A powerplant (engine/PSRU/prop) package for the SAL, or any other replica type for that matter, has to be the right weight to allow the correct CG, not overstress the airframe, provide good performance, and still allow a scale appearing nose. That would seem to be a pretty tall order, but fortunately we have several ways to accomplish all that.
The original prototypes had Ranger engines: a six cylinder, air cooled, inline engine built in the 1930-1945 period for use in military trainers. The Ranger puts out about 200 horsepower and has the advantage of being an aircraft engine with the perceived redundancy that brings. It's disadvantages are (not necessarily in order of importance) that it's too light (!!), too bulky to fit scale cowling, not especially powerful, and parts are getting hard to find.
How can it be too light? The problem is that the original P-51 had a pretty heavy engine, and the SAL is built in pretty much the same proportions. This means that to keep the nose length correct, we need quite a bit of weight. The original prototype SAL had a 120 pound prop and still had a nose that was noticeably longer than scale.
The various automotive V8 and V6 conversions address the size, weight, and power issues rather well. They allow a scale appearing nose due to their weight and dimensions, and the power can be whatever the builder wants. Redundancy is possible except for the spark plugs, and if a plug quits, there are 7 cylinders still working away.
To make an automotive engine work
a prop shaft reduction unit is needed to match the powerband of the engine to the usable prop rpm. This isn't a new thing; most large aircraft piston engines including the Allison and Merlin had reduction drives. Since the average auto engine has a usable power band from about 3000 to 5500 rpm (or so) a 2 to 1 reduction ratio is pretty good. That's approximate; PSRUs are available from about 1.6 to 1 up to 2.5 to 1. The higher ratios work well with larger props or higher rpm engines.
I plan to limit my engine rpm to 4800 and cruise at about 3400, so a 2 to 1 ratio gives a 2400 rpm prop for takeoff and about 1750 for cruise. Obviously changing the ratio changes the prop rpm up or down.
This is the Alternate Airpower PSRU mounted on a all aluminum Chevy. It's a chain drive and can use a hydraulic prop. As pictured it weighs about 450 pounds. With an iron block the weight would be about 550.
Below is the FEW (Fighter Escort Wings LS1/LS6 package. It too is a chain drive, but has it's own oil supply and pump. As pictured it weighs about 540 pounds.
The Northwest Aero PSRU above is mounted on a Chevy V6.
Before going any farther, Iíd like to make it clear that even though I will profile only GM engines here, itís not because I think theyíre the only solution. I considered GM engines because first, Iím most familiar with them and second, I didnít see any reason to to anywhere else, since GM engines bridge the spectrum from 250 horsepower V6s to 450 horsepower V8sÖ.all suitable weight wise and dimension wise for the SAL. Perhaps even more in favor of them is the fact that almost all the PSRUs in production are for GM pattern engines.
For my purposes I feel I need at least 300 horsepower at sea level for takeoff. I want takeoff power at no more than 4800 rpm and would like to cruise at around 3400 (more or less) engine rpm with over 200 hp. I could probably get by with less power, but since I live in Colorado and plan to fly out of a 8500 feet elevation airport, starting with 300+ is necessary to have anything left on a 80 degree day at that altitude. Either of the following V8s will provide that level of power. The V6 is marginal, but would probably do OK at low elevations.
From what Iíve researched, I will need a package (engine/PSRU) somewhere in the range of 575 to 640 pounds, depending on the prop weight. That will allow me to have a cowl length from the upper engine mount bolt to the front bulkhead of 62 to 64 inches. This provides a good scale appearance.
The first engine...
I looked at was the classic small block Chevy. This engine has been around without fundamental change since 1955 in displacements (stock) of 265 to 400 and horsepower ranges from 165 to 375. It has more heavy duty and high performance parts available than any other engine. There are probably more people experienced in repairing and modifying it than any other. Only recently has it been replaced in new cars and trucks, and itís still available factory new from GM parts departments. Parts of interest are aluminum heads, aluminum blocks, forged steel crankshafts and rods, forged pistons, and all manner of parts that can increase durability and move the power band to the range usable in an aircraft.
Weights are approximately 520 pounds for a 350 with iron block/heads; 480 for iron block/aluminum heads; and 380 for all aluminum. These weights will vary according to what accessories are used. Power levels can be from 250 up to 400 at 4800 rpm, depending on displacement and level of tune.
Above is a ZZ383 GM crate engine, sold new at GM parts departments. Below is a dyno graph of it's output.
was the 90 degree 4.3L V6. I found the V6 to be attractive but with drawbacks that eliminated it. The sound of the V6 is very close to that of a V12, since itís firing order is even (2-3-4-5-6-1) and basically it operates like Ĺ of a V12. Unfortunately, the power level is lower than the V8s and it doesnít have the aftermarket support. In a smaller or lighter airplane, it could be a nice engine making 250 horsepower or so at usable rpm.
The Northwest Aero PSRU above is mounted on a Chevy V6.
