GarryW wrote:
Mark Sampson wrote:
I'm speaking beyond my knowledge here, but are not the turbos on aircraft used to maintain power at high altitude (where warbirds no longer fly)?
Unlike a turbo'd automobile where the boost is for a burst of acceleration on demand.
Still a terrible tragedy- I'm sure that the pilots did their best.
While I'm not an aerospace engineer or A&P mechanic, that's only partially correct. I worked with stationary turbo diesel engines for years which while they never flew (on purpose), they are quite similar in operation. Turbos pressurize the intake tract when the engine is under load. In the case with my experience with industrial equipment, the engine was under load constantly while its power was being utilized...much in the same way aircraft engines operate. Automobiles are a different animal as you are only substantially loading the engine during acceleration or while towing a heavy load. Most times, most of the force keeping a car going down the road is the inertia of a 1+ ton vehicle rolling down the pavement, only a small amount of the energy the engine is capable of generating is being used (very light loading). In an aircraft, the engine has a constant, fairly high load that depends on several things like the pitch of the props or the angle of attack of the aircraft. The turbos are always supplementing the engine. The altitude comes into play since the air pressure is less the higher you go. The turbo helps to minimize that differential but as you go higher, the engine will still lose power...just not at as great of a rate. That said, a turbocharged engine that is rated at 1200hp I am assuming is rated at 1200hp on the ground...the higher you go, that number will still decline. For sake of discussion, at 20,000 ft a 1200hp normally aspirated engine may only be generating 800hp. Turbocharge that same engine and it will make 1000hp at altitude. If I'm incorrect, please someone with more knowledge correct me...I'm just going off what I've read and my understanding of physics as a mechanical engineer.
That said, the people claiming "Well, in WWII B-17's were able to make it back to base on 2 engines" is a bit of a logical fallacy. With fully operational turbos even at sea level, they had quite a bit more power than the warbirds flying today with disabled turbochargers. However, I'm sure in WWII configuration, even a bomber coming back after a bomb run was still significantly heavier than the aircraft flying today. Its all a balance of lift and thrust vs. drag and gravity (weight)...more to the point, there are more variables than just "if 'x', then 'y'".
I think you're on the right track, perhaps minus the power addition any of the turbosuperchargers would contribute during sea level operations.
Generally speaking, aviation engines use forced induction to "see" sea level pressures at nearly all altitudes. Since these engines are already supercharged (as part of the core design, in the case/block) there really isn't much "boost" that can be applied to these things at sea level, at least not in these engines built for reliability and longevity over all-out air-racer style performance.
There are certain circuits in the carbs that have to be activated (kinda like opening the secondaries on a car carburetor) which can only happen at certain manifold pressures (and is critical for correct engine operation / air fuel ratios, doesn't really mean they need that much power).
I can pretty much tell you for a fact that these B-17s with disabled turbosuperchargers are still incredibly over-powered beasts in their current roles. Dropping the armor, bombs, oxygen systems, primitive avionics and radio/nav equipment, etc probably leaves these things about 10,000lbs under their original gross / MTOW but that's just a WILD guess on my part. I know our TBM is about
6,000lbs under its original gross wartime weight and that's for a single-engined carrier plane!
In any event, at sea level BDL sits at an altitude of 173' AMSL.. those engines will essentially be generating their rated horsepower regardless of modifications to the turbos (there may have been some slight boost settings used for takeoff, but at sea level that may require higher-octane fuels and certainly cuts the life of the engines somewhat). Again though - considering the weight of the plane, it's not even worth mentioning.
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