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Why does the MTOW increase with floats?


Vev
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Why is it, if one fits floats to an RAA ac we are allowed an extra 50kg in MTOW?

 

Are the floats considered lifting bodies?

 

Doesn't this compromise the airframe structure strength?

 

Cheers

 

Vev

 

 

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Not 100% sure but floats allows increase in regulatory MTOW, not mtow of the airframe, it must have that capacity already. So no structural problem.

 

Same for BRS i think

 

Im guessing stall must still be 45kts

 

 

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Thanks JR .... You make sense about the structural strength.

 

However I'm still unclear why we can have a 50kg increase just because these are floats? What's the rationale behind this MTOW increase?

 

Cheers

 

Vev

 

 

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The real question is why the regulatory MTOW isn't the same as the structural MTOW. I understand there has to be a line in the sand somewhere, but gimme a break. 150kt cruise, yep that's ok. CS prop, yep that's ok too. Oh you want to fill up your fuel tanks? Sorry, you'll need to paint a VH on the side to do that.

 

Why not just have a list of approved aircraft? There's not that many!

 

 

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I don't get it either Nick.

 

Do our aircraft become more dangerous or we become less competent with 50kg more, the rules says not, that is, so long as these are floats or a hull.

 

I'm not getting the point here ... Really I'm not!

 

Cheers

 

Vev

 

 

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Thanks JR .... You make sense about the structural strength.However I'm still unclear why we can have a 50kg increase just because these are floats? What's the rationale behind this MTOW increase?

 

Cheers

 

Vev

As a guess. The aircraft must be able to operate within its structural limits at the new weight. The added weight is a concession to the upper limit. If the floats were added and the plane was constrained to the "ordinary" all up weight then you might have to reduce the weight by getting rid of something eg passengers, cargo, fuel, oil or engine. But there might still be a temptation to fuel it up, load it up and fly outside the structural limits so a concession has been made for float planes but only if they meet all the rules at the higher weight.

There has been an argument that since some planes have an envelope greater than 600Kg they should be allowed to operate there - the answer, of course, is - that is what GA is for.

 

 

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Personally I think it is a dumb rule. It shouldn't matter whether the undercarriage has wheels, floats, skis or tracks, so long as the air frame limits are not exceeded or compromised. The weight limit should be 600 kgs or 650 kgs for all, not different just because you land on water rather than terra firma. Any fool can overload an air frame no matter what the rules say or what the air frame limits are. Provide me with some genuine logic to the rule & I will bow to that.

 

 

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The real question is why the regulatory MTOW isn't the same as the structural MTOW. I understand there has to be a line in the sand somewhere, but gimme a break. 150kt cruise, yep that's ok. CS prop, yep that's ok too. Oh you want to fill up your fuel tanks? Sorry, you'll need to paint a VH on the side to do that.

If you want to blame someone it's not RAAus but rather the manufacturer of the aircraft.

 

They sell you an aircraft that doesn't fit within RAAus rules by making you fly with half empty tanks to squeeze it into those rules.

 

As to

 

Why not just have a list of approved aircraft? There's not that many!

As far as I know it RAAus was founded on the principle of home-built if not home designed aircraft (I might be wrong, I've only been a member for 2 years), so having a comprehensive list for all aircraft, some of which may be only one of their kind, would be a bit too complex.

As to the allowance for extra weight for BRS or floats - I don't understand it either.

 

 

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Guest Nobody

MTOW of a particular design can be limited for many reasons. It might be based on the wing structural loads or it might be based on the undercarriage strength or it might be based on the climb performance or some other requirement.

 

For example, in the certified world, when on floats the required climb performance is less.

 

FAR 23.65 reads in part

 

Each normal, utility, and acrobatic category reciprocating engine-powered airplane of 6,000 pounds or less maximum weight must have a steady climb gradient at sea level of at least 8.3 percent for landplanes or 6.7 percet for seaplanes and amphibians

The theory is that a seaplane has greater choice in choosing to takeoff into wind, generally operated at lower density altitudes and also is less likely to have obstacles in the takeoff path.

 

The undercarriage loads when opersting on land are given in Far 23.471 to 511 the water load in 23.521 to 23.537. When worked through the loads are typically less for the float plane than the land plane.

