Pilot_DAR wrote:
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.
carlmeek wrote:
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.
The braking effect may not be the number of blades as much as it is the fine pitch stop setting – three bladed MT props are often certified to spin to red line rpm in fine pitch with plane stationary and full throttle, i.e. pitch as fine as can be achieved without over speeding the engine on takeoff. This maximizes acceleration from zero speed and the noise on the initial takeoff roll is typically still OK given the lower tip speed of a three bladed prop. On approach the effect is like putting your manual gearbox car in second gear to decelerate versus staying in third gear. Been there, done that with my MT.
Discussion with a friend who runs his company in competition with MT surfaces the issue above. I find it a useful tool for braking with the throttle on final approach, but on my electrically actuated prop it gives rise to a very steep descent if the engine were to quit… until you can manually drive the pitch to coarse. That would take a while and you’re likely to be pretty busy at that point.
Just my observation – if there is opposing data that indicates its the number of blades instead, that would interesting to hear about.
Ignoring turbuleneces, at zero prop incidence 2 blades should give the same breaking effect as 3 blades with 50% rpm increase on the former?
Interesting question.
I have to admit that I also don’t understand how more blades should give more drag.
I always thought that a windmilling prop produces a force equivalent, but opposite in direction to the force it would if powered, but with the profile inverted. That appears to be wrong.
Cobalt wrote:
I have to admit that I also don’t understand how more blades should give more drag.
Trivial: bigger frontal area.
Ultranomad wrote:
Trivial: bigger frontal area.
Not quite that trivial. For a stopped prop, absolutely, but a stopped prop – even if not feathered – does not produce much drag.
The autorotation produces the drag, and as I understand it, the faster the prop is spinning, the higher the drag. As observable on free-speed wind [electricity] turbines, which spin faster and produce more energy with increasing wind speed.
In the same way, the rotating prop produces the thrust, and the faster it spins, the more thrust it produces.
So why does a three-blade prop produce about the same thrust at a given RPM as the two-blade prop it replaces, but much more drag when windmilling? Or if it is windmilling at a higher RPM, why?
I remember reading about an accident in a home built europa aircraft with a Subaru engine. The prop was belt driven from the engine and the belt snapped. The aircraft then proceeded to descend at over 3000ft per minute due to a very fast prop creating huge drag! As such the landing wasn’t too pretty.
The best explanation of this I have ever seen is from tgrayson on the jetcareers site. I copied it some time ago and paste below.
Windmilling props generate reverse thrust. The faster they windmill, the greater the reverse thrust. I haven’t done any diagrams on this, but I think a thought experiment can show why this is true. Consider a propeller generating positive thrust and having the engine quit:
* The drag on the propeller, which, by definition, acts opposite to the propeller rotation, will slow the propeller.
* As the propeller slows, the AoA seen by the propeller get smaller, generating smaller thrust.
* Eventually, the propeller slows to the point where the AoA is zero, but the propeller is still rotating.
* Since there is still drag opposing the rotation of the propeller, it slows further, which moves the AoA into the negative range, since the relative wind is now in front of the propeller.
* Since the aerodynamic force produced by an airfoil is perpendicular to the airflow causing the force, you suddenly have a component of this force acting in the same direction as the propeller rotation, and a component acting opposite the flight path of the aircraft, which makes it drag.
* The component of the aerodynamic force of the propeller that acts in the direction of rotation is not sufficient to counteract the drag of the propeller opposite the direction of rotation, however, so it continues to slow.
* As the propeller continues to slow, the negative AoA gets larger and larger and the force in the direction of propeller rotation gets stronger and stronger, and it eventually is strong enough to exactly neutralize the drag opposing the rotation of the propeller and the rotational speed reaches equilibrium.
* At the same time, as the negative AoA gets bigger and bigger, and the propeller rotation slows, the component of reverse thrust opposite the flight path of the airplane gets smaller, meaning that the lower the RPM, the lower the drag.
* Internal engine drag, since it makes the equilibrium RPM smaller, actually works to reduce drag, rather than increase it.
The reason why a turbine propeller generates more drag is that it tends to flatten more easily and there is less engine drag (they are free turbines) hence more drag is produced when spinning ie they spin faster.
JasonC wrote:
there is less engine drag (they are free turbines)
Correct, however the Garretts on the Twin Commander and KA100 are not free. Presumably auto feather and rudder boost are MEL items on most twin turbo props?
Thanks JasonC for the detailed explanation, it illustrates the full dynamics of it
The easiest analogy I can have is from my car when moving the gear from 6th position to 2nd at 70mph, I get huge RPM increase a speed drop (change in speed is negative acceleration = more drag) and few tires marks on the motorway, while I am not stopping as I always generate thrust in 2nd gear
Now think what happens in hill climb, decent or when you plug the reverse gear or on a american automatic gear car 
RobertL18C wrote:
Correct, however the Garretts on the Twin Commander and KA100 are not free.
Direct drive, need to have a negative torque sensing system. When that activates they are no longer constant speed. i.e. the blade will coarsen until the negative torque is below a set level. This is still more drag than feather.
