Would the LSA airframe be capable of a 180 hp engine with c/s prop?

[Dougla$]

The LSA Bearhawk is meant to satisfy the old lightsport requirements, such as the 1320 lbs. weight limit, 120 kt max speed, etc. But with MOSAIC coming, pretty much all Bearhawks barring the model 5 will qualify (can’t seat more than 4 people, though only one passenger is allowed, so most 4 & 5 owners would have PPL regardless). While the LSA & Patrol airframes look similar, there are some differences. The LSA doesn’t have flaps, one is slightly taller and longer/shorter than the other (the website lists one as being 6’3” tall and the other as 6’5”, and one being 22’3” long and the other at 22’8”, and I remember the taller one is the shorter, but I forget which one). The LSA also has a wing area of 171 sq ft and the Patrol is 180 sq ft. Both planes have the same wingspan. The LSA cruises slower, but that is largely due to the smaller engines.

Could the LSA airframe support having the larger engine of the Patrol mounted? I was browsing through some older posts comparing the two, but I didn’t see where anyone has mounted a larger engine. Perhaps the larger engine would severely impact the CG. But even if the same engine/prop can be mounted, would the LSA cruise as fast as the Patrol? One big reason I’m sold on the Patrol is its faster-than-average cruise. But the LSA is priced lower than the Patrol ($13k lower for QB kit, and $15k lower for build assist), so that’s why I’m partially interested in this model.

Edit: Forgot to add the empty weight of LSA is 800 lbs., and the Patrol is 1150 lbs.

[Utah-Jay]

You are obviously taking a deep dive into the Bearhawk world, I think you need to give Virgil a call.

But to answer your question, I do not believe so.

[jaredyates]

One thing that can happen sometimes with me, and I think might be happening to you too, is when we do a deep dive based primarily on numerical inputs like published specs, we can end up drifting a little off course from the big picture.

Let's reiterate three key aspects of the big picture:
-The LSA was designed around a 4-cylinder Continental with around 65-100 horsepower.
-The Patrol was designed for a Lycoming 4-cylinder with around 160-180hp.
-The most important factor in deciding which plane to build is deciding which plane you want to end up with.

Tony Bingelis has written some excellent guidance about the pitfalls of mismatching the engine in a homebuilt in his books and articles, but here are a few things to consider.

Weight
When a builder puts in too big of an engine, weight is a primary threat. A published number like empty weight is highly variable. If you were to put the weight of a 180hp engine into an LSA airframe, it would not have a total empty weight of 800 pounds. The biggest engine that I know of in an LSA is a Lycoming O-235, and that plane's empty weight is 890. The LSA was designed to accommodate a gross weight as high as 1500 pounds if you didn't need it to be limited to 1320. I can't imagine why any future builder would limit the max gross to 1320 considering the new mosaic rules, but even at 1500 gross, getting the empty weight of an LSA up to 1000 pounds is really going to start cutting into it being a useful airplane. (It's also going to be much less delightful to fly, which is a point often lost on engine oversizers). 2x200 lb people leaves you only 100 pounds for everything else including fuel. The design fuel capacity of 30 gallons would weigh 180 pounds (30x6). We build Bearhawks because they fly nice, and we want to carry a lot! The way we carry a lot is by keeping the empty weight down. Bigger engines weigh more, as do their props, oil coolers, required fuel and tanks, and all the rest. Weight starts to cascade as engine power goes up. The same thing happens with bigger Bearhawks by the way. What good is an extra 100hp if the empty weight is 500 pounds more? The engine weight alone wouldn't make the plane that heavy, but if the builder had prioritized performance, they could have kept the engine more reasonable, put way less stuff in the plane, and had a much more useful finished product. There was a conflicting application of priorities and it showed in the end result.

Speed
Setting aside weight concerns, another big problem with trying to put a big engine in an LSA is that the VNE needs to be limited due to the unbalanced control surfaces. It does have a lower published cruise speed and it does have less horsepower, but those are correlational more than causal. The LSA needs to cruise slower for flutter considerations, regardless of how much horsepower it has. On the spectrum of limitation-breaking sins, flying overweight and exceeding VNE are both dangerous, but over-speed is going to yield bad outcomes faster.

Range
Planes with oversized engines also typically suffer in their range, and this would apply here too. The Patrol has an expected fuel capacity of 55 gallons, vs 30 for the LSA. When you are sipping 4gph with a lil' Continental, 30 gallons is plenty. Push it up to 8 gph and now the 45% loss of capacity starts to matter.

Cost
$13k or even $28k is a lot of money, but I would encourage you to not select between an LSA or Patrol based on a published numerical price difference of the kit. I suspect that the published build assist price is lower in part because the smaller engine has less complexity. As with the empty weight, I think you'd see that published number creep up, especially if you are having to pioneer a new engine mount, cowl, baffles, and all the rest. Select the type of engine you want to use, or select the type of cruise speed you want, and let those choices drive you to the airplane that is optimized for those priorities. If you need to reduce the cost of the build, there are ways to do that. But building the wrong airplane because it was a 10% cheaper is not going to be a good value decision.

Anything is possible. I've flown a Cessna 150 with 160hp. It had its mission, but in many ways it was not as useful as the same type of plane with the engine that it was designed for. Airplane designers make a lot of compromises to end up with the most useful tool for a particular mission and design criteria. Dynon vs Garmin is a plug and play choice. As is Polyfiber vs Oratex. Engines are not plug-and-play choices. If the designer was doing things right and the available choices have not significantly changed, the farther you deviate from the designer's intended engine envelope, the worse of a finished product you'll end up with.

[Bcone1381]

I really like the LSA. I was on the fence when I chose the Patrol. And I see an LSA about everyday I’m at the airport. I’ll echo what Jared wrote regarding Mission and focus on a data point that performance number always leave out.

Baggage space. If your going to go an a picnic the LSA is fine. If you’re going camping in an LSA you better discover what the ultralight guys and gals are taking with them on the Applicacian Trail. I mean the Ultralight hikers are NOT hauling a 40 pound pack. More like 15lb. No tools. You and your passenger have to be okay with it. Just you camping? Then it’s another story because you can remove the rear seat. For that to sink in you need to go look at the LSA, Patrol and also a Companion and judge the craft from the baggage space point of view.