Hey real pilots. I have a question. In most real world piston aircraft is it common to not let the RPM of the engine get too low in flight? For example; would you ever set throttle to idle when descending or slowing down (or both) in normal flight? Or would you always try and keep as much power in as possible? Obviously you cut power just before touching down, but I’m talking about prior to that.
I ask because I know a lot of piston plane engines are not happy at low rpm on the ground, but perhaps it never gets low enough when in flight, even at idle, to be an issue?
Basically I just want to know if it’s unrealistic to cut the throttle as a means to “get down quicker”, or is that perfectly ok?
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The takeaway is pitch + power = performance.
Working backwards from that, you’ll find that for any given, stabilized airspeed, less power will give you a faster rate of descent and vice-versa. So if you want to get down faster, you can chop. You can modulate pitch and power to see the effects of this.
Some caveats to this are:
You don’t always want to get down quickly due to obstacles, weather, IFR minimum altitudes, passenger comfort, fuel economy, maximum speeds, etc. You often have to figure in an angle, not just a rate.
Some aircraft are pretty low-drag and a steep descent rate can cause trouble when it’s time to slow down to configure or join the pattern or approach.
Some engines don’t react well to pulling power quickly, like geared piston engines, and of course the whole “shock cooling” phenomena.
Different configurations and high-drag devices like flaps and spoilers can allow you to steepen the descent a bit without gaining additional speed.
All that aside, in terms of RPM, it generally won’t get too low if you’re moving through the air, even at idle. But if your idle for long, especially in colder OAT, some manufacturers or other SOPs recommend “clearing” the engine with a bump in power every so often.
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Thanks CharlieFox. That all makes sense. I guess it depends on engine, but in general probably best to avoid chopping the power suddenly or running at low rpm for extended periods.
I love these types of questions asked by sim only pilots and answered by IRL pilots, thanks both.
I think with the way that wear and damage modelling is going with some aircraft releases, these are especially important as we might have to deal with the consequences of our flying. After I bought the JF Tomahawk, I induced a spin and it took me a bit longer than it should to recover. As I neared my approach, I found that my flaps were damaged. I think this was the first time I’ve had to deal with something in the air that was my own fault.
Hi,
My two cents here.
I am flying a C172R in real life, and one of biggest concerns when starting to descend (from a cruise level) is not to stress the engine too much with abrupt temperatures changes.
I usually do not bring RPM bellow 1900-1800RPM for a normal planned descend. This will give you a nice rate of descend, but of course you need to plan your top of descend to arrive at a certain point with the desired altitude.
Also, keep the mix lean, so you are not cooling the engine too much.
Best regards,
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Great question! Arrow pilot here - in my Arrow, I only bring power to idle on the ground during my runup which consists of mixture full rich, RPM 2100, prop cycle three times, then pull power to idle and make sure engine doesn’t quit. Then back to around 1000 rpm…
In flight - usually won’t pull power to idle… power settings for descent from cruise and approach don’t call for this. But - If I want to get down fast - pulling power to idle will make the Arrow drop like a stone (1969 Hershey bar winged arrow 
)… And I’ve done this when I want to make a super tight downwind-base-final turn and I’m still around pattern altitude.
In fact - the power-off 180 calls for power to idle in order to execute the maneuver (which is the one that fails a lot of potential comm checkrides )
One note with the older PA28R-200s - they don’t like to have power pulled too soon when landing. In the trainer 172s I flew for primary training, you could pull power over the numbers and it would still want to keep flying. In the Arrow - pull power too soon even in the roundout and it will drop through ground effect and cause a nice plunked landing. I keep power in just before starting the flare in my Arrow and start pulling power to idle then. I’ve been known to have to give it a shot of power if I flare too early and it feels like it’s gonna plunk in. Sort of a feel/experience thing, I guess.
So - to answer your question - idle power usually - not really. If the situation calls for it - power to idle for sure and it’s not a big deal.
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When landing my 152 I will start decent into base turn go from 2400 RPM to 1700 RPM and keep it around that range as i get close or pass runway threshold, then will go full idle. Anytime below 2000 RPM/s the carb heat is turned on. Anytime in normal flight rarely will go below 2000 RPM, if I do carb heat always comes on.
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If you need to make an emergency decent due to engine fire you will go full idle but max our your decent.