 

The consequence of these is that if MTOW is not governed by in flight wing loads then the design can have a higher MTOW on floats than off.

 

 

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Plus, if you have a look at FAR 23 Appendix B, you will see:

 

For a seaplane version of a landplane, the landplane wing loadings may be used to determine the limit maneuvering control surface loadings (in accordance with B23.11 and figure 1 of Appendix B) if--

You'll probably have to go back to the old Civil Aeronautics Manuals to get the reason for the reduced loadings. http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgCCAB.nsf/MainFrame?OpenFrameSet

The wing still needs to be substantiated for the higher weight.

 

Water loads may be less than landing loads however I'd expect the floats to have at least one different set of attachments so fuselage strength may be an issue locally.

 

 

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I'm not in anyway learned in things aerodynamic but it does seem logical to me that floats provide added lift to a flying aircraft....after all they provide lift and get the aircraft onto the plane as speed builds up in the water. Water and air are both fluid mediums albeit with different viscosities.

 

Kaz

 

 

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I think the MTOW limits on RAA regulations (nothing to do with structural capacity) have been used histroically to include or exclude certain Aircraft or types of aircraft.

 

I agree without the extra weight its unlikely any seaplane would make it into RAA limits

 

 

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Guest ozzie

going back to basics the Wheeler Sea Scout flew a lot better than the land based version. Claimed the floats provided additional lift.

 

 

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"They sell you an aircraft that doesn't fit within RAAus rules by making you fly with half empty tanks to squeeze it into those rules."

 

Zibi

 

This is not limited to RAA. Most GA aircraft (if not all) are fuel limited at MTOW if you fill all the seats and cargo areas.

 

 

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"They sell you an aircraft that doesn't fit within RAAus rules by making you fly with half empty tanks to squeeze it into those rules."Zibi

 

This is not limited to RAA. Most GA aircraft (if not all) are fuel limited at MTOW if you fill all the seats and cargo areas.

C182 one exception with standard weights I've been told?

 

 

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C182 one exception with standard weights I've been told?

Ben

 

Pretty close but with commonsence - used as a 4x4 replacement, CoG and MTOW can be exceeded depending on what is put in the cargo area. Still fly as most will, but can exceed "legal" limits.

 

 

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FFS - The MTOW is what the aeroplane was designed and certificated for. The LSA rule allows the applicant to design for a higher MTOW if the aircraft has floats.

 

Strictly, the MTOW is the least of:

 

(1) The maximum weight at which the aircraft was proven to meet the structural requirements;

 

(2) The maximum weight at which the aircraft was proven to meet the stability and handling requirements;

 

(3) The maximum weight at which the aircraft was proven to meet the performance requirements.

 

An applicant can thus design for whatever weight he chooses. However, the rules designate a number of aircraft categories; for example if the aircraft is to be self-certificated by the manufacturer under the LSA rule, its MTOW will also be limited by whatever that rule allows. NOTE THE WORD ALSO . If the manufacturer anticipates selling the aircraft with a float capability, he may elect to justify it for any higher weight that happens to be allowed by the category.

 

So it has nothing whatever to do with whether floats add lift; it's what the rules permit (and don't ask me how people worked out those rules - they don't make sense to me, either) and how the manufacturer / designer chooses to work within those rules. Unless an increased weight is allowed by the aircraft's certification, you CANNOT increase the MTOW by adding floats.

 

 

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"They sell you an aircraft that doesn't fit within RAAus rules by making you fly with half empty tanks to squeeze it into those rules."Zibi

 

This is not limited to RAA. Most GA aircraft (if not all) are fuel limited at MTOW if you fill all the seats and cargo areas.

I know that, however I was referring to Nick's comment, which I suspect was about Jabiru (as he mentioned he flies one in some other thread) which is only fuel limited if under RAA rules, and can have much higher MTOW under GA. Thus my comment still stands, that it was the manufacturer that has designed a plane for a slightly higher category and is squeezing it into RAA (LSA) rules.

