LSA weight difference

[chumash]

Hello,

LSA max weight is 1320lb, unless it’s amphibious then it goes to 1420 lb max. Is the aircraft designed to 1420lb or is there a different airframe for amphibious? I’m not going to fly over max, but was asked by my son, and realized I didn’t know.

Thanks

[drseti]

Actually, the maximum gross weight for all LSA seaplanes (whether on floats or hulls, and weather amphibious or strictly water-operated) is 1430 pounds. The additional weight allowance is to accommodate the necessary extra weight of the flotation devices and their supporting structure. It works from a performance standpoint because the floats or hull are not only buoyant, but are also airfoils, so they generate additional lift.

The aircraft in which I got my seaplane rating was a J3 Cub on straight floats. The max gross weight of a J3 on wheels is 1220 pounds. The floats add considerably more than the additional 100 pounds that the landplane LSA rules would have allowed. But floats are also draggy. So, instead of the usual 65 HP Continental engine, that Cub had been upgraded to 90 HP. That gave it just enough additional thrust to overcome the extra drag from the floats. So, in the air, it flew just like a Cub. On the water, it flew just like... well, a boat!

[Warmi]

Same frame.

My understanding is the floats give you a bit more lift which offsets the additional weight but , from me experience, often dealers will tell you that the airframe was built to handle 1420 lbs or even more, floats or not ...

[3Dreaming]

The airplane structure doesn't have to support any more load, because the lift from the floats will support the extra 110 pounds. It is common mistake that people often assume that the airplane structure can carry that excess weight because of the weight increase for adding floats.

[drseti]

The significance of LSA weight limits is not how much the airframe can "handle", but rather the impact weight has on kinetic energy (which is highly correlated with safety). Adding weight increases KE, which is detrimental to accident survivability. But increasing lift decreases KE, which improves safety. Given the combination of greater weight and increased lift for a seaplane, KE is pretty much a wash, so safety remains the same as it is for an LSA landplane.

Without getting overly technical, suffice it to say that a lot of engineering went into both the ASTM consensus standards and the FARs. These numbers are hardly arbitrary.

[drseti]

was asked by my son

Your son is to be commended for asking the right question!

[chumash]

Thanks for the replies, makes sense now. I didn't know the floats added lift. Never been interested in flying amphibious aircraft, but my son is. He's starting his PP training soon.

[fatsportpilot]

The significance of LSA weight limits is not how much the airframe can "handle", but rather the impact weight has on kinetic energy (which is highly correlated with safety). Adding weight increases KE, which is detrimental to accident survivability. But increasing lift decreases KE, which improves safety. Given the combination of greater weight and increased lift for a seaplane, KE is pretty much a wash, so safety remains the same as it is for an LSA landplane.

Without getting overly technical, suffice it to say that a lot of engineering went into both the ASTM consensus standards and the FARs. These numbers are hardly arbitrary.

That's true but it's also true that lower weight means less stability when landing which is also detrimental to landing safety because it makes accidents more likely even if said accidents involve less kinetic energy.

Imo the real reason it's important to stay within weight limits (other than keeping your license) is that higher weight changes stall speed.

[Wm.Ince]

. . . . . the real reason it's important to stay within weight limits (other than keeping your license) is that higher weight changes stall speed.

Regardless . . . the critical angle of attack remains the same.

[3Dreaming]

That's true but it's also true that lower weight means less stability when landing which is also detrimental to landing safety because it makes accidents more likely even if said accidents involve less kinetic energy.

Based on my 40+ years of flying experience I would always choose light over heavy. Light is always more maneuverable, and I will always take that over any slight decrease in stability.

[drseti]

higher weight changes stall speed.

Well, that's true, of course. But it's a little more complicated than that, because stall speed also influences kinetin energy. The two are inexorably linked.

Consider the most common LSA accident scenario: the aircraft runs off the side of the runway
during takeoff or landing. (BTW, it's always the left side of the runway. I will leave the explanation of that as an exercise for the student.)

In this scenario, the aircraft is always at roughly stall speed when it departs the runway. And (thankfully), after crumbling the runway, the occuoants generally walk away unscathed. These are almost non-injury accidents. Chiefly because very little kinetin energy is involved. The strict LSA stall speed and gross weight restrictions ensure that.

Now, let's look at what happens when you increase weight. Kinetic energy increases three times, and injuries go way up! Here's why.

The kinetic energy equation is:

KE = 1/2 m v^2

Where m is mass and v is velocity. As stated, for this type of accident, v is roughly stall speed. Now a heavier aircraft means more mass, so of course KE goes up. That's strike one.

Because stall indeed varies with weight, a heavier aircraft increases v. That's strike two.

But the KE equation involves velocity squared. So that increase in stall speed (which came from increasing mass) acts twice. That's strike three, and you're out!

If you check the accident statistics for side-of-the runway excursions, you'll see that the heavier the aircraft, the greater the incidence of injuries (and even fatalities). That's reason enough for me to prefer slow and light over fast and heavy.

[3Dreaming]

Paul, explain this one. https://aviation-safety.net/wikibase/240606

[drseti]

I hadn't seen that one Tom. Not sure I can explain it from just what's in that report, but I can surely speculate.

The initial left turn in the flare is to be expected from gyroscopic precession, p-factor, and insufficient right rudder. The blast of power during the botched go-around would normally have been expected to bring the nose further left, so I'm guessing the student over-compensated with full right rudder. As for escaping injury, one might conclude that the drag of the grass slowed the plane down considerably before it hit the hangar. In any case, I would say the student was exceptionally lucky.

[JimParker256]

If you watch the video linked to the report, it appears that the student pilot reverted to "driving a car" instead of "flying the plane" once the plane veered off the runway... You can see him turning the "wheel" to the right in an attempt to avoid the hangar.

[Warmi]

Once he made up his mind to go around, I think he was already in a state of panic and simply froze up with his application of right rudder.