Constant Speed Prop HELP? Keep Throttle open all the time, control power with prop lever?
Posted by Impossible-Fig2072@reddit | flying | View on Reddit | 38 comments
So I kind of understand the principles of constant speed prop etc.
My issue is I have no intuition about it. Most of the time during my training, the instructor just gave me a good MP and RPM to aim for, for cruise, descents etc
I am reading online, some people are saying you should be leaving the throttle essentially always open, because closing the throttle essentially just restricts airflow to the engine, and make it less efficient. Instead, if you want less BHP, just keep the throttle wide open, and redue the RPM and lean accordingly. They essentially control the BHP with the prop/mixture lever and keep throttle wide open. (I think the caveat to this is that you have to be high enough in altitude so that you can keep the throttle wide open, without getting too much BHP)
Help me understand this? And a good practical resource would be very helpful. I want to be able to understand this intuitively instead of just memorizing MP/RPM/Mixture to aim for.
Jwylde2@reddit
How the system works...
The prop control is NOT a "prop pitch" control like some oversimplifying literature likes to describe it. Does it change the prop pitch when you change the control setting? Yes, but it's just reacting to the current air load against the power applied (throttle/manifold pressure) vs the commanded RPM. Change the air load or the power applied and the propeller pitch changes again.
Wait what?
From idle until you're on the governor, the throttle works the same as a fixed pitch engine (throttle controls both MP and RPM). The propeller blades are against the fine/low pitch stops. Once on the governor, if the propeller control is full forward, the propeller accelerates to its redline speed (or close to it). As you increase throttle from this point, the propeller speed remains constant while further increasing the throttle causes the propeller governor to increase the blade pitch, and will continue to do so until you (a) run out of power, or (b) the propeller blades are up against the coarse/high pitch stops.
If you still had additional power to go on the throttle with the propeller against the high pitch stops, further increases in power would restore the engine to working like a fixed pitch again (power would control RPM again, and would push the engine/propeller beyond the redline speed).
The propeller control sets the desired RPM. That's it. The pilot is blind to the propeller pitch as propeller blade pitch is managed by the propeller governor. The governor commands the blade pitch that yields an angle of attack that results in the desired RPM (set by the propeller control) at the current power setting (set by the power control/indicated by manifold pressure) and propeller air load. Blade pitch automatically changes upon the change of any one (or more) of these three parameters.
Command lower MP (reduce throttle setting)? Blade pitch reduces.
Command higher MP (increase throttle setting)? Blade pitch increases.
Command lower RPM (pull the propeller control back)? Blade pitch increases.
Command higher RPM (push the propeller control forward)? Blade pitch decreases.
Air load decrease (accelerate, headwind shifts to tailwind, or you initiate a descent)? Blade pitch increases.
Air load increase (decelerate, tailwind shifts to headwind, or you initiate a climb)? Blade pitch decreases.
Once you master it, it's a rather fun system to fly and easier to manage than a fixed pitch as the governor accounts for changes in air load automatically.
helno@reddit
HP = Torque x RPM / 5,252
The throttle controls torque the prop lever controls RPM.
So yeah you can reduce output via the prop lever. As you climb the MP goes down even at wide open throttle. You still have to manage it with throttle unless you are high enough.
Jwylde2@reddit
Assuming a normally aspirated engine, the MP will change with altitude.
Horsepower is, in fact, a byproduct of torque. A lot of people dismiss this fact.
Motoflyn@reddit
As said - POH is your guide. Someone uses a bicycle with gears analogy which really fits if you’re looking to understand the effect. I appreciate that you want more than numbers. Truly understanding all the how’s and whys is the best approach. So many people fly eyes inside when they are Learning ( tradition or other) but never really grasp exactly what’s happening, just that they hit the MP, RPM, or fuel flow they are targeting but not what is really going on. Knowledge needed but knowledge without understanding is really just memorization.
skibumatbu@reddit
Read the POH. In there is a section on power settings... for a given altitude and power 65%, 75% what the correct mp and rpm are. It also has numbers for best climb as well.
I was taught that when I level off I set MP, the RPM, then mixture. When I climb or decent I do it reverse and adjust as needed for other things.