 

 

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I know that, however I was referring to Nick's comment, which I suspect was about Jabiru (as he mentioned he flies one in some other thread) which is only fuel limited if under RAA rules, and can have much higher MTOW under GA. Thus my comment still stands, that it was the manufacturer that has designed a plane for a slightly higher category and is squeezing it into RAA (LSA) rules.

Most current aircraft design standards require that, without exceeding the aircraft's certificated MTOW, you must be able to fill all the tanks, with only the pilot (a standard weight pilot, usually 86 Kg nowadays); AND you must be able to carry fuel for one hour at normal cruise power, with all the seats full (to their placarded or stated occupant weight limits). If the tankage were limited so that you could fill all the tanks and all the seats, the available range at reduced load would be unnecessarily restricted.

 

 

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Guest Nobody
going back to basics the Wheeler Sea Scout flew a lot better than the land based version. Claimed the floats provided additional lift.

I doubt that the floats added significant lift but they do change the handling.

 

They add area to the front of the aircraft and often require additional tail surface area to maintain lateral stability. For example the maule m7 below.

 

 

They also add drag below the centre of gravity which can change the pitch balance. This face is more likely to explain the perceived improvement in handling.

 

 

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Most current aircraft design standards require that, without exceeding the aircraft's certificated MTOW, you must be able to fill all the tanks, with only the pilot (a standard weight pilot, usually 86 Kg nowadays); AND you must be able to carry fuel for one hour at normal cruise power, with all the seats full (to their placarded or stated occupant weight limits). If the tankage were limited so that you could fill all the tanks and all the seats, the available range at reduced load would be unnecessarily restricted.

Again, I'm not talking about not being able to load max weight passengers, plus full fuel plus full baggage but rather this bit: J230 has a MTOW of 700 kg (VH Registration) and 600 kg (RAA Registration) (source: http://www.jabiru.net.au/aircraft-kits/j430-kit).

 

 

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Years ago, Phil Ainsworth, when with Jabiru, wrote an article for the University of Sydney Aero-mech. school explaining why Jabiru decided to build their own engine when KFM pulled the plug on the 1600, and why they didn't just go for a Rotax then. http://www.aeromech.usyd.edu.au/AERO1400/Jabiru_Construction/jabiru.html It's very well worth reading.

 

Jabiru confirmed to me some time ago, when I sought information as to whether I could do anything to increase the MTOW of my STI (which is a 55-reg LSA) that the placarded MTOW was a result of the stall speed, not a limit imposed by the structural considerations. As I understand it, the UL was a development necessary to meet the lower European stall speed requirement.

 

However, the various regs. - which in regard to at least some of the limitations appear to have been developed by people in authority plucking a handy figure out of the air (pun intended) provide a 'box' within which the designer has to operate. Phil's article shows up rather well the compromises the designer has to grapple with.

 

 

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It makes one wonder how and why these MTOW were decided. Notwithstanding structural limitation of a particular ac, weight capping within the scope of RAA limits is hard to rationalise in my mind.

 

I completely get max stall of 45knts ... lower stall has an impact on safety over many levels and is also consistent with observing overall crash survivabilty ie seats, cabin integrity etc. higher stall will require greater consideration for strength to protect occupants.

 

Come Sept we will have access to VH aircraft with a switch over to RPL that will allow lawful flight of >1500kg aircraft, that is, following a flight review etc. This suggest to me a vote of confidence in our training and abilities to safely use heavier aircraft.

 

RAA provides us many wonderful privlages, one of which is self maintenance, which I personally enjoy and learn from. Additional the medical requirement help many that would otherwise be grounded. RPL will not allow owner maintenance (save owner built experimental) and drivers lic med will also lock out many too.

 

If we can now lawfully fly a RAA ac at 650kg, just because it has floats or a hull, why shouldn't we allow 650kg Aircraft across the board. I would argue that an additional increase in weight would provide more safety (greater strength and durability) ... Indeed why isn't RAA & CASA revisiting MTOW for RAA ac, in light of RPL caps, and consider higher weights that still observe 45 knot stall.

 

I know these consideration have been out there for a number of years, but surely the argument is become more irrational as time passes not to revisit this again in light of the RPL impacts.

 

Just my opinion .. Looking forward to hearing your views?

 

Cheers

 

Vev

 

 

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