Currently, in every aircraft with a constant speed prop, and in any stage of flight, the airspeed will go down when you lower the prop RPM. This isn’t correct behaviour as far as i’m aware?
Changing the pitch of the prop blades should work like the gears in a car, lowering the prop should decrease power but increase airspeed. So when climbing you put the handle fully forward but in level flight you lower the RPM to get better maximum speed.
I can never get this behaviour in the sim it seems?
What aircraft are you referring to?
Not sure if you are a pilot. But the memory aid is Prop ahead of Power.
When you want to increase the rpm you apply prop (blue lever) and push it in before you apply power (black lever throttle). When you reduce rpm you do the opposite. First reduce power then reduce prop rpm. The word speed in constant speed prop is related to the rpm not airspeed.
Take-off prop full, power full, after about 400-500 feet AGL you reduce power MP (Manifold pressure) and then you reduce prop rpm. Sometimes referred to as 24 squared, 24 in. MP and 2400 rpm.
In cruise flight you reduce further for fuel economy/airspeed (ground speed) trade off. Again power first then prop. Power to 22 in MP and prop 2200 or what ever is in the POH.
A constant speed prop keeps the rpm constant with changes in throttle/power (MP), up to a point. reduce the throttle too much and the rpm governor will not try to keep the rpm at 2200/2400 etc because the blades can’t be set to do that. Similar for increase in power the blades can’t be adjusted if there is too much power. Hence the “Prop ahead of Power”
The constant speed prop is not the same as gears in a car.
It is in a way comparable like how a car’s gears work. You use low pitch for climb performance, and higher pitch the get a bigger bite of the air, for better efficiency and performance.
It’s discussed here: http://www.askacfi.com/7489/propeller-pitch.htm
And even described like this on Wikipedia: Variable-pitch propeller (aeronautics) - Wikipedia
And more in-depth here: Aircraft Propeller Aerodynamic Process
People are basically using the same analogy.
The problem is in the Baron 58 and even in the JF Arrow the performance only gets worse regardless of altitude. It isn’t working according to the POH performance tables.
The only thing that happens is the FF goes down (which is correct), and the plane slows down.
Full prop speed isn’t always the setting for fastest cruise speed.
I now see it’s also being discussed on the JustFlight forums: Cruise RPM setting | Just Flight Community
The title of your thread is the constant speed prop is wrong, but from what you and others describe the engine.cfg for the engines and prop sections are not set correctly. I can kinda agree there. The default aircraft need a lot of tweaking.
I agree more with Brian in the link you posted.
Hi,
I am quite new to this so pls keep that in mind if my comments seem completely irrelevant.
I think I understand most of what is written. But not really the quoted sentence. With same throttle set it will always be fastest?
The prop blades change their angle in a constant speed prop. Fine pitch rpm high, course pitch rpm lower. So high prop rpm/speed does not mean the aircraft is being pull faster through the air.
Fine pitch (usually full forward, max rpm) is used for takeoff and landing. Coarse pitch (lever(s) usually pulled back, lower rpm) is used once cruise (or a cruise climb) is established. It doesn’t really (directly) relate to airspeed in any sort of ratio or formula.
However, you (OP) are absolutely correct that the behavior in the sim does not reflect reality, at least as far as I can tell. This is most noticeable in approach and landing; fine pitch (blades at the most flat) should actually slow the aircraft down. That does not happen.
I mapped out the Bonanza G36 last year and I did observe a speed increase at lower propeller rpm
https://forums.flightsimulator.com/t/mapping-the-performance-of-the-beechcraft-bonanza-g36/332575
At 74% (2007 rpm) the propeller angle has the best grip, yet fuel efficiency is the best when it has the least grip. (That’s mostly due to the Bonanza being most aerodynamically efficient at 98 IAS in the game)
I have not looked at how it is modeled, but in real life:
The simplest way to understand the effect of RPM is to look at engine horsepower. With a constant speed prop piston engine aircraft, remember that moving the RPM lever does not change manifold pressure, it only changes RPM. And in simple terms for a piston engine, if the manifold pressure stays the same and the RPM’s go up, the horsepower goes up. So regardless of whether you are climbing, flying straight and level or even descending, if you do not change manifold pressure (the throttle lever), an increase in RPM will increase power and hence speed.
If you look at an aircraft’s performance tables, for a set manifold pressure at a given altitude, higher RPM will give you higher fuel flow, more horsepower and thus higher speed.
So don’t think of it as changing the pitch, think of it purely as changing the engine’s RPM, and thus power, at a constant manifold pressure.
Just for clarification, for the MP to stay the same when increasing RPM, you need to increase throttle.
Power can’t increase without increasing fuel flow.
This doesn’t work. If you don’t touch the throttle, decreasing RPM will increase MP and vice versa.
