Is it bad to brake in the pattern with the prop?

FORANE

FORANE

En-Route
Joined
Mar 7, 2013
Messages
4,458
Location
TN
Display Name
FORANE
Home Airfield
0a9
I have a MT electric prop. I can change its pitch and induce drag slowing the plane in the pattern. If I pull power to idle and pitch prop to its low pitch limit, I hear popping somewhat like a jake brake. Is that detonation?
Is it bad to brake with the prop?
 
not detonation. Might be backfiring (actually afterfiring) in the exhaust. Also, it's not likely you could hear it but if you have a crank with moving counterweights they might be doing strange things.

in general though, there are a lot of OWT about "turning engines with props" and unless you have a geared engine you probably have nothing to be concerned about.
 
In my experience in aviation, nothing good comes of doing anything which causes a popping sound in the plane.
 
Last edited:
JHW said:
not detonation. Might be backfiring (actually afterfiring) in the exhaust. Also, it's not likely you could hear it but if you have a crank with moving counterweights they might be doing strange things.

in general though, there are a lot of OWT about "turning engines with props" and unless you have a geared engine you probably have nothing to be concerned about.
Click to expand...

Those OWTs had more to do with the radial engine and the crankshaft. Something to do with the pistons putting pressure on parts of the shaft where oil is the least so many times per revolution. You don't have to worry about that in these flat engines.

That being said, using the propeller to slow you down is just a bad idea in every way I can think of it.
 
When you close the throttle quickly or over run the engine with the prop like that you can get a rich mixture that doesn't get completely burned. You also don't have a lot of flow because the throttle is closed and very little air gets drawn in so the un-burned fuel mixture collects in the exhaust and when all of the conditions align just right it goes off with a POP!

It's often mistakenly referred to as a backfire but as noted above it's actually an after-fire which is an explosion in your muffler and it can damage it so it's best to avoid the operating conditions that cause it to occur.
 
Silvaire said:
When you close the throttle quickly or over run the engine with the prop like that you can get a rich mixture that doesn't get completely burned. You also don't have a lot of flow because the throttle is closed and very little air gets drawn in so the un-burned fuel mixture collects in the exhaust and when all of the conditions align just right it goes off with a POP!

It's often mistakenly referred to as a backfire but as noted above it's actually an after-fire which is an explosion in your muffler and it can damage it so it's best to avoid the operating conditions that cause it to occur.
Click to expand...
This is what I believed to be occurring.
 
FORANE said:
I have a MT electric prop. I can change its pitch and induce drag slowing the plane in the pattern. If I pull power to idle and pitch prop to its low pitch limit, I hear popping somewhat like a jake brake. Is that detonation?
Is it bad to brake with the prop?
Click to expand...

You aren't doing yourself or you engine any favors by using it as a brake and other methods of slowing are preferable like just reducing power and holding altitude or even climbing, but it's still preferable to using the back end of another plane.;)

The thing is you are transferring stress to the rod bolts and caps when you drive the engine with the prop. At least a Jake Brake maintains pressure in the top of each stroke to use pressure on the beam of the rod to absorb the energy. Without any ancillary equipment to do so, you actually are turning your engine into a vacuum pump, and it's not really built for that. Will you explod your bottom with a little bit? No. Will you explode your bottom end going flat pitch, closed throttle, and pointing the nose down for Vne and taking a ride down from 14,500 to SL? Yeah, you might, more likely though you'll just hear a hollow knock from the bottom end that will go away when you add power to the engine and come back as it levels or unloads. That's a fat end rod knock typically caused by stretched rod bolts.

So summary: Try not to do it, but if we're talking a low grade thing with a couple hundred RPM change, no real worries. Using it as a speed brake coming down from altitude, not good and should be avoided in all but emergency conditions. If I have smoke in the cockpit, **** the engine, I'll sacrifice it to get me on the ground quicker.
 
While the guys have done a good job of describing the perils of letting the prop drive the engine, in your specific case you have a leaking exhaust gasket. Get it fixed.
 
Dr. O said:
While the guys have done a good job of describing the perils of letting the prop drive the engine, in your specific case you have a leaking exhaust gasket. Get it fixed.
Click to expand...

