kwlf wrote:
there is a maximum rate of descent that cannot be exceeded in autorotation (you would need power to descend faster).
That is correct, you would over rev the main rotor to descend faster, and over reving is bad. I presume that this is because locally, air is becoming incompressible around the blade, so it must get out of the way, rather than be compressed. It can only get out the way so fast.
Silvaire wrote:
total combined blade area is constant for the same service regardless of the number of blades. However structural (strength and/or stiffness) issues typically make it necessary to increase individual blade area slightly on three blade prop, meaning the combined area of a three blade prop is usually a little greater than a two blade prop for the same service.
My MT three blade has blades of about the same total area as each of the Hartzell blades on the two blade I removed. I have not measured blade area, but you can just see by looking at it.
Revamping this old thread to post a question: would it do any harm to the engine when braking the plane with the prop?
Obviously it rapidly cools the engine. Would it be possible say below any specific CHT?
Background: I am trying techniques to slow down my plane on a high speed approach. When I adjust power just such to not get a load on the prop (lowest MP that doesn’t produce that “sound” where the prop is braking), aircraft clean (gear and flaps up) it just gets too fast. Yesterday on a 3.7 degrees approach I started with 118 knots and 1000 above landing runway I had 155 knots (all the way throttling back as much as possible). The extension speeds are much lower so I can’t just throw the gear out to slow down. So the only option that is actually effective is to use the prop (idle power, max RPM). But if that was doing any harm then I might just have to forget doing it.
What would happen if you pitched up to maintain 118kts and then reduced power. Then kept 118kts using that angle of attack. Could you get to gear and flap speed that way and control the speed using pitch, adding power gradually if needed in the descent.?
The only engines I’m familiar with that have mechanical issues with closed throttle operation are (apparently, so I’m told) the 1950s geared Lycomings. The gearbox on those is reputed to wear when back driven. Otherwise with an Lycoming O-540 I see no mechanical issue, and I don’t think shock cooling is a significant consideration for that engine either.
The gearbox on those is reputed to wear when back driven.
Diamond’s Austro engine has some problem with timing chain when shut and that’s why windmilling starts in the air are not allowed. It’s not exactly the same issue but kind of similar.
Same goes for chain drives , no different to being driven or driving.
If you drive by one side only one side of the chain is tensioned only. On the slack side somebody might install some tensioning device. Now it you drive from the other side you tension the other side of the chain which goes through the tensioning device and can break it and you no longer have chain tension taken care of. Very typical setup for timing belts in car engines. Those seem to build on the fact that the whole thing is only driven from one side only.
Now I´d like t know what sort of gears might be unsuitable for loads from being driven as compared to driving
Not sure this is the case but maybe the actual change is the problem. If there is some slack in the gears it takes a hit every time the load situation switches.
Quickly scrolling through this thread, most of the historical comments are about airframe and propeller effects, whereas @UdoR’s seems to be more focused on the engine, in his case a 6-cyl Lyco.
Although there is some measure of OWT in the adage about MP vs RPM (high RPM should be with high MP and viceversa) there are three effects of low-MP with high-RPM on non-geared Lycosauruses that are not negligible:
1- ICP (intra cylinder pressure)
2- Crankshaft vibration
3-Rearward loading on the crank thrust bearing
No 1 is clear: throttle closed at idle implies MP about 15" . Maintaining throttle mostly closed with progressively higher RPM (or much higher as in 2500RPM in a 160KTS descent ) will progressively reduce MP until it is close to zero (a vacuum) . The result is a negative pressure across the pistons (low ICP) which will unseat rings, which are designed for very high pressures in the opposite sense. Since crank pressure in such situation is about 30", this also means crank oil is being forced into the combustion chamber and blown out the exhaust. Now, this occurs anyway in most low-power situations during the induction cycle so it is not the end of the world, it just gets much worse if the throttle is closed or mostly closed at high RPM.
No 2 is not so clear since it depends very much on the engine and propeller configuration. I would say this issue is more prevalent on long cranks with tuned mass dampeners (typically 6- and 8-cyl Lycosauruses). Again, the crank configuration, dampener included, is designed for a certain (mostly positive) torque pattern and a negative one will introduce a crank torsional vibration pattern which can have a negative effect on those dampeners. This will wear their bushings more quickly than otherwise, and lead to a slight detuning.
As to No 3, it is about the crank being pushed rearward against the side of the bearing with least lube.
More details in this longish Pelican’s Perch article by John Deakin about props driving engines
The occasional use of prop braking for a few seconds should not be hurtful.
However, especially on 6-cyl engines, I for one, prefer to level-off (typically approaching the FAF on an ILS) , slow down, extend gear (or whatever drag means are available) and then speed back up to the gear limit thus avoiding high propeller RPM and closed throttle. In fact during approach I do not push the propeller control forward until IAS is slow enough that the propeller control has no effect anyway (prop at low-pitch stop), thus minimizing any braking effect.
