Let's talk about props...

[Cjserio]

Here's the situation...The aircraft's a Piper Arrow IV...Constant Speed prop...

Let's say we're at cruise at 6,000MSL and let's say we're at 2400RPM and maybe 23-24" of MP cruising around 130kts or so. It's time to descend.

Conventional "wisdom" (I use the term loosely) says you start bringing the throttle back a bit (maybe a couple of inches), pitch down and do a cruise descent...leave the prop alone. You're fine in the descent, cooling the engine a bit until it's time to slow down for the pattern...then you essentially have to chop the power back a bunch...when you manage to get the speed down, deploy the gear, get into the white arc and start bringing in flaps as appropriate to continue slowing. Eventually on final or short final you can bring the prop level full forward (high RPM) now that you're off the governor and land.

I'm doing some recurrency training with a new instructor and he's adamant about the procedure he's been using for decades. His procedure is essentially to creep both the MP and the prop RPM back somewhat simultaneously. Come back a couple of inches of MP, prop back 200RPM or so and pitch down. As it's time to slow in the pattern, bring the MP back a bit more, prop back a bit more as well etc.

His reason is that reducing the prop RPM with the MP will allow you to reduce your airspeed quicker which means you won't have to "chop the power" as drastically...which of course means reducing temperature stress on the cylinders.

This seemed counter-intuitive to me since I've always thought of the props having the highest drag due to windmilling when at the lowest blade pitch (full forward)...so reducing prop RPM when trying to slow the plane seemed backwards...but his statement is that on an arrow, the difference between pitches is minor (15 degrees from extreme min to max) and that reducing the prop rpm will have more of an effect on reducing the power/thrust it's putting out than flattening the blade angle can by increasing it's drag. Also, it's not going to be "windmilling" substantially anyway until you're nearing idle...and since we're trying to avoid having to chop the power back that far anyway, it's not a factor.

Our "test flight" to prove his theory to me today got rained out but I'm not sure what to think any longer. I feel like training nowadays is very lacking on how to USE constant speed props. There are hundreds of books on how they work but very little written on how to use them. Everyone seems to simply go full forward on takeoff and on short final...and only ever use 2500/2400/2300RPM depending on the cruise setting they choose. Surely the prop lever is a lever and not a switch for a reason.

How are YOU using your prop lever?

 

[deyoung]

If I'm descending from cruise, where the RPM is already back at a cruise setting, I leave it alone until it's time to go full forward for landing. I'm open to a good explanation for why a different approach might be better, though.

 

[eman1200]

I wonder how his theory impacts a go-around.

 

[MAKG1]

I wonder how his theory impacts a go-around.

Once the throttle is low enough, you push the prop full forward and it doesn't come off the stop.

Engine heat generation is roughly MP*RPM, so his approach is equivalent to pulling 4 inches of MP at a time instead of 2. Sure, it will work for slowing you down, and on an Arrow, it won't make any difference for engine longevity (not even chopping the power will do that….).

If it were me, I'd just pull off more throttle and keep the engine spinning as fast as it will. That keeps the oil pressure up, and the peak cylinder pressure down (though at low throttle, the peak cylinder pressure is not a problem -- you won't detonate).

I tend to leave the prop alone as well except for takeoff and landing.

Flat plate drag is for when the air is turning the prop, rather than the other way around, and the engine losses turn into drag because nothing compensates them. You'll get much more drag from the prop at coarse pitch, where the angle of attack is greater.

 

[coloradobluesky]

The instructor's way will work, so I'd do it that way. Then after I get his signoff, I'd go out and do it MY way (I don't pull the prop back on descent, no reason to, but I don't see it hurting anything). I think it is a good idea to pull some power and cool the engine somewhat before pulling all the power abeam the numbers (and do a power off landing--that's my engine out drill also). Like I say, you need your instructors signoff, so get used to doing it his way. Wont hurt a thing. Make you humble and versatile as a side effect.

 

[James331]

My first job was in a turbo arrow IV.

I used to bring the prop back a little for long descents, basically keep the speed up best I could, arrow is quick enough for what she is, but for pax it ain't a LR60, so whatever you can do to expedite helps


Careful how much power you chop on your way down, I usually like to keep it in the green outside of short final, if you have a engine monitor keep a eye on it.

If I'm descending from cruise, where the RPM is already back at a cruise setting, I leave it alone until it's time to go full forward for landing. I'm open to a good explanation for why a different approach might be better, though.

Doesn't, once you're way back on the power you can just put the prop forward, if you need to go around before or something put the prop forward

 

[CJones]

Not in an Arrow, but...

