Propellers = drag...?

[SgtPappy]

As I watched a special on Lockheed one Discovery Channel night, I got the the part about jet engines. At one point, jets were going so fast that the velocity of the gasses escaping the engine was actually less than that of the air entering it. They solved this with new technologies like turbofans, (sc)ramjets and compression chambers shaped like bottle noses.

Made me think. I think it's likely that WWII fighters were approaching the same kind of speed. The plane must have reached a speed at which point the prop would actually not be rotating fast enough to create a thrust velocity faster than that of the air hitting the plane. So the question is: Did WWII fighters approach such speeds where the propeller became a huge drag load? Is so, is that modeled in our game?

[Spatula]

I'm certainly no expert in this field, but i'm fairly sure that the faster a prop aircraft goes, the less efficient the propeller becomes. Taking this to its logical conclusion, my thoughts would be that it must reach a point where it can no longer produce useful thrust, and therefore is unable to accelerate the aircraft any further. So, it would then seems that it (the propeller) can't push/pull the aircraft to a speed where it creates more drag than thrust. Only to a point where it can no longer produce usable thrust. Drag on the airframe vs engine power may stop it ever reaching this point as well - but im ignoring airframe drag for now.

I guess a dive, using gravity to exceed the speed the propeller and engine could normally propel the aircraft to, would mean exceeding the the above point and then i would assume it would create more drag than thrust.

Like i said, im no expert here, and dont take my reply as a statement of fact, just my own quick thoughts.

[Cthulhu]

I'm certainly no expert in this field, but i'm fairly sure that the faster a prop aircraft goes, the less efficient the propeller becomes. Taking this to its logical conclusion, my thoughts would be that it must reach a point where it can no longer produce useful thrust, and therefore is unable to accelerate the aircraft any further. So, it would then seems that it (the propeller) can't push/pull the aircraft to a speed where it creates more drag than thrust. Only to a point where it can no longer produce usable thrust. Drag on the airframe vs engine power may stop it ever reaching this point as well - but im ignoring airframe drag for now.

I guess a dive, using gravity to exceed the speed the propeller and engine could normally propel the aircraft to, would mean exceeding the the above point and then i would assume it would create more drag than thrust.

Like i said, im no expert here, and dont take my reply as a statement of fact, just my own quick thoughts.

Basically correct

At high speed, the airspeed that the propeller's tip sees (due to combined prob rotation and forward speed) moves into the transonic range. Shock waves build up on the prop blades such that it's much like trying to turn the prop thru mollasis (not to mention the fact that the sudden change in pressure distributions on the prop wants to tear it to pieces). Special high speed props have been built, but the gains have been negligible. Basically there's a practical limit to how fast a prop can move thru the air. Jet engines get around this by keeping compressor airflow in the subsonic range and by compressing the airflow in stages. All those fancy inlet ramps, spikes, etc. you see on jets are there to help see that subsonic flow enters the engine.

And yeah, in a very high speed dive, the prop does effectively become a big brake.

[Serenity]

And yeah, in a very high speed dive, the prop does effectively become a big brake.

An interesting trick with this:

Next time you dive in AH, power off, cut your rpms back to minimum. See how much faster you accelerate and how much of a higher speed youll get.

[Cthulhu]

An interesting trick with this:

Next time you dive in AH, power off, cut your rpms back to minimum. See how much faster you accelerate and how much of a higher speed youll get.

I'm curious, wouldn't reducing rpms in a dive give you a coarser prop pitch and hence less drag?

Oh, and you left out that after doing all that, you should find yourself pointing straight down, 100ft off the ground, going Mach "JESUS!"

[splitatom]

i thought props could only go so fast and thats why they stoped making them

[AKHog]

Did WWII fighters approach such speeds where the propeller became a huge drag load? Is so, is that modeled in our game?

In an adjustable pitch system like most if not all of the ww2 fighters had it would be normal adjust the pitch of the propeller often in flight. Most of these systems can reach a fairly aggressive pitch angle, keeping the rpms low even at high speed. So to answer your first question, yes the prop creates drag, but this is negated to some extent as long as you can adjust to a high pitch and keep the rpms low.

To answer your second question, test this by doing what serenity suggests, and experiment with rpm and speed in a dive. If you lower your rpm [increase prop pitch] you should accelerate to a higher speed than if you keep your rpm at its normally higher setting. Also note when you land with a damaged engine/prop and it is not turning, you tend to float down the runway a lot longer than if you have an engine running at idle. Both of these things would tell me that prop drag is modeled.

[Serenity]

I'm curious, wouldn't reducing rpms in a dive give you a coarser prop pitch and hence less drag?

Exactly. Thats why in a power-off dive, you accelerate faster and reach a higher speed with low RPMs than high

[SgtPappy]

Thanks all. I'll try that AkHog. I'll try and get myself a good timer so as to really model the acceleration. Just for fun maybe I'll smack myself into the ground

[Cthulhu]

Exactly. Thats why in a power-off dive, you accelerate faster and reach a higher speed with low RPMs than high

You know I'm gonna start doing this when I inevitably glide my Dora back to base after tangling with buffs. Changing prop pitch might just reduce drag enough to get me home sometimes. Need to experiment.Too bad we're not able to completely feather the prop. I'm surprised you P-38 drivers haven't put this on your wishlist... or have you?

[Serenity]

You know I'm gonna start doing this when I inevitably glide my Dora back to base after tangling with buffs. Changing prop pitch might just reduce drag enough to get me home sometimes. Need to experiment.Too bad we're not able to completely feather the prop. I'm surprised you P-38 drivers haven't put this on your wishlist... or have you?

We/they have

Yeah, whenever my engine gets killed, the first thing I do is cut my RPMs while the prop is still spinning. It makes a GIGANTIC difference.

[bozon]

Exactly. Thats why in a power-off dive, you accelerate faster and reach a higher speed with low RPMs than high

Not in the initial stages of the dive.

The real problem that required to lower RPM in the dive was not to over-rev the engine.

[Gianlupo]

You know I'm gonna start doing this when I inevitably glide my Dora back to base after tangling with buffs. Changing prop pitch might just reduce drag enough to get me home sometimes.

You mean you never did that???

It's the basic drill for dead engine: lower RPM and hit alt+x to achieve best gliding speed (meaning the speed that will allow you to cover the most horizontal distance possible).

[Golfer]

You mean you never did that???

It's the basic drill for dead engine: lower RPM and hit alt+x to achieve best gliding speed (meaning the speed that will allow you to cover the most horizontal distance possible).

Use caution...I use Alt-X in climb and use the .speed function to set my preferred climb speeds depending on the airplane I'm flying.  It works great if you didn't manually set a speed in prior to your initial hitting of speed mode on the autopilot.

[Cthulhu]

You mean you never did that???

It's the basic drill for dead engine: lower RPM and hit alt+x to achieve best gliding speed (meaning the speed that will allow you to cover the most horizontal distance possible).

Just never bothered varying pitch. I fly a Dora, so I'm a masochist at heart anyway. But buff drivers like 999000 have made me into a top-notch glider pilot.