Maybe, wouldn't hurt to look, that's for sure, but typically the popping is from pulling so much vacuum in the induction that you end up so rich you're still burning fuel when the exhaust valve opens. There comes a point as well where you are pulling vacuum when the exhaust valve opens and that can lead to some real strange issues of reverse flow pulses out the induction during split overlap, but that's not what he's seeing, that requires extreme driving the engine with the prop operations.
 
Last edited:
If you are making noise in the traffic pattern, it is bad.

People hate small airplanes for only one reason......NOISE!!
 
Formation flyers use the prop to slow them down all the time. Is it hard on the engine? IMHO anytime you use the engine to slow down the plane it has the potential of stressing areas not normally stressed. Caution is a good thing here. Having said that I use the prop to slow down my RV-10 (IO-540) when descending by slowly pushing the prop control forward, and pulling power at the same time. The engine will speed up to 2650 and slow the plane down nicely. I don'tdo this all the time, just on occasion.

After fire is not going to hurt the plane. The popping is taking place outside the plane, not in the exhaust, hense the term "after fire".
 
JHW said:
like happens on every landing of every propellor-equipped airplane ?
Click to expand...
I throw the prop forward on short final, when it's already at its mechanical limits. There is no noise, there is no extra stress, and there is no potential for overspeeding the engine.

Geico266 said:
Formation flyers use the prop to slow them down all the time.
Click to expand...

:confused: In all of my formation time, I've never had to adjust the prop to come in to or maintain position.
 
It is a rare RV that doesn't pop on short final. When we pull our throttle back to idle, after pop is the norm.
 
Geico266 said:
...After fire is not going to hurt the plane. The popping is taking place outside the plane, not in the exhaust, hense the term "after fire".
Click to expand...

"after-fire" means "after" the combustion chamber. It happens in the exhaust, not out in the open air. Whether or not it can hurt anything depends on the system. Some airplanes have mufflers that are nothing more than a thin walled can, some have internal baffles and some airplanes have no muffler at all. With straight pipes you will only get mild individual pops and virtually no chance of any damage but with a canister muffler such as the Hanlon-Wilson and various other types used on a variety of GA aircraft the volatile mixture can accumulate and result in a significant explosion that absolutely can cause damage.
 
I'm a little confused by all this. I fly a slippery airplane (CTSW) with a geared Rotax 912 engine. I usually fly final at idle and the engine RPM is always 200-300 higher than ground idle. Is this an issue?
 
Old Geek said:
I'm a little confused by all this. I fly a slippery airplane (CTSW) with a geared Rotax 912 engine. I usually fly final at idle and the engine RPM is always 200-300 higher than ground idle. Is this an issue?
Click to expand...

No. That's how it's supposed to be.

Ground idle is different from flight idle.
 
Cpt_Kirk said:
I throw the prop forward on short final, when it's already at its mechanical limits. There is no noise, there is no extra stress, and there is no potential for overspeeding the engine.
Click to expand...
how does your landing rollout compare with a prop feathered vs props at pitch?
 
Silvaire said:
"after-fire" means "after" the combustion chamber. It happens in the exhaust, not out in the open air. Whether or not it can hurt anything depends on the system. Some airplanes have mufflers that are nothing more than a thin walled can, some have internal baffles and some airplanes have no muffler at all. With straight pipes you will only get mild individual pops and virtually no chance of any damage but with a canister muffler such as the Hanlon-Wilson and various other types used on a variety of GA aircraft the volatile mixture can accumulate and result in a significant explosion that absolutely can cause damage.
Click to expand...

You are describing "back fire", ...... "after fire" is outside of the exhaust... according to Titan aircraft engines. I had an "after fire" issue due to an over pressurization from my fuel pressure regulator.

I agree, an exhaust system with a muffler can cause damage to the muffler should it back fire.
 
Last edited:
Silvaire said:
"after-fire" means "after" the combustion chamber. It happens in the exhaust, not out in the open air. Whether or not it can hurt anything depends on the system. Some airplanes have mufflers that are nothing more than a thin walled can, some have internal baffles and some airplanes have no muffler at all. With straight pipes you will only get mild individual pops and virtually no chance of any damage but with a canister muffler such as the Hanlon-Wilson and various other types used on a variety of GA aircraft the volatile mixture can accumulate and result in a significant explosion that absolutely can cause damage.
Click to expand...

