flaps lower the stall speed because they

 

1. change the shape of the wing and raise the coefficient of lift or

 

2. They increase the wing area, both raising the coefficient of lift and lowering the wing loading.

 

I suggest you put the formula for Coeficient of lift into an Excel spreadsheet and put in the numbers for wing area found on Gary's site. You will be supprised by the results.

 

Tomo said "Baph, regarding VNE, it's obtained just like any other aircraft you sit in."

 

VNE can be obtained either theoretically or experimentally, nothing on Gary's site to indicate which method has been used.

 

Appologies for the long post.

 

Baph

 

 

[Guest Maj Millard]

Please keep in mind that ANY airspeed indicator, unless it is a very expensive Helicopter one, or one fitted to the space shuttle, will often be inaccurate at the high and low ranges, and will be optimized for accuracy, at one particular range. The last time I did a comparison check on the altimeter fitted to my aircraft, which is typical of what we use in ULs, it was found to be most accurate in the 55-60 Kts area, and became quite inaccurate at higher speeds and even more inaccurate at the lower end. The error at the low end was in the 9-10 Kt area.

 

I would take any speed claims in the 20 Kt area with a large grain of salt !!. It may not be the manufacturers' fault, he's just reading what the needle is telling him, and may be oblivoius of the above.

 

I can say however that on a few occasions in the past, I have seen the needle parked on the lower stop (below 20 Kts) with the aircraft still flying....but it was in a Slepcev Storch with full flaps !!.......................................................................................Maj...

 

 

[Tomo]

Thanks Baph, and others.

 

But if you go back to the beginning a bit - I stated I didn't really care what it stalls at, I like aeroplanes that fly - not stall. Regardless of what the "numbers" are the Sierra is mind boggling - I know cause I tried it. I seriously encourage each of you to do to so, instead of sitting here bashing each other around. I'm Not sure what the Cheetah is like though as they are different.

 

Another thing that you may have all forgotten, it is a '19' registered aircraft at the moment - experimental. Not sure what your dictionary states that to mean, but mine says this: (a new invention or product) based on untested ideas or techniques and not yet established or finalized

 

Arguing is pretty pointless then isn't it?

 

Wait till the certification is over, and see what they put down for the numbers then. You can then argue with the certifiers over it.

 

I do understand about aerodynamics btw - and forgive me if I'm wrong, but aerodynamics has a lot to do with stalling!! Because if it weren't an airfoil of sorts it wouldn't ever 'un-stall' = fly or stop flying = stall. ;) An airfoil or aerodynamics just defines the way of how it goes about doing it efficiently.

 

 

[Guest Baphomet]

Tomo, there isn't anything personal in this, or directed specifically at you. My position is this. If someone turns up at a fly-in with a new design, they have built for themselves as a one-off, and claims rediculous performance figures, I really couldn't care less. If however, an aircraft manufacturer does it, I take it very seriously. If it turns out that the manufacturer has delberately 'guiled the lilly' so to speak, to gain a competetive advantage, then it should be exposed. Make no mistake about this, quoting a stall speed of 20knts for this aircraft has the potential to kill someone. In defense of the manufacturer, he isn't the one making the claims (apart from the web site, and he needs to chage the reference) in fact his response on this forum about the stall speed lists numbers that are more believable.

 

You are right, we should be waiting for the certification process to run its course.

 

Baph