Max Take-Off Weight

[Skippydiesel]

With reference to;

Sonex Legacy (A) powered by Rotax 912 ULS, with wing fuel tanks.

My Sonex, yet to fly, is required by Recreational Aircraft Australia (RAA) to have placard which, amongst several things, must have the Maximum Take -Off Weight (MTOW) for the aircraft.

In Australia, this is usually determined by the aircraft/kit/plans supplier. Sonex as you know, have a sliding scale for its aircraft, depending on engine choice (Rotax not getting a mention).

Further complicating the matter, is the the unusual feature of my Sonex - wing tanks!

Wing tanks usually confer greater MTOW, of the so enhanced aircraft, compared with the same aircraft that has a fuselage tank (mostly to do with turbulent/rough air penetration & associated stress).

More complication; RAA does not allow aerobatics in aircraft registered with it - again this suggest a higher allowable MTOW (structural stress).

RAA allows MTOW to 600kg, subject to manufactures agreement

I would very much like to take the opportunity of having my Sonex approved to 600kg. This will allow for more fuel/baggage - all important with our vast distances, with few refuelling points.

[NWade]

You mention wing tanks often allowing a higher MTOW. But the reason for that is more complex than it may seem.
Weight in the wings has less of an impact on fuselage-to-wing-spar loads than weight carried in the fuselage. This is why in some aircraft (such as gliders manufactured in Europe) the aircraft manual specifies a maximum “weight of non-lifting parts”.

But structural loads on the airframe (both fuselage stucture and wing spars) are only one of several factors in determining MTOW/Gross weight. Power loading, stall speed, and other factors must be considered.

Based on some statements that Sonex Aircraft has made in the past, I’ve always been under the impression that power-loading is a big factor in limiting gross to around 1150 or 1200 pounds. While it may not seem like a big deal during ideal conditions, the power loading can greatly impact your ability to execute a go-around or deal with high density altitude conditions.

—Noel

[Kai]

Yup,

You’ve got a point. We have been banging our head against the wall with or national CAA up here as well.

The flight manual for the Legacy (A model) says 1100 lbs mtom for anything except the Jab 33A. The limit is not so much structural, but also dictated by flight performance (accelleration, climb performancee etc) and a possible unstable flight condition with loaded rearward CG location. With the (not always!) 120 HP J33A, you can go to 1150 lbs (522 kgs). Bear in mind tht the flight manual for the Legacy was written when the VW-1, the Jab22A, and the Jab33A, was all that was available for the aircraft. Peculiar, as structurally the aircraft are all the same. One can only speculate that the Sonex is very overbuilt!

Much later, since installation wise the J33A and the R912S are fairly close weight wise (but well within the 200 lbs limit), and that the R912S develops much more torque on the propshaft than the J33A, our national Air Sports Federation was willing to accept both types as applicable for the 522kg limit as long as the g-loads and safety factors in the utility class were observed. And no aerobatics with any type of additional fuel tank- because the kit manufacturer did not test this.

All the above when the aircraft was to be registred as an LSA (outside the CAA sphere of influence).

However, if you elect to build the thing as an EXP (a CAA class of aircraft), you can apparently do more or less as you please. This results in a motley collection of powerful UL Power engines hung on the nose, not to mention the Corvairs and the converted Honda engines. And you can set your own takeoff weight. All without any structural changes- there went the g-load limits and the safety factors!

The logic just is not there.