But at no point is my power kept open. Cruise is a MP of 25 inches or so with an RPM of 2400. Would you keep the power maxed on a fixed pitch prop? No. That likely will red line the engine. Same for constant speed
usmcmech@reddit
At 5000 MSL 25” MAP is basically wide open.
skibumatbu@reddit
Depends on the engine, I guess... 25 is cruise for me. If it was wide open I'd never be able to climb!
WhiteoutDota@reddit
Wide open means full power. What do you mean you'd never be able to climb?
skibumatbu@reddit
You can push the engine past maximum manifold pressure. I have a turbo in mine and can easily over boost if I give it too much
Accomplished-Ice-604@reddit
29.92” - 5” =24.92” unless you have a turbo.
skibumatbu@reddit
I have a turbo
TheEvilMonkey7@reddit
Some engines have a pretty much infinite combination, with some restrictions and recommendations. Always consult the POH or engine manual to verify, some engines have no go combos. It’s kind of like riding a bicycle, a high gear (high rpm) allows you to use less effort but you have to move faster. Low gear requires more effort but at a lower speed to achieve the same output. Depending on conditions you may find a combination that gives best power, best performance of the prop blades, and allows engine to operate efficiently (operating temps).
Personally, naturally aspirated io-520a, I prefer a set RPM about 2350 and adjust MP for power. High altitude, may end up at max recommended cruise RPM of 2500 to maintain power. At lower altitudes and desire to reduce noise I’ll run 2000-2200 rpm.
Check AircarftPower app, great if you don’t have nice avionics with HP percent. I use it and use an offline google sheet to calculate the fuel burn for full rich, best power, and best economy based on data extracted from the engine performance charts and HP at current engine settings from the app.
Also, search for “oversquare myth”; you get articles like this one.
org000h@reddit
There's a couple of youtube video's floating around that do a good job of explaining it.
Taking a step back - if we isolate the engine, it's more or less converting the linear movement of a piston into spinning the crank. Make those pistons bounce faster, crank spins faster.
Let's add some resistance to that spinning crank, eg a propeller. The crank will slow down, cylinders won't be able to move as fast. You can add more fuel & air to give them a bit more "shove" - it's now a bigger bang in the cylinder. They can "push" against that resistance a bit more now, and potentially go faster too.
That "shove" is your manifold pressure.
Spinning crank is connected to your propeller via a CSU. This is like the gearbox in your car. If the engine is doing 2500rpm, and you want to do 10mph, you want to be in 1st or 2nd gear. If the engine is doing 2500rpm and you want to do 60mph, you want to be in 4th or 5th gear. The CSU is more like a constantly variable transmission.
It's the same principles in the plane but the measurements are reversed, we're now measuring the RPM (speed) of the propeller instead of the wheels - this is affected by how fast the airplane is going through the air, and we're measuring the manifold pressure of the engine instead of rpm.
So I want to take off - I want maximum engine and propeller - the red, black and blue levers are all forward.
I want to do a leisurely cruise (eg 60mph in 6th gear where nothing is working too hard) - I take my foot off the gas (bring the throttle back), make sure i'm in the highest gear (prop set to a coarse setting) and give the engine the least amount of fuel it needs (mixture leaned out).
imitt12@reddit
I'm but a lowly student, but that honestly sounds like a hell of a lot of information to keep track of until you have it down as reflex. And then having to duplicate that for multi operations... It makes sense now why turbines are popular for multi, either you have a turboprop where you don't have to worry about mixture, or turbines with only thrust levers.
inseine79@reddit
In a couple of flights, its second nature. I don’t even think about it anymore.
Metharlin@reddit
I get the thinking, but the choice of engine (normally aspirated, turbocharged recip, or turbine) is made to meet the design mission for the plane, not based on pilot workload.
mustang__1@reddit
Eh.... Take off you bump the prop and mixture to make sure they're full forward. Then go full throttle. Gear up, flaps up. Pull throttle back to max continuous power, pull prop back to max continuous power. Reduce both to climb power. Cruise as desired. It's just another step, like carb heat, lights, etc.
