Propellers = drag...?

[BoilerDown]

I've known about the low RPM trick for a while, but what gets me is when I make it back to base (if I make it back), I tend to overshoot because I have less drag than the usual throttled-engine landing that I normally make.  And then wreck it just past the far end of the runway 

[RTHolmes]

yeah min rpm/max pitch for gliding, max rpm/min pitch for dive bombing as a brake. I always try to leave a 60s or so engine power for the landing - for control/trim (I dont manually trim) but mostly for braking.


ok heres a good one - take a spit XIV to 21k, level off and when you get to 350ish keep the throttle at max and wind rpms back all the way. rpm bottoms out at about 2650, makes no difference to your speed, but fuel flow goes from ~140gph to ~80gph.   5 perkies for the best explanation

[Gianlupo]

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.

yeah, you have to be quite masochist to fly a Dora! (I love the plane, unfortunately,it just doesn't suit my way of fighting)

Good one, Holmes, never knew about that.... who knows if it works even with other planes....

BoilerDown, just play with the RPM setting to get the approach right. When I come near the runway I begin alter the RPM to "slow down" the airplane and not overshoot the strip. Try it, you'll be amazed seeing how increasing RPM slows down the airplane.

[BaldEagl]

I've known about the low RPM trick for a while, but what gets me is when I make it back to base (if I make it back), I tend to overshoot because I have less drag than the usual throttled-engine landing that I normally make.  And then wreck it just past the far end of the runway 

I raise the RPM again on final approach then play with it (and flaps) to maintain best final approach speed.

[BoilerDown]

BoilerDown, just play with the RPM setting to get the approach right. When I come near the runway I begin alter the RPM to "slow down" the airplane and not overshoot the strip. Try it, you'll be amazed seeing how increasing RPM slows down the airplane.

So nice to see that avatar again   Anyways that's a good idea and I should have thought of it myself, I'll use it next time, thanks!

[Gianlupo]

So nice to see that avatar again   Anyways that's a good idea and I should have thought of it myself, I'll use it next time, thanks!

You're welcome. Both for the advice and the avatar.

[BnZ]

yeah min rpm/max pitch for gliding, max rpm/min pitch for dive bombing as a brake. I always try to leave a 60s or so engine power for the landing - for control/trim (I dont manually trim) but mostly for braking.


ok heres a good one - take a spit XIV to 21k, level off and when you get to 350ish keep the throttle at max and wind rpms back all the way. rpm bottoms out at about 2650, makes no difference to your speed, but fuel flow goes from ~140gph to ~80gph.   5 perkies for the best explanation

Fewer RPMs=engine turning few times per minute=fewer sips of fuel per minute for each cylinder.

[Chalenge]

Fewer RPMs=engine turning few times per minute=fewer sips of fuel per minute for each cylinder.

Fewer rpms equates to more prop pitch which means its moving more air per revolution.

[RTHolmes]

Fewer RPMs=engine turning few times per minute=fewer sips of fuel per minute for each cylinder.

true, but my question is why doesnt increasing the prop pitch to max reduce rpms below 2650...

[FireDragon]

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).

If you get a radiator hit you can get a full sector...Ive done it in many planes.... I like to get a little alt as engine starts to over heat and get  speed up as high as possible.... then shut engine off and quickly feather the prop I use shift x and nose down.... based on currant alt and distance to closes base....  as the engine cools down restart engine and go till it hits read line......repeat as needed

[dtango]

true, but my question is why doesnt increasing the prop pitch to max reduce rpms below 2650...

Because you've reached the maximum blade pitch angle and the associated opposing torque the propeller can produce.

Constant speed props use the concept of varying the torque needed to spin the prop in order to maintain a constant RPM.  The prop governor adjusts blade pitch to vary the amount of torque needed to turn the prop which controls the RPM.  To reduce RPM while the engine is running, the governor increases blade pitch to increase the torque needed to spin the prop which results in slowing down the rotation of the crankshaft. 

There is a limit to how much the blade pitch angles can be varied.  Constant speed props have an min and max blade pitch angles set by physical mechanical stops.  What you are seeing is that we reached a maximum blade pitch angle that can be acheived due to the mechanical stop and thus no more increase in torque to reduce the RPM further while manifold is wide-open.

