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Post by Anonymous on Feb 3, 2019 3:38:10 GMT
Conceptually, from the standpoint of aerodynamics only (ignoring power, material strength, etc.), could a propeller-driven aircraft break the sound barrier? Every answer to this on the Internet that I have seen so far has been something like, "it's not feasible," or, "nobody's ever done it before."
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Post by tepidbread on Feb 3, 2019 6:05:57 GMT
I would say technically yes, Although, practically no. I am by no means an expert but a propeller has very high tip velocities that cause massive amounts of mach drag to the prop and immense stresses along the entire blade. Technically the biggest difference between a turboprop engine and a commercial turbofan engine is the bypass ratio and lack of shroud around the blades in addition to the gearing. A propfan is a combination of the two. As far as I can see there are no aerodynamic limitations that would prevent an prop aircraft from reaching the speed of sound. The only things that prevent this are material, noise, power and efficiency considerations. Technically a turbofan is just a turboprop with smaller and more numerous blades. So one could argue that prop aircraft already exceed the speed of sound.
However, supersonic props are a novelty that no engineer in their right mind would bother considering for any application, reasonable, or unreasonable. As evidence of this let me list some of the problems:
"The XF-84H was destabilized by the powerful torque from the propeller, as well as inherent problems with supersonic propeller blades."
"The XF-84H was quite possibly the loudest aircraft ever built (rivaled only by the Russian Tupolev Tu-95 "Bear" bomber), earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger". On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away. Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run. Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews. In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H."
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Post by airc777 on Feb 3, 2019 11:38:26 GMT
Sure they can, they do it all the time. All you need is a ducted prop.
Alternatively there's probably been someone somewhere whose accidentally gone transonic in a dive.
But seriously though, this thing and these things might be of interest to you:
I can tell they're not what you meant, but the fact that none of these went supersonic and we aren't building them anymore should tell you something about how much easier it is to just duct the prop call it a jet and go much faster.
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Post by AtomHeartDragon on Feb 3, 2019 13:48:19 GMT
Technically a turbofan is just a turboprop with smaller and more numerous blades. And a duct.
Also, I don't know how important it is for bare workability of an engine, but supersonic jet engines (regardless of their exact type) tend to have complex intake geometry to deal with the supersonic flow they encounter.
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Post by walterscientist on Feb 4, 2019 13:44:56 GMT
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Post by AtomHeartDragon on Feb 4, 2019 18:14:12 GMT
That's the point. They first need to make supersonic airflow subsonic, accomplished via intake geometry.
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Post by shurugal on Feb 20, 2019 0:01:45 GMT
Conceptually, from the standpoint of aerodynamics only (ignoring power, material strength, etc.), could a propeller-driven aircraft break the sound barrier? Every answer to this on the Internet that I have seen so far has been something like, "it's not feasible," or, "nobody's ever done it before." The biggest barrier to propeller-drive aircraft breaching the sound barrier is what happens when the propellers are exposed to supersonic airflow. Since the tips of the propeller have a higher linear speed than the parts of the propeller closer to the hub, the prop will go supersonic from the tips in to the roots. This results in a large speed range over which part of the prop is supersonic, while the rest is subsonic. The supersonic region experiences the shockwave forces associated with trans and supersonic flight, while the subsonic region does not. This results in extremely unbalanced stress on the blade, which will quickly destroy it. NASA, during a research project into this subject, discovered that a very short, very fat propeller can survive the transition into supersonic flight. The problem with this is that short fat propellers are extremely inefficient. Propellers are, after all, just miniature wings spinning in a circle at high speeds. Wings with low aspect ratios (short span, wide chord) have very high drag for the lift (or thrust) they produce. This is due to a phenomenon called a wingtip vortex. Wingtip vortex size is determined, in part, by a positive relationship with the chord of the wing: the larger the chord, the larger the vortex. The part of the wing in the flow of the vortex produces significantly less left, as the airflow in this part of the wing is bleeding energy to the vortex instead. When you have a short wing with large vortices, then a very large percentage of your wing is spending energy making the vortex, instead of making lift. This is why propellers are traditionally long and skinny - to keep that vortex as far away from the part of the prop making thrust as possible. The short answer is "yes" you can design a prop that can drive an aircraft at supersonic speeds. But by doing so you sacrifice efficiency to such extremes that there is no point in doing so: A properly engineered turbojet or turbofan (with a properly engineered intake) is a much better way to get the same end result. It's kind of like autogyros: they blend the major drawback of a plane (you have to keep moving forward to keep flying) with the major drawback of helicopters (you can't go fast) - so they don't see any serious use. |
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