TheShellCorp@reddit
https://avweb.com/features/pelicans-perch-46but-my-mechanic-says/
Read this (and all the other articles too. They're excellent.)
ltcterry@reddit
The POH and/or engine manual will have a chart showing all the possible combinations of prop and throttle you may safely use.
You will likely have 3-4 options each for 55%, 65%, and 75% power. Pick what you want/need and set accordingly.
Metharlin@reddit
This.
If you really want to optimize, pick the MP/RPM with the lowest RPM setting. Then climb until you reach the MP for that combination at WOT. At that altitude, retard the RPM to the correct value and lean.
For example, if in your POH 55% power can be achieved at either 18" MP and 2500 RPM or 25" MP at 1800 RPM (btw, I made these up just for illustration), pick the 2nd combination. Climb to an altitude where you hit 25" at WOT, then pull the prop lever back to 1800 RPM and lean.
Why is this better? Because there are internal frictional and pumping losses that increase with increasing RPM. The slower you run the engine (for a fixed hp output) the more of the engine's power is being delivered to the prop.
The reason for the climb is similar. The engine operates most efficiently at WOT.
toraai117@reddit
What airplane are you cruising around at 25/1800 in?
toraai117@reddit
The reason they say the throttle stays full (in a non-turbo) is because at around 3-4k feet the throttle is usually wide-open, and if you are cruising up high, your MP will already be low enough for a cruise power setting.
Thus, your transition from climb to cruise power is made with the prop and mixture.
Climbs and descents at altitude are often again just prop and mixture unless you need to get down in a hurry
wt1j@reddit
Just do the most basic research on this. A quick google search. A you tube video. A short even. Then come here and ask a question that isn’t banged out in 10 seconds while streaming, and those of us who fly constant speed props will be happy to help.
indecision_killingme@reddit
It’s very similar to a CVT transmission, but most people don’t understand those.
Have you ever ridden a mountain bike? When you shift the gears and it gets harder or easier to pedal? That’s what’s happening with the prop lever.
Take off Power will almost always be full forward. Most POH is have a descent RPM setting. Your instructor should give you a recommended practice cruise setting.
For cross country work, anything above 5000 the throttle is wide open in normally aspirated engine. Then I pick a prop setting appropriate per the POH.
I’ve seen some POH say to use the lowest prop setting at which the Engine runs smoothly for cruise. Some experience with engines and not having a noise canceling headset helps here. You just kinda listen and ask yourself if the Engine sounds pissed off.
Your gallons per hour will go down as you pull back on the prop liver, you’ll lose a small amount of speed too.
Bottom of line, read your POH. A copy of the power setting pages on your iPad is t a bad idea.
CrosseyedCletus@reddit
Think of it like engine power (MAP) and gearing (prop setting / RPM). It makes sense to keep the throttle at full if you’re climbing high in a non turbo because power available will decrease with altitude until the engine is longer able to produce 75% power due to altitude loss.
rangespecialist2@reddit
To oversimplify this. Think of this as a manual transmission car. Uphill use lower gear so the car can go uphill easier. In a plane it's a shallow pitch so the plane can go uphill easier. In straight and level in a car it's a higher gear so it's more efficient. Higher speed when the engine is at lower RPM. You can get away with this because the cars engine doesn't have to overcome the extra load of climbing the hill. The equivalent for an airplane is a steeper propeller pitch so the prop takes a larger bite of air with each revolution of the engine/prop.
In both car and plane, you dont do high gear/high blade angle together with high throttle setting during the climb. In a car you will stall the engine, in a plane you will probably destroy the seals on the prop governor and the engine makes a scary sound like it's struggling.
Now that you know the concepts, the POH gives exact prop/throttle settings to fly at.
Pop quiz. Do you know why they call it a constant speed prop?
Manifestgtr@reddit
A good thing to do is study it until it makes sense to you…until you know the “why” instead of just the “what”. That’s the point at which the concepts become real knowledge
KaanPlaysDrums@reddit
For a very basic level of understanding, think of it like swimming. If you move your hand through the water like a slice, it’s very easy, but it also doesn’t move you much. If you angle your hand to bite the water more, you get a lot more torque but it’s harder to do.