Thx

[Murray Parr]

In the B model plans it states 100Hp and up is 1150lbs Max

[sonex1566]

Good evening everyone,
I'm going to put my two bob's worth into this. Based not on any engineering principals rather than the performance of my own aircraft which is a scratch built legacy Sonex. I originally had an AeroVee fitted, I registered and test flew the aircraft at 544kgs with Recreational Aviation Australia (RAAus) which was the old weight limit until the later weight rise. I flew it a few times at that high weight on the 80hp VW, but it was a bit too marginal for my liking. The climb rate was 'curvature of the earth' type stuff. The cruise speed was fine, but it really wasn't good enough. I personally weigh nearly 100kg, due to the fact that I am a knuckle dragging earth moving type with solid muscle between my ears. It doesn't help that I don't have any petite people as friends! The 500kg as stated in the Pilot's Handbook really is the most sensible. I have now fitted the Jab 3300 which is spectacularly better. I have flown it quite a bit with a passenger, which brings us up to the 544kg limit (more or less!). The heavier Jab 3300 helps here as it is 8kg heavier in the nose than the VW, but the CG still moves back a bit. I can't remember how much, it is still very much in the allowable range, but I definitely need to trim the nose down a bit. Even with the extra hp I would not entertain the idea of going up to 600kg, even if the aircraft is a structurally sound design, there is only so much those little wings can do! I worry that if you have a placard on the dash saying '600kg MTOW', then some poor sap sometime in the future will do just that and kill themselves.

The Sonex website has a bit of info regarding MTOW's, I really think that they do know what they are talking about. Sorry to rain on your parade, but if you can get a chance to fly one you will see why I am concerned.

[GraemeSmith]

"two bob".....wow - I know what you mean - not sure how many others do.... ;-) My 2 cents (as I emigrated).

A Legacy Sonex with an 80hp AeroVee is NOT a homesick angel. Sonex's number of 1,100lb (499kg) is the maximum I would want to remotely consider flying the plane at to have reasonable climb performance and obstacle clearance. Sonex have some clearly enumerated reasons as to why they recommend this number. Some are about legalities of USA LSA certification - but others are about what is just plain sensible.,

My aircraft weight after a lot of scraping out and re-equipping with lighter options 633lb (287kg) - giving an actual usable of 467lb (212kg).

--

In ALL small GA that I fly - I have a personal minimum to try and only use 80% of the usable to give me some "padding" for hot and high days or the obstacle that grew taller than last time it was measured. So that puts me at 374lb (170kg) to use.
Dressed I'm at 200lb,
small spares and oil 12lb,
parachute if doing acro 15lb (spares and oil left behind - take off 12lb = nett +3lb),
full tank of fuel (16.7 US Gall) 100lb.
I'm quickly at 315lb

Plane plus that scenario 948lb and JUST Acro legal with full tanks (still have to burn a touch off though to get the CG).

Or is I DO go maximum gross (so no acro) I can carry a 152lb passenger. Though it had better be a long and unobstructed runway as I am exceeding personal minimums.

--

Not saying my way is RIGHT. But I do have an opinion that trying to squeeze more usable into the airframe is not wise.

If you need more usable - a Sonex is the wrong plane for your mission.

YMMV.

[GordonTurner]

This seems to be a risky topic in which to express an opinion. Doing the math however, reveals that a 1300 pound airframe with 120hp has a much better power to weight than an 1100 pound airframe at 80 hp. So much for that one.

There are three categories of Part 23 certification:

Normal – +3.8Gs and -1.52Gs;
Utility – +4.4Gs and -1.76Gs;
Aerobatic – +6Gs and -3Gs.

If you extend the 1100/1150 MGTOW from utility to normal you easily fall within 1300 pounds MGTOW.

Similarly if you use formula available online for extrapolating likely stall performance you find only a small increase in stall speed from 1150 pounds to 1300 pounds. Possibly this will put you out if LSA performance limits, but my calculations using several formulae didn’t show that result.

Landing gear. No way around it. The landing gear/brakes and various structural components will experience greater loads at the same touchdown sink rate. Or upon hitting the same “bump” on the takeoff roll. So here you have to make your own risk assessment.

I believe you have a responsibility to flight test any number you intend to use through the entire cg range.

I plan to set my MGTOW higher than the manufacturer of the kit recommends, test it fully, and then rarely if ever actually use it. It I have already defined specific instances when I do plan to use the full amount I define.

