Propeller Arrangements

KJ_Lesnick · November 13, 2015, 06:28:27 AM

Contra-Rotating Propellers

I'm curious about the effects of contra-rotating propellers on performance: I know it counteracts the helical airflow from the first propeller blade which negates torque by negating the helical flow results in the flow going back faster. I wouldn't be surprised if it added additional speed to the flow as props basically (like any engine) accelerate airflow.

As I've heard of contra-rotating aircraft that used either a single engine (Macchi M.C.72, Fischer P-75) and twin-engines (Douglas XB-42): I'm curious if the thrust is doubled?

Push & Pull

I've seen some aircraft (HP V/1500; Dornier Do 335) that have utilized a propeller in the front, and one in the back: I'm curious if the advantages are the same as a contra-prop?

PR19_Kit · November 13, 2015, 08:00:30 AM

The MC.72 actually had two engines, one mounted behind the other, and each drove their own prop independently.

KJ_Lesnick · November 13, 2015, 08:22:09 AM

PR19_Kit

I thought so too, but I checked Wikipedia: I guess they were wrong. Big surprise

jcf · November 13, 2015, 10:12:20 AM

The AS.6 was considered a single 24-cylinder engine. Yes, it is technically two AS.5 V-12 in tandem, but
the induction system, ignition and valve timing etc. was setup for it to run as a V-24. They had horrendous
problems with fuel vapour explosions in that long intake trunk.

Hobbes · November 13, 2015, 12:43:52 PM

Quote from: KJ_Lesnick on November 13, 2015, 06:28:27 AM

As I've heard of contra-rotating aircraft that used either a single engine (Macchi M.C.72, Fischer P-75) and twin-engines (Douglas XB-42): I'm curious if the thrust is doubled?

If thrust were doubled everyone would have been using contraprops from the moment they found out about the thrust advantage.

Propeller thrust is a product of propeller area, blade pitch and speed. It is limited by the amount of power you put into the propeller.
If you keep the same engine, there's no way going from one large prop to a contraprop with the same area will increase thrust much.

There is one exception: if your single prop is speed-limited by tip speed, going to a contraprop (more blades, so can be a smaller diameter for the same prop area) can increase thrust.

KJ_Lesnick · November 13, 2015, 04:19:03 PM

JCF

QuoteThe AS.6 was considered a single 24-cylinder engine. Yes, it is technically two AS.5 V-12 in tandem, but the induction system, ignition and valve timing etc. was setup for it to run as a V-24.

So it was sort of a single engine and sort of a twin-engine?

QuoteThey had horrendous problems with fuel vapour explosions in that long intake trunk.

That doesn't sound good...

Hobbes

QuoteIf thrust were doubled everyone would have been using contraprops from the moment they found out about the thrust advantage.

Yeah, that's a good point (unless weight doubled)

QuotePropeller thrust is a product of propeller area, blade pitch and speed. It is limited by the amount of power you put into the propeller.
If you keep the same engine, there's no way going from one large prop to a contraprop with the same area will increase thrust much.

There is one exception: if your single prop is speed-limited by tip speed, going to a contraprop (more blades, so can be a smaller diameter for the same prop area) can increase thrust.

Uh, it isn't always limited by tip-speed? I figured that by the time you were doing around 300-350 mph you were basically reaching tip-velocities that would be transsonic/supersonic at the tips with maximum RPM (the reason is due to the incoming airflow and rotational velocity).

BTW: When did we understand the blades actually were going supersonic?

wuzak · November 13, 2015, 05:00:32 PM

Quote from: KJ_Lesnick on November 13, 2015, 04:19:03 PM
JCF

QuoteThe AS.6 was considered a single 24-cylinder engine. Yes, it is technically two AS.5 V-12 in tandem, but the induction system, ignition and valve timing etc. was setup for it to run as a V-24.
So it was sort of a single engine and sort of a twin-engine?

It was a single engine in teh same way that the V-3420 was a single engine, except that the AS6 could have one half spinning by itself, and that's how it was started.

In both cases there was only one supercharger and one set of accessories. The difference was that the V-3420 had the crankshafts geared together, so they couldn't turn independent of one another, but the two crank shafts of the AS6 were separate and not connected.

Here is a video of the Macchi MC72 with AS6.
https://www.youtube.com/watch?v=gLRLhZJsCh4

The start up is shown at ~52s. The forward prop, driven by the rear crank, starts first. I believe this is because that is the crank to which the starter was connected, and it is also because the supercharger and accessories were connected to the rear crank. That is, the front crank couldn't operate without the rear crank working.

PR19_Kit · November 13, 2015, 06:19:05 PM

Quote from: wuzak on November 13, 2015, 05:00:32 PM

except that the AS6 could have one half spinning by itself, and that's how it was started.

Exactly my original point......

And it worked well enough to break, and hold to this day, the world piston engine seaplane speed record. Just a pity the only 1/72 model of the MC72 is such a bag of bolts.

KJ_Lesnick · November 13, 2015, 07:00:24 PM

wuzak

QuoteIt was a single engine in teh same way that the V-3420 was a single engine, except that the AS6 could have one half spinning by itself, and that's how it was started.

In both cases there was only one supercharger and one set of accessories. The difference was that the V-3420 had the crankshafts geared together, so they couldn't turn independent of one another, but the two crank shafts of the AS6 were separate and not connected.[/quote]That seems kind of like two engines almost working as one...