Last, and best to me...
the LSx (LS1, LS2, LQ4 Vortec, and now the LS7) family of engines. The LS1 was first introduced in the í97 Corvette and later in the í98 up Camaro/Firebird. The LS1 is 346CI. In about 2000 the LQ4 became the V8 of full size GM Pickups and had aluminum heads and an iron block with 364CI (6.0L). The LS2 replaces the LS1 and is very similar but is 364CI like the LQ4 but with an aluminum block. The LS7 is the new 427CI (!!!) for the Corvette Z06. Not a lot of information is available about the LS7 as yet. These engines are very similar and share common fitment of parts. All but the LQ4 are all aluminum and weigh about 410 pounds with accessories (no exhaust manifolds). The iron block of the LQ4 brings it up to about 520 pounds.
Above, the LQ4. It's the truck version of the LS engines with 364CI and up to 345 stock horsepower. The only drawback is the iron block, but that's easily overcome by using a LS2 aluminum block that will accept all the Vortec parts. That brings the weight to the same as the others. The LQ4 style engine has the most compact dimensions for accessory mounting, but is a little taller than the others because of the different fuel injection manifold. Fortunately there's still enough room at the top as long as at least a 7" offset PSRU is used. You'll notice the sump on the oil pan is at the rear, which is wrong for aircraft use since the engine is mounted "backwards", but that can be fixed by using a GTO front sump oil pan. From what I've found, getting up to 400 horsepower at usable rpm is easy with this engine even without increasing the stroke and CI. The LS7 is a 427CI engine similar to the LS2 but with a larger bore and longer stroke. It also has a dry sump oil system (internal, no belts) and different heads. Rumor has it that LS2 heads will fit the LS7 block, allowing the use of the dry sump system and large displacement with a more low rpm powerband. Sort of the best of both worlds.
The GM crate LS2, above, comes with everything but accessories and is new production.
Either the small block or LSx...
type engine will make up to 400 reliable horsepower. Both are produced in marine versions that run for hours at high power settings. The small block can be built as low or high tech as desired, having been built with everything from ignition points and a carb to full electronic fuel and spark control. The same can be said for the LSx, since the aftermarket is currently making distributor drives and carb manifolds for them.
My inclination is to use the electronics but with dual switchable aftermarket computers to control everything, and dual switchable fuel pumps. That allows for automatic mixture control and full ignition control including timing retard in case of detonation. IMHO, it's a better way to go, and just as importantly, it's already been done.
News is that there are three new LSX engines: the LS7, LS4, and L92. All are all aluminum and except for the LS4 are externally almost identical. All will weigh in at around the 410LBS of the LS1.
The LS7, at present only found in the Corvette is a 427CI, advertised 500HP engine that has much in common with the earlier LS types. The block is externally the same as the LS2, but it has a factory dry sump lube system.
As you can see itís a very simple and compact system. By using the LS7 crankshaft, it can be adapted to any of the LS engines. This will require either using the LS7 accessory drive setup or modifying the others due to the fact that the LS7 crank pulley being placed farther from the block.
The best thing about the dry sump system here is that the pump is entirely internal, consisting of a scavenge stage and a pressure stage. Itís driven by the crankshaft in the same way the LS1 pump is, it just has the extra stage. A limiting factor is that with only one scavenge stage, it only picks up oil from the pan, and so has only limited inverted capabilityÖlasting until the oil is exhausted from the external tank. The good side is that this should allow for a few seconds of negative g (thatís enough for me, anyway) and would eliminate transitory oil starvation from short periods with the pickup uncovered.
On the surface, it appears a different tank would be needed to allow for a flop tube, since the line to the pressure side is at the bottom of the stock tank. This isnít entirely bad, since the factory tank is about $900.
Using the block from the LS7 and various parts from other LSX engines would allow tailoring the powerband to better suit aircraft use without breaking the bank.
The L92 is the new 6.2L (around 377CI) truck engine. Itís factory rating is around 400HP and is the most attractive to me, due to the fact that itís already built to produce power at less RPM than the car engines. It has LS7 style heads but with smaller ports (part of the reason its power comes at a lower speed) and a factory steel crank. Itís also set up to use the same accessory drive setup as the earlier truck enginesÖgood, since the alternator is tightly mounted inside the outline of the engine and high, out of the way of the engine mount tubes.
It appears that the existing serpentine belt drive could be used with the elimination of the power steering and idler pulley between the water pump and alternator. A shorter belt could then be routed directly from the water pump pulley to the alternator. Possibly the position the power steering occupies could be used for an oil pump for the PSRU if desired.
The LS4 is used in front wheel applications and possibly would be more compact but would probably have a different rear flange and crank. I haven't found much info except that it's 5.3L and rated at 303HP.
The accessory drive looks good on the LS4, but it's hard to tell how difficult it would be to eliminate the power steering and air conditioning.
A lot more information will be emerging in the next few months, but it appears we'll have plenty to choose from, expecially since virtually all these LSX engines share interchangeability of parts such as heads, intakes, crankshafts, oil pans, and camshafts. This should make building an engine to do exactly what's needed very possible.