Effect of propeller as a brake

vic wrote:

Instead look into truck engines and their extreme loads on all components and still doing hundreds of thousand hours with minimum oil changes or 50 hour checks – ridiculous.

There are some very imporant differences between aircraft piston engines and truck engines. One is that the truck engine is not a 70 year old design, the other is that there are no axial loads anywhere in a truck engine or its gearbox while most of the loads on an aircraft engine are axial.

@vic look into the truck engines of the same time as when the aircraft engines were designed, which is in the 50ies.

They didn’t even have cartridge oil filters and needed oil change every 5000 kilometres.

Let´s keep it realistic, reduction gears on aero engines are typically 1:2 while in truck or car gearboxes you can have 1:5 or maybe 1:10 , first up to top gear. So when going downhill and throttle closed what do you get on these gears – a load of thrust from the other side of teeth, coupled to helical gears too, with axial loads from this as well. Certainly loads are a lot lower when throttle closed, maybe one third or quarter of total power. So I would not have any headaches about reverse loads on reduction gears in a aero engine, OWT definitely. Piston pushing a conrod or the other way – does not matter at all, see your car or truck in deceleration downhill or any time on the road. Piston ring flutter from this, same verdict I´d say. Axial loads on the crankshaft reversed when “engine braking” by propeller – I´d belive the axial faces on that crank section to be same size for both directions , so would not matter – unless somebody can tell sizes to be different, plain bearings all along these cranks. In all these matters don´t forget all forces to be only a fraction of full power forces so no concern really. There are more arguments from Deakin I do not support but he is no engine man, I would have to look up his ramblings, not this time. Vic

vic wrote:

For example like prop driving the engine and geared types, in this condition my guess the gears get only one third of loads from windmilling the crank as compared to having all power on them in climb

There is more to it than that. There are harmonic forces in engines (evidenced by the need for counterweights, and/or propeller clocking on some engines. At the pilot level, we are generally unable to detect nor appreciate these loads, but they may be there. In some engines, it can be felt in the cockpit, and avoided by RPM change, or has a limitation (yellow “avoid” RPM range). For my experience flying geared engines, I carefully avoid reducing power such that the prop could be driving the engine, unless training/authoritative information I have tells me it’s okay.

Though “things happen”, and occasionally a quick descent is needed, I do try to avoid them in piston engined planes. I have great respect for air traffic controllers, who nearly always do a great job. But, occasionally, they give a clearance or restriction which I determine to be operationally inappropriate, and I decline it.

I was landing at an airport which is both a busy fixed wing and helicopter base. I have flown both types from that airport over the years. When turning slant final as cleared, I was “cleared to land on pad one”. I declined, explaining I was flying a Cessna 182 today, and would rather use the runway. The controller laughed and apologized. No one is perfect, and it’s not always required that pilots bend over backward to comply with a “difficult” clearance.

Pilot_DAR wrote:

I declined, explaining I was flying a Cessna 182 today, and would rather use the runway.

You could’ve told him that you’d give it a try, but if you put the Cessna 182 to pieces it’s him to blame.

I don’t buy the sucking on piston rings. Even on idle still fuel is injected into the cylinders, an aero engine doesn’t have fuel shut down like an auto.

The combustion pressure should still be peaking around 300 psi (20 bar) according to what I found on the internet. Maximum on T/O power should be around 1000 psi (65 bar). To me these values sound reasonable.

The propeller is working “against” the engine because it is having a (positive) compression. The engine is mostly working as an air pump under this condition. 30 hp of braking force could well be for high RPM.

So there’s never a vacuum in the cylinders. The cylinder sucks in air. If MP is 10" then it sucks in fewer air mass than at 29", but then the piston goes “up” (compressing) and pressure rises way above environmental pressure. There’s no “sucking” on engine parts.

Under any circumstances, be it high power or zero power output, every piston “brakes” the engine when going up towards compression. But it is compensated after ignition (after top dead center). Under load (engine power), the piston is accelerated downwardly. But the braking action on upward movement before stroke is the exact same thing every single piston cycle. In fact it is even less stress for the engine when idling than on high power.

I don’t see any sucking on piston rings, that would be any different from normal operation.

In fact, when the propeller “brakes” the engine the only different thing is the missing (or smaller) explosion that accelerates the engine. All the rest is the same.

