Effect of propeller as a brake

The disk area ratio is higher to absorb the extra power the power plant is capable off. (the blade area versus the diameter) πD^2/4. This translates into more drag when the engine is no longer driving the propeller. Bit like bigger tyres on a car can deliver more stopping power (force) but they have more rolling drag. (less efficient)

At idle thrust there is very little airflow driving the turbine which turns the propeller. The blades flatten to maintain RPM which makes it a large flat disk free spinning disk which provides net negative thrust. Obviously the prop (in a PT6) is not physically connected to the hot section of the engine it is driven by a viscous coupling of air.

Free power turbine…

Dash 8 props turn into large barn door like speedbrakes at power idle and max np (prop rpm). Basically, if you can see it, you can land there. 200+ knots to the outer marker is possible. On the ground, one almost never touches the wheelbrakes thanks to beta mode. And, while not done normally, one can self push back, just keep your feet far away from the pedals/brakes ;)

In so far it is imperative to feather the engine for any abnormals and get rid of the barn door!

Can be used as aerodynamic air break or engine break depending on rpm, angle of attack and prop radius

So you can view it as a flat disk or rotating air intake, high rpm at zero angle of attack should give you a nice air-break in TurboProp with large props?

The effect is pretty noticeable on a Twin Otter. I remember it feeling like someone put their foot on the brakes when they went to max RPM.

The effect is quite noticeable in piston aircraft as well. My wife has commented that she dislikes the “braking effect” when I put the prop in high rpm during an approach.

In many turbo props (PT6 in particular), the blade angle will adjust into a more flat pitch with the power lever selected to idle, this is in preparation for operation of Beta, which is the entry point to use of reverse thrust. A braking effect in the air is an intended feature, you either carry power down final if you don’t want the braking effect, or, if the engine has stopped, feather the prop. Different props on PT6’s have noticeably different idle braking effects – ’depends on the prop. A PT6 is free turbine, so the engine does not care if the prop is turning or not. I have flown PT6 powered aircraft with the prop feathered, and the engine still running happily at idle.

In a direct drive piston engine, it’s more a factor of the prop model (number of blades). When I changed props on my flying boat to a three blade MT, the increase in idle braking effect was nearly alarming, from what had been simply noticeable with the two blade Hartzell. Now, in that aircraft, a forced approach requires that the prop be selected to full coarse, and then it’s okay to glide a normal steep approach. On the surface, that prop can be selected to reverse thrust, which is very desirable for maneuvering on the water. It’s not a “Beta” system though.

For any geared piston engine (the first letter of the engine designation will be a “G”), the propeller should never be allowed to drive the engine – no idle and braking effect to be done.

For any geared piston engine (the first letter of the engine designation will be a “G”), the propeller should never be allowed to drive the engine – no idle and braking effect to be done.

I’m inclined to think that’s a bit of an OWT. I owned a Skylark (with a sweet 175 hp GO-300-C engine) for a few years, and there was no such stipulation in the operating limitations section of the owner’s manual. The operating details section merely recommends that “the glide, with closed throttle, should not be longer than necessary” and “normal landings are made power off with any flap setting”. I think that goes for many engines whose lubrication may be sub-optimal at idle speed.

I loved that Cessna despite the ridiculous undercarriage. The engine and gear train never gave any trouble but sadly the airframe had spent too many years by the sea before being flown from Brazil to Spain, so we scrapped it. I still have a few parts free to a good home.

The braking effect on my RV10 is very considerable, and it has a 3 blade prop. I was demonstrating this to another pilot and for a bit of fun decided to see what speed I could turn final. 140 knots at 1000ft and still got it back to 70 over the hedge. Without adjusting the RPM, just pull it back to idle and it feels like standing on the brake pedal.