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Putting Props full forward to slow the plane down

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Putting Props full forward to slow the plane down

Old 29th Dec 2019, 12:08
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Putting Props full forward to slow the plane down

On approach, you can put your props full forward to slow the plane down (like an airbrake sorta).

Can somehow explain the theory behind how this works?

Putting the props full forward at say 220 knots sets the angle of attack for the prop to a less than efficient angle, creating more prop induced drag?
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Old 29th Dec 2019, 12:34
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Bad practice as you have the props now driving the engine,which can affect the whole drive train/gearboxes..Increases noise significantly,engine wear on pistons,negative torque on turboprops,and generally shows a lack of airmanship and `mechanical empathy` with your aircraft......Much better to plan a slow-down by easing the power off earlier ,then increasing prop rpm on short final.....do you slam your car gearbox down thru` the gears everytime you brake....?
Your passengers will appreciate a smoother,quieter flight,as will the engines....treat every flight,and your car as if you are taking your favourite grandma for a trip......
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Old 29th Dec 2019, 14:20
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Hello!
You are quite right, at idle power and hi-speed the prop tends to windmill creating drag, increasing the RPM you increase the drag.
It is definitely noisy and uncomfortable for the occupants, but it isn't always a problem for the engine, it depends on the engine type (piston or turbine, free turbine) and the engine can be integrated with a Negative Torque-Sensing system that increases the prop pitch to reduce the effect if necessary.
All the Turboprop planes I flew (PC 6, Be350, ATR72, Sb2000) used to slow down really well without the need of increasing the prop, but it was an option if needed and I don't remember it as a problem (within some limits).
More often was used for steep approaches or to help to stabilize the approach in turbulent and windy conditions.
Just my two cents
Evan
PS
I do downshift to help braking every day in my car...
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Old 29th Dec 2019, 16:34
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Originally Posted by slate100 View Post
On approach, you can put your props full forward to slow the plane down (like an airbrake sorta).

Can somehow explain the theory behind how this works?

Putting the props full forward at say 220 knots sets the angle of attack for the prop to a less than efficient angle, creating more prop induced drag?
I've always reserved the "props forward" until short final, and I understand the reason being to configure for a go-around, rather than a"braking" maneuver. Putting the props near flat at 220 doesn't make a lot of sense to me for the reasons stated previously in this thread. Happy New Year.
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Old 29th Dec 2019, 17:15
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How does one actually overcome the torque on a running engine, is so wouldn't the compressors quit and the engine flame out?
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Old 29th Dec 2019, 17:35
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I was of the opinion the idea was for the props to be ready for the go-around . Nothing to do with slowing down.
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Old 29th Dec 2019, 17:50
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Originally Posted by fantom View Post
I was of the opinion the idea was for the props to be ready for the go-around . Nothing to do with slowing down.
Both effects are true consequences. The "idea" of which one is intended lies in the pilot's head.
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Old 29th Dec 2019, 20:10
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Don't do that it in the C421 - it won't take that too kindly at all.
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Old 29th Dec 2019, 20:37
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Originally Posted by fantom View Post
I was of the opinion the idea was for the props to be ready for the go-around . Nothing to do with slowing down.
It depends on the plane if I remember properly on Be350 and old ATR was required, but PEC ATR and Sb2000 kept prop at low rpm during approach for noise reduction and went automatically to hi rpm when selecting GA power (or manually selecting hi rpm for the reasons of before).

How does one actually overcome the torque on a running engine, is so wouldn't the compressors quit and the engine flame out?
Not in a free turbine design.

Don't do that it in the C421 - it won't take that too kindly at all.
Never flew the plane, trust your words.

My two cents
Evan



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Old 30th Dec 2019, 01:06
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Under boosting a piston engine can be just as bad as over boosting. In ground school had a piston of a Wright Cyclone displayed where the top of the piston had a massive hole in the crown where the top had been "sucked" out by under boost.
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Old 30th Dec 2019, 01:47
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The basic theory is 100% correct. Increase the surface area facing the airflow as close to perpendicular as possible, increases the drag and will slow you down if all other factors remain equal.

Has nothing to do with bad airmanship or bad habits IMHO, simply whether your airplane can deal with it or not. The turboprop I flew before (geared turbine, not free) did not really care and the manuals listed it as a method of deceleration available if needed. No adverse effects listed. Used very situational, sometimes to allow us to keep high speed down to 4-5 miles to skip ahead in the sequence if the controller could fit us in. Lots of fun, 250kts until 5nm and stabilized by 3nm. Props full forward helped a great deal.

Would not try it on others planes without doing my homework first, but that should be standard anyway.
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Old 30th Dec 2019, 02:32
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The OP didn't ask whether it should be done - the question is why or how it works as a brake.

Answer - it is like putting a car or truck into low gear to "engine-brake" going down a hill. The aircraft is absorbing energy from the slipstream/airspeed, in order to drive or spin an idling engine as an air pump instead of a power source. Instead of the engine driving the prop and adding energy to the propwash.

AND also producing drag directly by making the prop blades a "flat disc" - the opposite of feathering the blades to reduce prop-engine drag.

In either case it makes it harder for the aircraft to move through the air, thus reducing speed, or holding a speed while being able to increase descent rate.

It can be hard on the engine, because it is in effect trying to make the drivetrain run backwards (which does not happen literally - it is just that the forces are trying to do that). I.E. producing "negative torque" in the various moving parts. It can be especially hard on a reduction gearbox, as in turboprops.

Generally speaking the aircraft/engine manuals will provide guidance on just how much this effect can be used - if at all. MP and rpm limits for pistons; torque for turboprops, maybe engine temps for both.

As mentioned, a torque sensor may be installed, to overrule you as the pilot, unflattening the blades automatically to avoid negative torque. Or you may be left to watch the torque gauge yourself to keep it positive.

On the whole, I agree that unless one has screwed up the descent planning, it is better to leave the props in cruise pitch (higher torque), which leaves more elbow-room to reduce power (fuel flow) without nudging too close to zero or negative torque. Eventually slowing to flap or gear speed - where one can add drag using those, instead of straining the engines shafts and gears. Once that drag is in, THEN one can flatten the blades to full forward on final, for speed reduction and preparation for a possible go-around, while still using substantially more than idle power (i.e. substantially more than zero torque).
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Old 30th Dec 2019, 05:03
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If over running on a formation join up (complete balls up) judicious use of MAP and fine pitch acted as a brake and avoided use of the speedbrake which would otherwise signal to the instructor flying chase your error. Of course, you had to assume he wasn't sighting you through his prop disc, else he would note the disparity in RPM.
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Old 30th Dec 2019, 19:38
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Having spent far too many hours flying the DHC-6 Twin Otter, I can speak to this. The Otter has fully reversible props, and they will go to full reverse in flight, if demanded. However, the props are so powerful I always felt it would stop dead in flight and stand on its nose, and tumble... if you were silly enough to do that.

The technique if you were silly high and needed to get into a very narrow space was to twist the throttles and very carefully squeeze them back through the gate. It was like hitting a brick wall. The blades would act like a huge flat disc and the rate of descent was scary.

The one place you could use it was in stopping quickly when landing into a short field. As the wheels got to about a foot off the runway, pull the throttle straight through into full reverse and the Otter would sit down like a rock and stop on a sixpence. However... do not do this too high or you will be a steaming pile of rubble. Either way it was a trick to impress the bystanders. Stopping was never the problem, getting airborne was the constraint.
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