Ghost of Merlins Past
In researching on aircraft engines, I find myself pawing over documentation that often seems to lead to the iconic Rolls Royce Merlin. And what an icon it was. It emerged in 1933 and was improved continually until it was done in by post-war piston radials and eventually the jet engine. The Merlin delivered as much as 1720 HP in some variants and its power density was generally under a pound per horsepower. (Modern aircraft engines are closer to 1.5 pounds.) It had the sweetest exhaust note in the history of civilization.
It was so superbly successful that I've occasionally wondered why no one ever downscaled it for the light aircraft market. But a cursory examination answers that question: It was hopelessly complex and expensive to build. That doesn't matter in a war economy, but it's everything in peace time.
Fast forward 65 years and you can see the vague ghosts of the Merlin in Adept Airmotive's newly announced engines. The announcement came out of left field this week and at this point, I can't even put a number on its chance of success. But after speaking in detail with Adept's Richard Schulz for this podcast, it's clear that the company has some good ideas.
The 120-degree V design appeals because the load paths are more easily resolved with less structure than in a conventional opposed engine. Because rods can share a journal, the crank is short and stout, not long and thus subjected to wicked torsionals. This makes for smooth running and a smaller physical footprint.
But like the Merlin of old, the price to be paid is a complex, heavy block casting and potentially complex valve gear. And that would be true, except materials science and machining advances of the last six decades—which Adept is making full use of—have all but rewritten the book on manufacturing economy. Engines like these can be made in shorter runs, with more precision and less handwork and fewer parts than the engineers at Crewe could have dreamed of in 1938.
The Adept engines have proposed power up to 320 HP, which hits the middle of the most profitable segment of GA. They're an interesting amalgam of the latest automobile and motorcycle technology: Water cooled, sequential pulse injection, ceramic coated cylinders with a single plug and direct fire coils, dual overhead cams with bucket-under shim adjustors on the valve stems and acoustic knock sensors. If you own a motorcycle made in this decade, you've been using this stuff for years.
All of this means fuel octane is a non-issue. Even with compression ratios of 9.7 to 1 on the turbo version, Schultz says early tests suggest there's plenty of detonation margin. The real eye opener is the fuel specifics: consistently under .4 is claimed, with .37 in range. If your Cirrus did that well, you'd be steaming along at 190 knots on 15.5 GPH.
Of course, Adept has a long road to travel before it ever produces a certified engine. (It will aim for European certification first.) And all of this data is gauzily preliminary. We haven't clapped eyes on the thing to confirm it yet and it will be awhile before we can. Adept appears to be adequately if not lavishly funded. The engines haven't flown yet.
And let's not forget that in 2003, with over-the-top PR, Rotax/Bombardier introduced a pair of V-6s of somewhat similar design. They sank without a trace, primarily, I think, because the OEMs just wouldn't get serious about them, although there were probably other factors, too. The engines weren't that light and weren't that efficient.
Adept may benefit from better timing. For one, the fuel panic is beginning to reach full song and CNC economics have advanced in the past seven years. It could very well be that the GA market is genuinely interested in paradigm shifting engines rather than just saying it is and then not buying what's offered.
Of such stuff is great spectator sport made.