But yeah just read the POH. I recommend screenshotting the power settings charts and throwing them in your documents folder in foreflight for easy access.
rdrcrmatt@reddit
I’d be happy to take a few minutes on a zoom/teams/google video call with you to explain it. DM me.
LostPilot517@reddit
In LOP operations, fuel flow directly controls power. There is a method similar to what you are talking about, where you leave throttle wide-open, set your prop control, then rapidly cut mixture to LOP, to a set fuel flow corresponding to a percent power. This runs the engine cool, clean, and lower fuel flow. The issue is the red-fin you're avoiding, and the need to monitor, and having highly accurate engine monitoring, and balanced injectors.
You should operate your aircraft by the manufacturers recommendations.
https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf
Clunk500CM@reddit
Do you have a bicycle with gears?
What helped me to "intuitively" understand constant speed propellers was to think of it in terms of geared bicycles.
The gears on a bike make it possible for you to pedal (RPMs) at a constant rate, changing the gears (Pitch Angle) based upon the terrain so your pedaling speed (RPMs) doesn't change.
Going uphill (Climbing) use a lower gear (lower Pitch Angle), going downhill (descending) use a higher gear (higher Pitch Angle) in order to maintain the same pedaling speed (RPMs).
VelocitySUV@reddit
Mike Busch has a good, but long, video about running “oversquared”. And I flew my plane at full throttle from takeoff until about 4-3000 feet MSL when the spread between RPMs and manifold pressure was large. Full throttle at takeoff, adjusted prop at 1000 feet for the climb, adjusted the prop again in cruise, and then adjusted the mixture. It ran great the entire time and never had a problem. Anyways, check out the video I linked from Mike Busch.
https://youtu.be/A1fmNFWNHD4?si=bS8X7MxfuljriqUW
InvestigatorOne2@reddit
Read your POH a couple of times cover to cover. What helped me was the AFH Transition to Complex Airplanes and my POH. There's no magic or intuition for it, if you ask me. Almost every phase of flight has a procedure with appropriate MP & RPM combinations defined in the POH.
Reading Chapter 7 (Propellers) of the Aviation Maintenance Technician Handbook also helped me understand the system better from a mechanical/functional perspective. Highly suggest if you have any background where something like that might help/interest you.
TxAggieMike@reddit
This book may be of interest to folks…
Positive Flying: Flight-tested Numbers and How to Fly Them for Precision Performance
https://a.co/d/avVw6j0
TxAggieMike@reddit
Some reading that may contribute to the conversation …
https://www.avweb.com/flight-safety/technique/pelicans-perch-15manifold-pressure-sucks/
https://www.avweb.com/features_old/pelicans-perch-16those-marvelous-props/
swedishlightning@reddit
Best practice will depend a bit on your flight profile. If you are hopping from pattern to pattern at 3000ft between airports 10miles apart for training, just pick a valid combination from the POH and don’t worry about it too much.
But if you are doing a reasonable XC, then it merits some consideration. You are correct that flying with the throttle full forward and managing power with RPM/Mixture is the best way to balance speed, economy, and engine longevity. Mike Busch has quipped (paraphrasing) that he keeps his throttles full-forward for all phases of flight and if he could figure out a way to land that way, he would. Here is an article that expounds on this.
WelderNo4099@reddit
I had to watch a bunch of YouTube’s to finally get it. This one helped me - https://youtu.be/-Ba_IpCyKIw?si=vVJZJMzLYGHMuC_3
rFlyingTower@reddit
This is a copy of the original post body for posterity:
So I kind of understand the principles of constant speed prop etc.
My issue is I have no intuition about it. Most of the time during my training, the instructor just gave me a good MP and RPM to aim for, for cruise, descents etc
I am reading online, some people are saying you should be leaving the throttle essentially always open, because closing the throttle essentially just restricts airflow to the engine, and make it less efficient. Instead, if you want less BHP, just keep the throttle wide open, and redue the RPM and lean accordingly. They essentially control the BHP with the prop/mixture lever and keep throttle wide open. (I think the caveat to this is that you have to be high enough in altitude so that you can keep the throttle wide open, without getting too much BHP)
Help me understand this? And a good practical resource would be very helpful. I want to be able to understand this intuitively instead of just memorizing MP/RPM/Mixture to aim for.
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