I may be wrong. I’m not a true expert. But I don’t see any stress on the engine – other than that it could be shock cooling. So one should not do that when the engine is running on say 400 degrees Fahrenheit, because without the heat output and the huge amount of air flowing through the engine it is cooled rapidly. So in question is the tolerances of the parts. But on say 300°F this should do absolutely no harm to a not-geared engine like the Lyco 6-cylinder.

How can a crankshaft know if it is driven or driving ? There are a few dampers on it which is good and the crankshaft is not very long with flat engines, unlike straight sixes, eights don´t happen anymore I think. Plus the forces on the crank are just a third, rather less than that , so why should one worry about crank being driven when the load is definitely way lower than when driving , harmonic forces or not, same for both ways no different ? As to dampers, with straight sixes in cars they got sort of external substantial flywheels on front end, mounted on a rubber-vulcanised hub on the crank. There is a real matter of dampening torsional forces with cranks that do 5000 – 7000 rpm, not these 2700 rpm just above idle speeds. Propllers can be seen as external flywheels and I´d think they may help smoothen out power pulses from big bore engines. Try running a big radial without a propeller on front: It will start up like a Ferrari engine and destroy the blower drive gear by extreme acceleration ! Remember they got a powerdriven blower at rear spinning at 8-10 times faster than the crank and any speed change will be 10 fold in the blower drive. Vic

Pilot_DAR wrote:

I was “cleared to land on pad one”. I declined, explaining I was flying a Cessna 182 today

LOL

UdoR wrote:

So there’s never a vacuum in the cylinders. The cylinder sucks in air. If MP is 10" then it sucks in fewer air mass than at 29"

For context the below is for NA spark-ignition piston engines. Some additional comments for TC.

Well, technically a vacuum is a lower pressure than atmospheric. I was actually referring to a relatively high vacuum like <20% atmospheric.
While the engine is running there is always a vacuum in the induction manifold: the higher the more closed the throttle is. MP can be 0.5-1.5" less than atmospheric at WOT depending on altitude an RPM or as low as 1" at high RPM and closed throttle.

The cylinders “suck” air during the induction cycle because their internal pressure is a bit lower than MP, otherwise there would be no airflow into the cylinders. So yes: there is a vacuum inside the cylinder during the induction cycle except on TC engines at high-power.
UdoR wrote:

But the braking action on upward movement before stroke is the exact same thing every single piston cycle. In fact it is even less stress for the engine when idling than on high power.

The concept is simple: there is always a pressure loss as fluid flows through any circuit (tubes, filters, valves …) or else it simply does not flow: by definition.

but then the piston goes “up” (compressing) and pressure rises way above environmental pressure.

For Lycosauruses 7:1 or 8:1 compression ratios, when you have a MP lower than 3" (high RPM, closed throttle), the compression cycle will result in a maximum pressure before ignition that will be lower than atmospheric.

There’s no “sucking” on engine parts.

I think you are referring to the vaccum in the cylinder at low MP and its effect on piston and rings. Remeber a normal crankcase is vented and has a pressure only slightly higher than atmospheric. So in the pistons you have on one side the cylinder pressure, and on the other side the crankcase pressure.
Rings are normally seated by high pressure in the cylinders, when it is very low, they are unseated, facilitating undesired dynamics. A good explantion is in this paper but a basic understanding is in this pic:

Just picture it with pressure being reversed. Call it “sucking” or not, they will be forced out of their normal position.

every piston “brakes” the engine when going up towards compression.

True.

But the braking action on upward movement before stroke is the exact same thing every single piston cycle. In fact it is even less stress for the engine when idling than on high power.

Untrue: it depends on throttle and RPM, and you are mixing braking action during compression phase vs braking action during induction phase. Why would the latter be lower at high throttle than low throttle? Also remember idle implies low-RPM (no braking action) whereas the condition being discussed is high-RPM with throttle closed: definitely not idle.

All the rest is the same.

Absolutely not: read above.

You are welcome to take the time reading the multiple online sources on internal cylinder pressure on this type of engine throughout the four cycles.

vic wrote:

How can a crankshaft know if it is driven or driving ?

This is a “thing” on a Rotax and FP prop. Descending, you can always find an air speed and throttle combination where the windmilling torque exactly equals the torque produced by the engine. Allegedly, this is not a good situation for the gear because the power pulses from the engine creates a situation with relatively high frequency changing of sign of the torque on the gear, a clickety clack situation. I don’t know how destructive it actually is, but it cannot be a good point of operation over longer periods of time.