You are describing the post prop strike regulatory position, which I do not think has any engineering basis.
I disagree. First of all it doesn’t matter if it’s due to regulation or technical substance, the cost is the same. Second, the gearbox engines are designed with this situation in mind and tested for it. The gearbox has to shear at a given torque which means no danger to the engine. If a direct drive engine is suddenly stopped, you cannot know what the forces on the engine were, there is no limiter. Thus the teardown approach.
I just had the simplest case of a prop strike in a Lyco O-320 C172 (no, it wasn’t me piloting). Both the prop was still serviceable and the engine had no damage due to the strike. Prop repair costed around 1000 € and engine teardown around 8000 €, plus VAT. That is a major advantage of a geared engine if technically merited or not.
100UL → 100LL → diesel → torque → Porsche → gearbox → prop strike
All logical, isn’t it? 
I will return to my diesel Jet A1 theme vs Avgas.
One ray of hope for aeroplane diesel engine developement is the UAV market. The Predator UAV started out with a snowmobile engine and the US defence department is eager to find a universl fuel. A one product fits all. To an extent Jet and Diesel can be switched about. Research has gone into heavy fuel development for UAV’s – read Jet A1, and they have looked at all sorts of ideas, some of them plain nonsense. HCCI was one offering – homogenous charge compression ignition. This does away with the fuel injector but so far has been fiendishly difficult to master.
Common rail fuel injection is now very developed and reliable, though most of my diesels in the plant at my home are the old distributive fuel pump type. For me the boxer set-up is most likley to succeed as it shortens the engine and allows a hefty journal bearing as opposed to an in-line setup, but I am not an engineer; I am a chemist that does a lot of engineering these days. Many years ago the German company Zoche had an X type engine fueled by diesel which I think from memory was a two stroke.
And lo here it is http://www.zoche.de/ Does anyone have any info. I wrote the paragraph below BEFORE I found the web page. It was not a cut and past.
Potentially, a turbo-supercharged two stroke diesel offers the answer of light weight and high power. This it the way that WAM have gone and I can see why. Benefits of diesel, lower weight due to smaller displacement, turbo- supercharging to aid scavenging, and reduced torsion stress on the crankshaft. More bangs per rev. Getting rid of the heat is still a problem but better than cranking up the RPM to con-rod crushing levels, and cherry red cylinder heads and exhaust valves.
Personally I do not like gearboxes. KISS.
I do not need to repeat the benefits of the supply side of Jet.
One ray of hope for aeroplane diesel engine development is the UAV market.
The USAF Predator A uses a Rotax 914, and the Predator B (AKA Reaper) uses a turboprop. The US Army specific version (Warrior or Gray Eagle) has used a Thielert diesel. I suppose it would be obvious to the observer that this would change and something called the Lycoming DEL-120 of around 200 HP is now being publicized. However I have yet to hear of this engine in relation to GA applications of interest to the forum – maybe I am missing something.
And lo here it is www.zoche.de/ Does anyone have any info.
There is very little (one could say no) progress. It is a genius concept, I have never seen anything that beautiful when it comes to aero engines. Unfortunately, it’s a hobby project for Zoche and the guy isn’t getting younger and suffering from Parkinson. He and his partner are pacifists and refuse anything remotely related to military. They will probably never bring their engine to market but their ideas might survive.
better than cranking up the RPM to con-rod crushing levels, and cherry red cylinder heads and exhaust valves.
Hmm, last time I checked cherry red cylinders and exhaust values are more an issue of the current air cooled big bores and not the high-rev modern water cooled engines 
Yes, Mr. Zoche is a nice guy, I like him too. But it’s pretty clear that his engine will never see a market. I do not think he saw it as a “hobby” though.
Rotax solved the cherry red cylinder heads and exhaust vales with water cooling. Fine it works but adds to the complexity, and is something else to go wrong. You still have a lot of very busy metal flying around. That is why I am not keen on them.
Shame about Zoche. I think he started this decades ago. It is a good concept though and I hope someone, hopefully with deep pockets picks up on it.
Extract from the US AOPA magazine:
Shame about Zoche. I think he started this decades ago. It is a good concept though and I hope someone, hopefully with deep pockets picks up on it.
Yes, indeed. Given sufficient drive and the associated funding, this development could gain a significant market share. Every delay reduces its chances, as Thielert, SMA, Austro, and perhaps even DeltaHawk mature. The “modular” concept (half row, one row, two rows) allows offering a consistent model range, so that mixed fleets (flying schools!) could benefit of reduced spare parts stocks.
Surely the basic “one row” four cylinder with its 150 HP should appeal to the market of experimental/homebuilt/permit planes, so that a fair volume of sales could be realised even before the (expensive, and time consuming) certification?
