Fanjet efficiency questions.

[PETTIFOGGER]

So my questions remains. If this fan that is in effect nothing more than a shrouded propeller (fixed pitch, at that), providing about 80% of the thrust and regularly manages to get these aircraft to cruise at almost sonic speeds - then why aren't all aircraft designed with this ingenious high speed device?

I suspect it's very simple - the sheer excess in horsepower is providing these numbers. The abandoned propfan experiments showed that it did not suffer in speed compared to the shrouded fan, so it's certainly not the case that a prop can't be just as fast or effective.

I have a sneaking suspicion that if you designed an aircraft with exactly the same amount of horsepower powering either a shrouded fanjet, or an unshrouded fan/prop, they would probably be exactly as fast and fly as high.

Hi Adam,
At a quick glance, the mathematics do not appear to support your suspicion. You may wish to check:
Turbofan Thrust
Turboprop Thrust Propeller Thrust
Propeller Analysis
rgds

[Mark1234]

Originally Posted by AdamFrisch

I have a sneaking suspicion that if you designed an aircraft with exactly the same amount of horsepower powering either a shrouded fanjet, or an unshrouded fan/prop, they would probably be exactly as fast and fly as high.

Nah, your intuition is wrong. Very wrong. Most likely because you're failing to consider that static thrust isn't the full picture. VinRouge actually pointed this out, but in simple terms:

Remember Mr Newton and his f = ma equation?

Let's say the prop shifts a huge amount of mass - but the aistream is 300kts. Once the aircraft airspeed reaches somewhere in the vicinity of 300kts, it doesn't matter how big 'm' is, theres no 'a', so F=0. Thus you don't go fast.

If the jet's moving a much smaller mass at 600kts, it still has some F at 400kts 'cos both 'm' and 'a' are positive terms.

Props go great at lower airspeeds - jets are faster.