One cannot make a propeller driven aircraft fly faster than the speed of sound, because the prop blades act like an airbrake
approaching sonic speeds.
Instead of airfoil blades extending straight out, they are swept
back, like the wings of a supersonic fighter jet?
Are you asking why an aircraft propeller couldn’t be made to operate at supesonic speeds?
If so, well one reason might be that there is no market for them. We already have perfectly adequate propulsion that will get to supersonic in turbo-jet engines and rockets.
Another reason is that high-speed, high-performance aircraft generally do not have just one wing. Examples are all over in commercial jets. Set next to a window by the wing and watch the high-speed, low camber, high aspect ratio, cruising wing be replaced before your very eyes by a low speed, high camber, low aspect ratio, landing wing.
This sort of thing would be a real technological challange in a propeller blade.
It’s been done, on the Republic XF-84H, but it wasn’t worth the trouble. Performance was miserable, and the noise made not just the pilot but the ground crew sick. But trying things on the government nickel just to see what would happen was characteristic of the aerospace industry in the 1950’s - my favorite example is the Navy’s supersonic seaplane fighter that was all the way into flight test before they stopped and tried to figure out what to use it for.
Even for subsonic aircraft, propeller RPM’s are limited by tip speed. If the tip, which is the fastest-moving part of the blade, goes supersonic, its efficiency drops.
The Hawker Sea Fury had a 5-bladed prop. With fewer blades on that powerful engine, the prop tips went supersonic. This caused a number of problems (which naturally I can’t recall right now), prompting them to add another blade to the prop.
I know that the tips of a prop cannot reach the speed of sound because as Mach 1 is approached, the resulting shockwaves slow the airplane down through reduced efficiency. I don’t see how the airplane itself could move through the air faster than it’s prop tips (and we’re assuming here that the airplane is using the prop for propulsion, it has occurred to me that you can go faster than your prop if the engine has failed :rolleyes: ).
Found this cite which covers why a prop plane can’t exceed the speed of sound healthily. There have been some reported cases of prop planes exceeding the speed of sound in the process of destroying themselves, but I don’t know if they’re official reports and I think most of the pilots died. I don’t know if that counts. I mean, you do want the plane and pilot to be usable again, don’t you?
I’ve been reading about props lately and it seems that there is an ideal blade width that gives the best efficiency for an engine putting out X rpm. Also, the longer the blade, the more thrust you get but there’s a limit to blade length, because you can’t let the tips get to close to the speed of sound. Once you reach optimum blade length and width, the only way to increase speed is to add more blades but you have a reduction in efficiency because with more blade surface, you also get more drag.
A way to get around this is using Q-tips, a blade (by Hartzel, I think) on which the tip is bent over at a 90 degree angle. The angle reduces tip vortices and noise, so I think it allows a longer i.e. more effective blade but they still can’t exceed the speed of sound. So I guess they already do sweep back the props.
Sorry if I covered information you’re already familiar with, and if I made a mistake, please correct me. (Unless it’s in spelling- I’m hopeless in that category.)
But to David Simmons, I think that there would be a market, however small, for a supersonic prop plane for the speed demon who just wants to go fast. Turbines, after all, are prohibitively expensive.
Not exactly. Drag increases very quickly through the transonic regime (i.e. as parts of the aircraft “go supersonic”. Local airflow, such as over the wing upper surfaces, may exceed free stream airflow, causing some parts of the aircraft to break the sound barrier before others.) As the aircraft accelerates, drag soon matches available thrust (propeller, jet or rocket makes no difference), at which point the aircraft is at its top speed.
In the specific case of a propeller exceeding the speed of sound, the same holds true even if the rest of the aircraft is nowhere near Mach 1. As the tip goes supersonic, drag on the propeller rises dramatically, soon matching available engine torque.
I’d imagine that a prop plane designed to exceed the speed of sound would also be prohibitively expensive. You’d probably need a turboprop engine to get enough power at a reasonable weight, at which point it would be roughly the same cost as a turbojet. And designing a propeller to operate efficiently both supersonically and subsonically would be a trick.
In general terms, propellers are most efficient at moving a large mass of air relatively slowly (efficiency being related to prop diameter). Turbojets are best at moving a small mass of air very quickly. That’s why propellers are better on relatively slow aircraft and turbojets are best on very fast aircraft. Turbofans, which are basically a ducted propeller mounted on the front of a turbojet, are a compromise intended to maximize efficiency both at high and low speeds.