Beginning the descent, I leave the prop at cruise setting (2350 RPM) and pull MP back to 19". At 500fpm descent, that puts me in the top half of the yellow arc on airspeed through the descent. I keep pulling MP back as I get lower to keep it at 19". When I level out, I pull the MP back to 13" and pull the nose up a bit to stay at TPA. Around midfield downwind, I'll be at flap speed, so I add half flaps. About abeam the numbers, I pull power back to about 10" MP (I still haven't touched the prop control yet, but it's usually off the governor at this point) and go into about 300fpm descent. Turning final, I'll put in full flaps and push the prop forward in case of go-around. I'll usually hold about 8-10" of MP with 300 fpm descent all the way to the flare when I'll pull power to idle.

If I do the process right, I never add power back in after starting the initial descent.

 

[Tarheelpilot]

If you're having to chop the power to get configured for landing then you're doing it wrong and it has nothing to do with your prop rpm.

As far as your instructors recommendation, I don't see any benefit. It does sound like a real pain in the butt.

 

[txflyer]

I only push the prop in when the MP has come off enough to let the RPM get below the green.

Otherwise you get the sonic noise in the pattern and every pilot on the ground laughs or shakes their head.

 

[tmyers]

You are going to gain about 1" of MP for each 1000' you descend. To make life simple I pull MP back one inch for each one thousand feet of altitude difference to pattern altitude. Nose the airplane down at say 1000 FPM decent. You will be descending at about 160 kts with and will arrive at pattern altitude back at your cruising MP. This will pick up any speed lost during the climb as well.

Only reason I would slow down in the descent would be in moderate or greater turbulence.

I had an Instrument instructor that wanted me to fly my full approach at 20" and 2000 rpm. I hated that because the prop is way out of wack if you need to go missed and have forgotten to push the rpm to full. I just told him I didn't like it and he said to leave it at 2400 rpm and pull the MP back enough to slow the airplane. That is what I do now.

 

[Cpt_Kirk]

If he mentions "shock cooling," tell him he's full of it.

Sometimes I do as you originally stated, OP. His method it tedious and absolutely useless. It takes a small issue and turns it into a big one.

Sometimes, I'll pull the prop back to the lowest allowable level in the green arc, and do things from there, but that's a long and drawn out explanation. Just do it the way you were doing it.

 

[James331]

If he mentions "shock cooling," tell him he's full of it.

Sometimes I do as you originally stated, OP. His method it tedious and absolutely useless. It takes a small issue and turns it into a big one.

Sometimes, I'll pull the prop back to the lowest allowable level in the green arc, and do things from there, but that's a long and drawn out explanation. Just do it the way you were doing it.

So shock cooling isn't a issue for piston aircraft?

Guess all these analyzers have alarms and meters for it just because they have extra screen space to burn, go tell the glider tow guys and the DZ operators how they are full of it

 

[Cpt_Kirk]

So shock cooling isn't a issue for piston aircraft?

Guess all these analyzers have alarms and meters for it just because they have extra screen space to burn, go tell the glider tow guys and the DZ operators how they are full of it

I'll have the e-mails out in the a.m.

I sure hope they respond.

 

[EdFred]

So shock cooling isn't a issue for piston aircraft?

Guess all these analyzers have alarms and meters for it just because they have extra screen space to burn, go tell the glider tow guys and the DZ operators how they are full of it

Not to the extent, if at all as, people claim it does for NA engines. No one ever talks about shock heating, and you heat up way quicker when you go from idle to takeoff power than the other way around.

You need to cool things much much quicker than air can in order to crack anything. I have monitors and alarms as well, but it isn't for cooling.

I don't pull power to descend, and I don't mess with the prop until I'm base or short final. I roll in trim for 500-700fpm descent, and gain back some of what I lost in the climb.

 

[bobmrg]

Once the throttle is low enough, you push the prop full forward and it doesn't come off the stop.

Engine heat generation is roughly MP*RPM, so his approach is equivalent to pulling 4 inches of MP at a time instead of 2. Sure, it will work for slowing you down, and on an Arrow, it won't make any difference for engine longevity (not even chopping the power will do that….).

If it were me, I'd just pull off more throttle and keep the engine spinning as fast as it will. That keeps the oil pressure up, and the peak cylinder pressure down (though at low throttle, the peak cylinder pressure is not a problem -- you won't detonate).

I tend to leave the prop alone as well except for takeoff and landing.

Flat plate drag is for when the air is turning the prop, rather than the other way around, and the engine losses turn into drag because nothing compensates them. You'll get much more drag from the prop at coarse pitch, where the angle of attack is greater.