Take note: I have not flown my plane yet and any opinions expressed here are likely to change once I have. Do your own homework, these are just internet opinions from an amateur builder.

[Skippydiesel]

All great stuff gentlefolk and thank you for entering the debate with such thought provoking comments.

My question actually comes from a real life scenario - I know of a European aircraft manufacture/kit supplier, who started production of aircraft with a 65L in fuselage tanks and a MTOW that complied with Euro regulations of the time (I forget the actual number but think it was a little below 500 kg).

Importation of these kits into Australia (MTOW 600 kg) saw representation from agents/owners to increase MTOW. With no structural changes the MTOW was increased to 544kg.

The various aircraft flew, without incident, for the next 20 years or so. Over this time, newer variants went up to 75L, in fuselage capacity.

The manufacturer introduced a a wing tank option, 2 x 50L (no in fuselage fuel tank) There are no structural changes from the original variants. If you purchase the 100L wing tank variant you now have (in Australia) a MTOW of 600kg.

[Skippydiesel]

All great stuff gentlefolk and thank you for entering the debate with such thought provoking comments.

My question actually comes from a real life scenario - I know of a European aircraft manufacture/kit supplier, who started production of aircraft with a 65L in fuselage tanks and a MTOW that complied with Euro regulations of the time (I forget the actual number but think it was a little below 500 kg).

Importation of these kits into Australia (MTOW 600 kg) saw representation from agents/owners to increase MTOW. With no structural changes the MTOW was increased to 544kg.

The various aircraft flew, without incident, for the next 20 years or so. Over this time, newer variants went up to 75L, in fuselage capacity.

The manufacturer introduced a a wing tank option, 2 x 50L (no in fuselage fuel tank) There are no structural changes from the original variants. If you purchase the 100L wing tank variant you now have (in Australia) a MTOW of 600kg.

So, at least in one case, a significant MTOW has been approved, without structural changes to the aircraft, by simply (ha!) substituting wing tanks.

[Kai]

What is going on here?

The latest all european microlight class (read:FAI ULM- ultra leger motorise) was originally weight wise limited to 450 kg mtom for a two seater land plane (1 lbs = 0,454 kg). The Sonex Legacy was designed to these limits (due to its Italian financial backing).

When the ballistic shute became an option, CAA’s upped the mtom to 472.5 kg. In both instances all planes, were required to demonstrate a payload of 2 pcs ICAO pax (a total of 186 kg). Exceed the mtom and you would neither get insurance nor certificaion. Get caught (on a ramp check or a crash) and you would get a fine.

Enter the US LSA class- europan manufacturers saw great opportunities in a hugely expanded market, and began manufacturing somewhat heavier airframes that did not need the extremely sophisticated materials employed in the scraped ULM’s. After a while it was almost impossibe to get anything registered as a european microlight. The european market started to object (notably Germany and the Checks).

After a lot of meetings there was finally a new european class, the ELSA (european light sport aircraft). Weight wise it is limited to a mtom of 600 kgs for a two seater land plane, and +6, -2 G (basta! gone are the utility-, normal-, and all the other classes). Subject to manufacturer approval, of course. BUT: the aircraft as such would also be required to demonstrate a certain percentage of the mtom as payload. Most european nations say 37%, a few 40%. No aerobatics, no night VFR, no IFR. And nothing that is not on the manufacturers plans/scope of supply.

As far as I know, the Legacy and the B are structurally identical. Sonex says mtom 1150 lbs with the J33A, don’t they? And +6, -2G??

So if you want to register a new B model with a max 120HP, 200 lbs engine, you are in Europe limited to an empty flyable weight of 724,5 lbs. And in many countries you need at least the mandaory acoustic SW switch in the wing leading edge. If you also want to extend the instrument panel with an AOA, fine!

Mind you, this is all for the 600 kg elsa class. This class is regulated by national air sports authorties. Go EXP, and you are under the CAA: a totally different situation!

Thx