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A few other clarifications for the thread in general....

(1) The drag produced by a propeller slicing through the air as well as drag rise due to compressibility affects with prop tip speeds nearing mach 1 do not directly impact the overall drag of an airplane because this drag is acting generally tangentially to the direction of the airplane's travel.  They greatly influence the propeller efficiency however but not overall drag of the airplane.

(2) Propellers can produce negative thrust which can greatly impact the overall drag of an aircraft.  This is typically what is meant by prop-drag.  This occurs when the blade angle of attack becomes negative.  Prop blades are like wings and produce lift at some angle of attack.  This lift basically acts in the direction of airplane travel.  We call this thrust.  If the blade angle of attack becomes negative the propeller produces thrust generally in the opposite direction which then adds to the overall drag of the airplane.  3 factors govern blade angle of attack: 1) blade pitch angle, 2) forward velocity, 3) prop rotational velocity.  There are a variety of scenarios when blade angle of attack can go negative depending on the interplay of these 3 factors.  And yes prop-drag is modelled in AH as pointed out by everyone regarding the reduction of RPM's with engine out or idle.  This serves to decrease drag produced by a windmilling propeller.

Tango, XO
412th FS Braunco Mustangs

[Cthulhu]

If you get a radiator hit you can get a full sector...Ive done it in many planes.... I like to get a little alt as engine starts to over heat and get  speed up as high as possible.... then shut engine off and quickly feather the prop I use shift x and nose down.... based on currant alt and distance to closes base....  as the engine cools down restart engine and go till it hits read line......repeat as needed

I do the same thing. But depending on initial altitude you can go a lot further than one sector. I  run the engine ~3 seconds on/5 seconds off at a reduced throttle setting/low rpm setting, "dancing" just short of engine seizure. You can limp quite a ways doing that.

What Gian was talking about was gliding home with no engine, not a damaged radiator. I hardly ever lose the engine, but the Dora's friggin' radiator is made outa glass.

[Cthulhu]

Because you've reached the maximum blade pitch angle and the associated opposing torque the propeller can produce.

Good write up dtango. 

I set out this morning to tackle this question, but after I read some of the crap I wrote:
"the airflow vector the prop sees (the prop's local angle of attack) is the resultant of the rotational component and the free stream component."
I said screw it and went back to surfing the web.
Pretty sure your explanation is more intelligible than mine would have been.

I do have a question though. Is it possible that he hasn't reached the mechanical stop at all? But that instead, by increasing prop pitch (and torque load) to a high value, has increased torque load to the point where it exactly balances the engine's max output torque. Any attempt to increase prop pitch (lower rpm) will produce an increase in load beyond the engine's capability, thereby slowing the engine down, which in turn supply's less torque to the prop, which in order to maintain the desired prop rpm must reduce prop pitch, which in turn reduces the torque load on the engine, which in turn allows the engine rpm to increase, which produces more torque, which allows the prop to go to a coarser pitch, which...... ad infinitum. I think what we're seeing is a dynamic system that's stabilized.

Just a thought

(I know there's a reduction gear between engine and prop, but I've left it out for simplicity. It really doesn't play a role in the mechanical ballet I described above)

[dtango]

Interesting theory Cthulhu.  However in this case we've dialed the RPM setting for the governor back to the lowest it will go yet the RPM's remain higher.  In this situation if indeed the engine has maxed out in it's torque it can produce than additional torque added by the prop would continue to slow the crankshaft down.  With the RPM's not even as low as govenor allowed, the governor would continue to want to slow the crankshaft down vs. trying to speed it back up to a particular RPM to maintain some equilibrium with torque available from the engine.

However the dynamically stabilized concept you describe is pretty much how a constant speed prop works to keep that RPM constant .

Tango, XO
412th FS Braunco Mustangs

[SgtPappy]

Does the crankshaft slow down due to the drag induced by the propeller's pitch? I got really lost in that explanation. 

Additionally, if we're aiming for such high performance and more thrust, wouldn't it be smarter to pitch the blade more to attain the performance? It bites more air after all. Unless of course my drag theory is correct in which the drag of the props simply cannot allow the prop to spin fast enough to create as much thrust as possible.