Take a look at figure 12-3 in the Airplane Flying Handbook and tell me again how coarse pitch increases drag.

Bob Gardner

 

[Sac Arrow]

I only mess with the prop twice on a normal flight - initial reduction for climb, then for cruise. I don't touch it again until I'm on the ground, or if I have to do a go around. Or if I'm doing a staged climb. 2300 or 2400 works fine on a Turbo Arrow.

 

[MAKG1]

Take a look at figure 12-3 in the Airplane Flying Handbook and tell me again how coarse pitch increases drag.

Bob Gardner

When the engine is producing power, it has the same L/D physics as the wing. Coarse pitch means greater angle of attack.

The figure you're referring too is for when the engine is not running, and only includes parasite drag. That's very different, as the angle of attack is negative when the wind is driving the prop.

With a running engine, induced drag is important. And it is much higher at coarse pitch and the slower speeds being discussed in this thread.

With the feathering props discussed in that figure, feathering a running prop will produce a lot of drag and next to no thrust -- it will be a stalled airfoil. Feathering a dead prop will just stop it.

 

[Marcelo]

Here is a nice video I found on Props. I was having some trouble visualizing the mechanism itself and this really shed some light on the matter:

https://youtu.be/TR7KZdHLkE4

 

[EdFred]

When the engine is producing power, it has the same L/D physics as the wing. Coarse pitch means greater angle of attack.

The figure you're referring too is for when the engine is not running, and only includes parasite drag. That's very different, as the angle of attack is negative when the wind is driving the prop.

With a running engine, induced drag is important. And it is much higher at coarse pitch and the slower speeds being discussed in this thread.

With the feathering props discussed in that figure, feathering a running prop will produce a lot of drag and next to no thrust -- it will be a stalled airfoil. Feathering a dead prop will just stop it.

Go up in a C/S prop.
Pull engine to idle.
Pull prop all the way back.
Note glide distance.

Repeat process only with prop all the way forward.

You get more glide distance doing it the first way. Guaranteed. Just did this last night testing power off 180's.

 

[mkosmo]

So shock cooling isn't a issue for piston aircraft?

Guess all these analyzers have alarms and meters for it just because they have extra screen space to burn, go tell the glider tow guys and the DZ operators how they are full of it

Considering that they don't crack when you shut down should be pretty good evidence of that in itself, but let's not make this another ****ing match about shock cooling myths.

 

[MAKG1]

Go up in a C/S prop.
Pull engine to idle.
Pull prop all the way back.
Note glide distance.

Repeat process only with prop all the way forward.

You get more glide distance doing it the first way. Guaranteed. Just did this last night testing power off 180's.

Yes, that's all true.

And the engine is not producing thrust.

Things are very different when the relative wind is pushing the prop. You can't get a positive angle of attack under that circumstance.

The appropriate test is at slow cruise power, not idle. Even then, you're measuring L/D, not drag, and I doubt you can get a non-feathering prop on the back side of max L/D.

This thread is NOT about engine-outs. It's about reducing power to descend. You don't do that at idle.

Please explain how a spinning feathered prop can possibly produce minimum drag.

Want a counter-example? All of us with CS props pull them back during run-up. What happens to the RPM at constant throttle? That's induced drag.

 

[EdFred]

Yes, that's all true.

And the engine is not producing thrust.

Things are very different when the relative wind is pushing the prop. You can't get a positive angle of attack under that circumstance.

The appropriate test is at slow cruise power, not idle. Even then, you're measuring L/D, not drag, and I doubt you can get a non-feathering prop on the back side of max L/D.

This thread is NOT about engine-outs. It's about reducing power to descend. You don't do that at idle.

Please explain how a spinning feathered prop can possibly produce minimum drag.

Want a counter-example? All of us with CS props pull them back during run-up. What happens to the RPM at constant throttle? That's induced drag.

I think we were all talking about different things initially.

 

[bobmrg]

When the engine is producing power, it has the same L/D physics as the wing. Coarse pitch means greater angle of attack.

The figure you're referring too is for when the engine is not running, and only includes parasite drag. That's very different, as the angle of attack is negative when the wind is driving the prop.

With a running engine, induced drag is important. And it is much higher at coarse pitch and the slower speeds being discussed in this thread.

With the feathering props discussed in that figure, feathering a running prop will produce a lot of drag and next to no thrust -- it will be a stalled airfoil. Feathering a dead prop will just stop it.

Point taken.

Bob

 

[MAKG1]

I think we were all talking about different things initially.

I think that's true.

In an engine out scenario, coarse pitch is your friend, definitely. Makes a hell of a difference in a 182 (yes, I've done your experiment).