Limit to 120 KCAS at sea level only with custom EMS - still an LSA?

[fatsportpilot]

If my reading of the regulations is right then a plane can be an LSA as long as it flies no faster than 120 KCAS at sea level at standard atmospheric conditions. That means it is fine to go faster at higher altitudes whether true or indicated or calibrated. In practice this happens with planes using the 914 or the new 915iS by 5 or 10 knots. I can't find anything about an upper limit of speed at higher altitudes.

There's a custom EMS called SDS (I found it at sdsefi.com) which replaces the electronics in a fuel-injected engine, and it's programmable. The EMS could be programmed to limit the RPM at sea level so the speed wouldn't exceed 120 KCAS, but enabled the full power at higher altitudes (like 500 or 1000 MSL). I know that weight and clean stall speed would become limiting after a while but it might make it possible to get 150 or even higher KTAS at cruising altitude. Would it be considered an LSA if the plane itself limits the speed at sea level automatically?

I'm purely curious and I don't plan on making this kind of modification but I would love to know if a plane like that would still fit in the LSA category as an experimental, or if it would have to be an E-AB.

I thought about this after reading https://sportpilottalk.com/viewtopic.ph ... 500#p58392 where it's said that a placard limiting the speed to 120 wouldn't be enough and that the aircraft itself has to have the limitation. I don't think it matters if the limit is aerodynamic or digital though.

[3Dreaming]

The 120 knot limit is with maximum continous power. I think programming to increase power as altitude is increased is going beyond the limits intent.

[Warmi]

The 120 knot limit is with maximum continous power. I think programming to increase power as altitude is increased is going beyond the limits intent.

Yeah but I am pretty sure that’s what he is going for , get around the limit while maintaining a legal fig leaf..

[fatsportpilot]

The 120 knot limit is with maximum continous power. I think programming to increase power as altitude is increased is going beyond the limits intent.

I thought it was specifically at sea level. Is that what the limit is intended for? Already existing LSAs can exceed 120 knots sometimes even by a lot, but only at altitude. I've seen ads for planes that can make it to 140 KTAS with the 915iS engine but which are still LSAs. Are they also going beyond the limit's intent or am I misunderstanding it? In my own mind this seems like a logical extension of planes that can naturally go above 120 at altitude.

I don't condone trying to get around limits in a legal grey area so if this does actually bring an airplane out of the LSA category and into E-AB then I would not fly it without the appropriate license. I'm curious about whether or not I could fly a plane like that with a sport certificate or if I'd need a different one. Not trying to get around a regulation and fly something I'm not supposed to!

So my question is "is this an LSA" and not "how can I trick the FAA to let me fly this".

[fatsportpilot]

Btw if this should have been posted in the FARs section then that's my bad and I hope it can be moved there.

[Warmi]

It is clear that the limit of 120 knots at max continuous power was meant to be just that ... whatever extra you get at altitude is just derivative of that fact that you are flying at altitude and not because your engine is programmed to play tricks with your power curve and I think that’s what Tom was convening in his post.

Now , having said that, I think this was a pretty stupid rule to begin with and even FAA seems to be recognizing this fact and are working on establishing new rules which appear to be based more on formulas like wing loading etc rather than hard speed and gross limits - Dan Johnson seem to be implying that new rules should go into effect sometime in 2023 or 2024 - if I were you I would rather wait for new rules rather than try to play tricks with the engine.

https://www.bydanjohnson.com/faas-propo ... te-report/

[fatsportpilot]

Now , having said that, I think this was a pretty stupid rule to begin with and even FAA seems to be recognizing this fact and are working on establishing new rules which appear to be based more on formulas like wing loading etc rather than hard speed and gross limits - Dan Johnson seem to be implying that new rules should go into effect sometime in 2023 or 2024 - if I were you I would rather wait for new rules rather than try to play tricks with the engine.

https://www.bydanjohnson.com/faas-propo ... te-report/

Yeah I'm keeping a close eye on that MOSAIC rule. I won't even buy a LSA (renting until then) until the new rule is solidified. Hoping that the Sling TSi will fit as a MOSAIC LSA! That thing looks sweet and is cheaper than I expected. That's why this Q is purely hypothetical.

[3Dreaming]

The 120 knot limit is with maximum continous power. I think programming to increase power as altitude is increased is going beyond the limits intent.

I thought it was specifically at sea level. Is that what the limit is intended for? Already existing LSAs can exceed 120 knots sometimes even by a lot, but only at altitude. I've seen ads for planes that can make it to 140 KTAS with the 915iS engine but which are still LSAs. Are they also going beyond the limit's intent or am I misunderstanding it? In my own mind this seems like a logical extension of planes that can naturally go above 120 at altitude.

I don't condone trying to get around limits in a legal grey area so if this does actually bring an airplane out of the LSA category and into E-AB then I would not fly it without the appropriate license. I'm curious about whether or not I could fly a plane like that with a sport certificate or if I'd need a different one. Not trying to get around a regulation and fly something I'm not supposed to!

So my question is "is this an LSA" and not "how can I trick the FAA to let me fly this".

Flying faster at altitude is not the issue. Increasing the power above what is available at sea level is. The 120 knot limit is established with maximum continous RPM and manifold pressure, ie maximum continous power. Programming either of these to increase with altitude means that you didn't use maximum power to establish the limit.

[drseti]

Vh is a design spec rather than an operating limit. Yes, it is defined at sea level, in a standard atmosphere, at max continuous power.