Some of it also happens at the interface of the pipe and atmosphere as there is where it gets the O2 to ignite off, before there it's above UEL in concentration.
 
Cpt_Kirk said:
No. That's how it's supposed to be.

Ground idle is different from flight idle.
Click to expand...


The little ctsw has ground idle and flight idle settings? I thought it was a fixed pitch, piston
 
I never go full pitch on the prop until RPM's have fallen out of the green arc and she's low MP. Maybe on base, definitely on final.

We don't want the dreaded whoop!whoop!whoop! in the pattern. :nonod:

Noise ****es people off. Even pilots sometimes.
 
"Lycoming suggests we maintain at least 15 inches MP during a descent and set the propeller to the lowest cruise RPM position to prevent piston ring flutter. Lycoming further warns against descent speeds in excess of high cruise and descents faster than approximately 1,000 fpm. We should start the descent with a leaned cruise power setting, gradually enrichen the mixture as we descend, and keep an eye on the engine instruments. Adjust the power and airspeed to maintain normal engine operating temperatures."

http://www.avweb.com/news/pelican/186778-1.html
 
JHW said:
how does your landing rollout compare with a prop feathered vs props at pitch?
Click to expand...

In a single engine? I'll tell you if you can let me know how to feather it.

In ME, the difference was negligible with one "shutdown" or zero thrust.

Are you saying I shouldn't bring it forward on short final? Sometimes I do, sometimes I don't. It all just depends.
 
Geico266 said:
You are describing "back fire", ...... "after fire" is outside of the exhaust... according to Titan aircraft engines. I had an "after fire" issue due to an over pressurization from my fuel pressure regulator.

I agree, an exhaust system with a muffler can cause damage to the muffler should it back fire.
Click to expand...

Don't know what Titan has to say about it but "backfire" for me has always meant on the intake side. Much less common and potentially more problematic but normally associated with start up rather than operating conditions. As I said earlier it's all semantics because most people just refer to any popping noise from an engine as back-fire anyway so it's not something you'd want to constantly nit-pick over.

What you are referring to as "after-fire" which is characterized by a burbling poppety-pop, although caused by the same general conditions, is not something I'd be worried about. It's the big KA-POW from inside the muffler that I'm talking about.
 
Cpt_Kirk said:
In a single engine? I'll tell you if you can let me know how to feather it.

In ME, the difference was negligible with one "shutdown" or zero thrust.

Are you saying I shouldn't bring it forward on short final? Sometimes I do, sometimes I don't. It all just depends.
Click to expand...
no, I'm saying that at some point on every landing the prop turns the engine unless the prop is feathered
 
Cpt_Kirk said:
In a single engine? I'll tell you if you can let me know how to feather it.

In ME, the difference was negligible with one "shutdown" or zero thrust.

Are you saying I shouldn't bring it forward on short final? Sometimes I do, sometimes I don't. It all just depends.
Click to expand...

Idle lever all the way back to cutoff feather

Or red knob back to cutoff feather

You'll float MUCH further.

A few caravans have had engine failures/ran out of gas, only to have a great normal power off setup for landing, however with that prop feathered they float all the way down and overshoot the runway.


Piston or turbine, I go forward on the prop on final, just incase I need it for a G/A or if I get some funk on final and sink out.
 
JHW said:
no, I'm saying that at some point on every landing the prop turns the engine unless the prop is feathered
Click to expand...

I normally pull my prop knob out to 2100 rpm during descent until final then go full forward with prop and mixture. I can actually pull my prop knob full aft in case of an engine out and my rpm will drop down to 600 rpm at 80 kias with 31 degrees max pitch. It just takes way too long to slow down under normal conditions and 2100 is as low as I normally go. Great for the 2 nm/1000' glide. I would say at that point my prop is not driving the engine since I am below my normal ground idle rpm of 740.
 
You must log in or register to reply here.
Back
Top