Jump to content

Theoretical speed from prop pitch?


Recommended Posts

If a propeller is turning 3,000 rpm and is 48 inch pitch is the theoretical forward speed 136 mph? (3,000 rpm x 60 minutes x 4 ft / 5280 ft = 136.3636 mph)

 

Obviously drag and slip will reduce this.

 

I ask because then a 400 mph airplane would need at 3,000 rpm a prop with over 12 feet of pitch and I don't know if that is possible.

 

Just a curiosity.

 

 

Link to post
Share on other sites

Well I'll bite. No because a propellor is not a screw boring through the air. It's a wing creating lift. Think of a yacht, a well designed and trimmed sail can sail way faster than the wind speed.

 

 

  • Like 1
Link to post
Share on other sites

Yes, your basic calcs are on the right lines. It is a screw working its way through the air but fallowdeer is also correct, the lifting aspect of the blades is a more efficient way of doing the same thing, than using flat (i.e non airfoil shaped) paddle blades.

 

IIRC, as a rule of thumb, and since air is compressible, you might allow 20% for slip, more in draggy situations, and less for clean airframes.

 

Any amount of pitch is possible but what happens is that with very high pitch angles, during take-off the airscrew blades will remain fully stalled until considerable airspeed is achieved. That means lots of horsepower to even be able to get airborne, and then remaining in ground effect to build airspeed until the relative airflow angle becomes sufficient to unstall the blades.

 

That requirement for high horsepower is what might be expected anyway for a 400mph airplane, but careful piloting is critical, any attempt to climb while the blades are stalled and producing heaps of drag can, and have been (in racing types of planes particularly), catastrophic.

 

That is where variable pitch propellers come into their own ...

 

Welcome to the forum, we need more Enigma, or should that be Enigmas ... or is it Enigmata?

 

Damn ... you've been here five minutes and created an enigma already ...

 

 

  • Winner 1
Link to post
Share on other sites

Enigma, you are quite right and it is only with a pitch-adjustable prop that you could do this and take-off too. At 400 mph, the pitch would indeed need to be near 12 ft for the airfoil to work as fallow suggested.

 

And the tips are probably over the speed of sound... so thats why prop planes hit a limit.

 

 

Link to post
Share on other sites

So "a component of lift" + the theoretical screw advancement of the pitch of the propeller - the "slip" (20% more or less) = forward speed.

 

Thank God and those that have gone before for empirical data.

 

 

Link to post
Share on other sites
So "a component of lift" + the theoretical screw advancement of the pitch of the propeller - the "slip" (20% more or less) = forward speed.Thank God and those that have gone before for empirical data.

Well then there is the load factor; the coarser the pitch, the more load on the engine (like a "higher gear"), the more the rev drop, the less the forward speed, so you have to factor in what the rpm increase/decrease will be.

 

 

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...