Maximum continuous power is maximum continuous power, regardless of altitude. For a fixed pitch prop, it is defined by RPM, not manifold pressure. For example, in a Rotax 912S series, it is a limit of 5500 RPM. Rotax has determined that higher than that will reduce engine life. So, if you choose to run your Rotax 912S all day long at 6000 RPM, and happen to cruise faster at sea level on a standard day, you're not breaking any Vh FAR. You're merely breaking a perfectly good $19,000 engine (and compromising safety of flight).

But actually, you are breaking a different FAR. You are required to have placards in an SLSA, ELSA, or E-AB, one of which lists max continuous power. These placards constitute operating limitations (remember the O in the ARROW acronym?), and violating operating limitations is not only unsafe, it is a violation. Note that the violation is not from going too fast, but from careless and reckless engine operation.

[drseti]

I would love to know if a plane like that would still fit in the LSA category as an experimental, or if it would have to be an E-AB.

Any homebuilt aircraft can be registered as an E-AB. If it meets the FAR 1.1 LSA design and performance specifications, it is also an LSA. So a plane can be both an LSA and an E-AB. It isn't either-or.

[fatsportpilot]

Flying faster at altitude is not the issue. Increasing the power above what is available at sea level is. The 120 knot limit is established with maximum continous RPM and manifold pressure, ie maximum continous power. Programming either of these to increase with altitude means that you didn't use maximum power to establish the limit.

I didn't realize that it was talking about maximum power that the engine manufacturer considers the maximum. I thought it just meant whatever the power is when the throttle is fully in (ie that if you had a stopper that prevented the throttle from going in more then that would count as a limit to the maximum continuous power too). Thanks for clearing that up!

It would certainly be beyond my own capabilities but that makes me wonder why a manufacturer doesn't make an engine that is designed to operate particularly poorly at sea level and much better at moderate altitude without using an electronic limiter and sell it as a "go fast" LSA engine. Maybe because that would cause the regs to change if it was done on a commercial scale.

[fatsportpilot]

Vh is a design spec rather than an operating limit. Yes, it is defined at sea level, in a standard atmosphere, at max continuous power.

Maximum continuous power is maximum continuous power, regardless of altitude. For a fixed pitch prop, it is defined by RPM, not manifold pressure. For example, in a Rotax 912S series, it is a limit of 5500 RPM. Rotax has determined that higher than that will reduce engine life. So, if you choose to run your Rotax 912S all day long at 6000 RPM, and happen to cruise faster at sea level on a standard day, you're not breaking any Vh FAR. You're merely breaking a perfectly good $19,000 engine (and compromising safety of flight).

I used to hear complaints that the 120 limit was a problem because it was for full throttle (bad for the engine after too long) and that safe RPMs would never hit that so the limit would end up being worse in practice. But that's not correct? If a plane can safely cruise all day at 120 knots at 5500 RPM but can hit 125 with full throttle at 5800 RPM for 5 minutes then it's still an LSA?

Sorry if these questions are dumb. I'm pretty new to this.

[drseti]

If a plane can safely cruise all day at 120 knots at 5500 RPM but can hit 125 with full throttle at 5800 RPM for 5 minutes then it's still an LSA?

That is correct.

Sorry if these questions are dumb. I'm pretty new to this.

There are no dumb questions. That's because dumb means mute. Since you asked the question, you're obviously not mute.

[FastEddieB]

Maximum continuous power is maximum continuous power, regardless of altitude.

Just to muck things up, how does that jibe with this AOPA review of the Carbon Cub LSA:

"The Carbon Cub SS’s pilot operating handbook allows full power to be used up to five minutes in takeoff and climb and limits pilots to just 80 horsepower in cruise flight. The manufacturer relies on the honor system for compliance, however, and there’s nothing about the engine or airframe to prevent pilots from flying at higher power settings (and greater speeds) than the POH dictates. The Super Sport Cub is limited to the LSA maximum 120 KIAS in level flight—and the tundra tires on the aircraft I flew were terrific speed brakes. At 2,500 engine and propeller rpm, the aircraft showed no inclination to go beyond that limit in level flight."

Bear in mind this is with "a four cylinder, 340-cubic-inch Lycoming engine with dual electronic ignition that produces up to 180 horsepower."

[fatsportpilot]

Maximum continuous power is maximum continuous power, regardless of altitude.

Just to muck things up, how does that jibe with this AOPA review of the Carbon Cub LSA:

"The Carbon Cub SS’s pilot operating handbook allows full power to be used up to five minutes in takeoff and climb and limits pilots to just 80 horsepower in cruise flight. The manufacturer relies on the honor system for compliance, however, and there’s nothing about the engine or airframe to prevent pilots from flying at higher power settings (and greater speeds) than the POH dictates. The Super Sport Cub is limited to the LSA maximum 120 KIAS in level flight—and the tundra tires on the aircraft I flew were terrific speed brakes. At 2,500 engine and propeller rpm, the aircraft showed no inclination to go beyond that limit in level flight."

Bear in mind this is with "a four cylinder, 340-cubic-inch Lycoming engine with dual electronic ignition that produces up to 180 horsepower."

From my understanding there are some limitations that are of the aircraft (or POH) itself like the MTOW, number of seats, SLC for power, fixed pitch or ground adjustable prop, fixed landing gear, and etc. Then there are some limitations of the pilot that they're trusted to comply with like VFR, day flying only, actually honoring the MTOW, 10,000 MSL or 2,000 AGL maximum whichever is highest.

The Carbon Cub SS is assuming that the maximum speed falls into the second category and I guess that's what you're asking about rectifying? I think most people assume it falls into the first even if the Cub does not. My original question I guess was a way to sort of move it into the second category but trusting software instead of trusting a pilot's word in which case if the Cub is correct then we as sport pilots have a lot more options.