? re: Sparrowhawk, Compression, Engine Life, Perf.

[killbilly]

The 152 I've been flying has the Sparrowhawk mod, which, as I understand it, means the motor is making about 125hp. I spoke with the owner briefly this morning about other matters and asked him about increased fuel burn with the greater horsepower, and he said that the conversion is achieved by running higher compression in the cylinders and that the fuel burn wasn't much higher than on a stock motor.

That raised several questions later when I got off the phone with him...here they are...

1) If running higher compression, you need higher octane fuel, yes? At least, that's what I've always known in cars. Is your standard 100LL high enough for an engine like that or should one run an octane booster? Related to question 2, but do you have to treat it any differently when flying?

2) With higher compression, (and this is a guess) do you end up running higher CHT? Should the engine be run a little richer at altitude instead of the lean-to-the-slightly-rich-side-of-peak that I've been taught?

3) Does a Sparrowhawk mod overall shorten the life of the engine? Or is it negligible in the grand scheme of things?

4) Finally, with the increased power, is there a relationship to how much extra weight the plane can carry? Is it a linear relationship? Does a 10% horsepower increase mean a 10% increase in weight it can carry? I ask because the POH I have doesn't indicate anything related to a horsepower increase, and I don't have immediate access to the one in the plane right now.

Just curious. It's a really nice airplane and I enjoy flying it - the extra oomph is noticeable, and renewed my possible interest in owning one someday.

 

[USMCmech]

1) If running higher compression, you need higher octane fuel, yes? At least, that's what I've always known in cars. Is your standard 100LL high enough for an engine like that or should one run an octane booster? Related to question 2, but do you have to treat it any differently when flying?

The O-200 was orginaly designed for 80/87 avgas, and 100LL is overkill in the stock engine. The C152 with a stock O-200 has an mogas STC for unleaded anyway. So you should be just fine with 100LL at the higher compreson ratio.

2) With higher compression, (and this is a guess) do you end up running higher CHT? Should the engine be run a little richer at altitude instead of the lean-to-the-slightly-rich-side-of-peak that I've been taught?

Probably.

Your CHTs are probably slightly higher than stock, but not enough to worry about. Your normal leaning procedure should be fine, unless there are specific instructions from the owner.

3) Does a Sparrowhawk mod overall shorten the life of the engine? Or is it negligible in the grand scheme of things?

Possibly, but it's probably a negligable difference in the long run. Having it flown regulary (even by students) is already doing the best thing possible for the engine.

4) Finally, with the increased power, is there a relationship to how much extra weight the plane can carry? Is it a linear relationship? Does a 10% horsepower increase mean a 10% increase in weight it can carry? I ask because the POH I have doesn't indicate anything related to a horsepower increase, and I don't have immediate access to the one in the plane right now.

Theoriticaly, the added HP will let you carry more weight. However, unless the STC says that there is a new higher gross weight, you must stick with the origonal.

If the holder of the STC wanted to test fly and prove to the FAA that the added weight could be carried, the STC would include a new Max takeoff weight.

 

[killbilly]

Theoriticaly, the added HP will let you carry more weight. However, unless the STC says that there is a new higher gross weight, you must stick with the origonal.

If the holder of the STC wanted to test fly and prove to the FAA that the added weight could be carried, the STC would include a new Max takeoff weight.

Thank you for the information.

Does that mean that the STC varies from plane to plane, owner to owner? In other words, would reading a "generic" STC indicate whether or not there was a higher MTOW or is that something owner seeks as part of the STC?

 

[USMCmech]

Does that mean that the STC varies from plane to plane, owner to owner? In other words, would reading a "generic" STC indicate whether or not there was a higher MTOW or is that something owner seeks as part of the STC?

No, the "manufacturer" of the STC may or may not ask include the change in max weight. In most cases the added expense of flight testing will not be worth the added value.

 

[nosehair]

AFIK, engine horsepower isn't the factor to consider for weight. The airframe structure (the wings) are designed to carry the certificated weight only. You would need additional structure and FAA flight testing to add max weight.

 

[tgrayson]

Finally, with the increased power, is there a relationship to how much extra weight the plane can carry? Is it a linear relationship? Does a 10% horsepower increase mean a 10% increase in weight it can carry?

You’ll have to be precise about what you mean “to carry.” Legally or physically? Physically, you’re limited by the wing bending moments. The aircraft fuselage generates very little lift, so whatever you cram in the cabin causes it to “sag” with respect to the wings. Some aircraft require that any weight in excess of a certain amount must be in fuel in order to move some of the weight out to the wings to reduce this sagging issue.

Assuming you have enough structural strength to handle the weight, the aircraft will fly with any weight, as long as you maintain enough airspeed. However, at every airspeed your aircraft will be draggier and thus require more power; for a given amount of power, your climb rate will be lower.

Since your modified aircraft has higher power, assuming the stock max gross weight, you will have increased climb performance. If you’re willing to accept the stock performance, you could certainly carry more weight. Limiting factors may be structural, as described above, climb gradients required by Part 23 during takeoff or balked landings, or maybe stall speed limitations for single-engine aircraft.

Assuming that climb gradient is the limiting factor, that’s determined by excess thrust, rather than excess power. But assuming we keep our climb speeds unchanged, an increase in climb rate will produce an increase in climb gradient. That being the case, climb rate is proportional to Excess Power/Weight. If you increase the excess power 20%, then, at first glance, you could obviously increase weight 20% for the same climb rate. However, what isn’t apparent is that increasing weight also reduces excess power, due to higher induced drag, so there’s a double penalty for weight. That means it won’t be linear.

As for legally, your aircraft will have to be certified, of course, for the higher weight.

 

[DE727UPS]

"The C152 with a stock O-200"

The 152 does not have an O-200 as a stock engine.

I did the Sparrowhawk mod on a 152 once. The prop part of the mod gives you 5 hp by allowing 2800 rpm. The high compression cylinders give you 10 more hp. 100LL is the approved fuel and it will work just fine. Autofuel is approved for 150's and some 152's, but is NOT approved for the Sparrowhawk mod.

There are higher temps in climb with the higher compression. The STC calls for a limitation on 2800 RPM at low speeds to deal with the temp problem. The STC includes "better" engine baffling which was intended to improve cooling. I never had a problem with high temps. As far as improved performance, there is no need for Sparrowhawk to "prove" to the FAA that the mod increases performance. It was easier to just say "this STC mod meets or exceeds Cessna book figures".

I do believe fuel flow was slightly higher.

If you google Sparrowhawk, you can get some of the info from their website.

The mod is a good deal if you are in the process of overhauling your cylinders. The prop mod is pricey. You can do just the cylinders, if you want, and you get 10 hp out of it.

 

[mhcasey]

I remember asking a guy about a Malibu I saw in a hangar that had been given the turboprop conversion. He explained that despite the increase in power, the certified (or is it certificated?) max gross (and payload) actually decreased with the mod.

By the way Bill, if you lean the club's Sparrowhawks properly you won't burn more than 6gph in cruise - even fully loaded (in my experience). I still always plan 6 though...