Now if we could only send the tips supersonic and scare the hell outta people on the ground…
MAP does change with changing RPM for a normally aspirated engine or turbocharged engine with fixed waste gate.
As above, increase in RPM will decrease MAP if throttle is not moved. Second you are only partly correct since you are not taking propeller efficiency into account, yes you’ll produce more power flooring everything, max. MAP and max. RPM as BHP = Torque (MAP) x RPM, this does not necessarily mean the Thrust Horse Power (THP = Thrust x TAS) increases. There is more power going to waste, not being converted into useful thrust when flying at a higher or lower than optimum blade angle for the phase of flight.
It is a change of pitch, all other variables remaining the same a change in prop RPM directly relates to a change in blade angle as that is exactly how the constant speed unit controls RPM. Similarly, all variables remaining the same, a change in throttle position (MAP) also induces a change in pitch. The resulting change in airspeed from changing MAP, RPM, or both also induces a change in blade angle. It is all a fine balance between MAP, RPM and airspeed to obtain optimum propeller performance.
This is only a rough rule of thumb though, there is no direct relationship between MAP and RPM as in it might be perfectly fine to fly 24 inch MAP on 2200 RPM on certain aircraft. Of course flying full throttle with RPM lever full back will certainly overboost the engine, otherwise the “prop before power” rule is not to be taken too literally. The “rule” is useful for power changes, for example:
Increasing power: from right to left - mixture rich - rpm increase - throttle increase.
Decreasing power: from left to right - first decrease throttle - then reduce RPM - then adjust mixture.
Not really, no. For optimum propeller performance you would need to maintain an angle of attack on the propeller blades close to the optimum lift over drag ratio (around 2/4 degrees AOA, similar to an aircraft wing). Flying at fine pitch (high RPM) at higher speed or vice versa will result in lower or higher than optimum angle of attack, reducing propeller performance. Done correctly the balance between MAP, RPM and airspeed will result in the propeller getting roughly the same “bite” of air during climb (low speed) as during cruise (high speed).
On a fixed pitch propeller, depending on which phase of flight the propeller is optimised for (climb or cruise) the “bite” of air will only be right for that particular phase of flight. Most propellers are optimised for cruise performance as the aircraft spends the majority of the time there, in such case the “bite” of air will actually be bigger during climb as compared to cruise (exactly opposite to your theory).
The whole comparison between a constant speed propeller and the gears of a car is a gross oversimplification. The bite of air theory as used in one of the articles is also not entirely correct. The “bite of air” the propeller takes is basically the angle of attack, not the blade or pitch angle (blade or pitch angle means the same by the way, contrary to what is written in one of the articles). Its difficult to explain but you need to visualise the path the propeller blade is moving to understand what “bite” of air the propeller is really taking, a higher blade angle does not necessarily mean a bigger “bite of air” if the aircraft is flying at a higher speed.
Not completely on topic but it might clarify some propeller principles:
It was meant to be a simplification. My point was in the sim it seems you always get the best performance with prop handle full forward. And i don’t think that is correct.
Depends on what you mean by performance? You won’t get the highest efficiency (specific fuel consumption) with everything floored, depending on the aircraft type highest speed might actually be reached at max MAP and max RPM in exchange for fuel efficiency. A constant speed propeller primary purpose is to increase fuel efficiency, not increase overall speed and performance.
Highest Brake Horse Power is reached at max. MAP and max RPM since BHP = Torque x RPM. The actual Thrust Horse Power is calculated by the formula THP = thrust x TAS, the difference between the two is the drag. A lower than optimum blade angle caused by flying with high RPM might offset the increase in BHP and therefore reduce maximum airspeed, but not necessarily.
Biggest problem is that practically speaking you are limited to the MAP you can set at a low RPM as not to overboost the engine, in other words you can’t set anywhere near maximum power at low RPM. Did you actually check with an approved AFM?
So it could be that at full forward prop the angle of attack of the prop is optimized for maximum speed?
I will have to do some more testing i think to compare with the POH.
Thank you for the insight i learned a lot.
Not really, it could be that the increase in engine power possible at high RPM offsets the reduced efficiency (extra) drag from the propeller by not having the most optimum angle of attack. A constant speed prop primary focus is to increase efficiency (reduce fuel consumption), not necessarily boost performance or max. attainable speed.
Its all about the interaction between the propeller and engine, there could also be cases where the propeller becomes so inefficient that it completely offsets the extra power produced by the engine and therefore a lower RPM might increase the maximum attainable speed.
In other words, its nice that the engine produces more power at higher RPM but if the propeller cannot convert this into useful work then it won’t benefit performance. On the other end of the spectrum if the engine is able to produce way more power at high RPM, then some of that power will be lost due to drag by the propeller not running most efficiently but the net effect might still be an increase in thrust causing higher attainable speed in exchange for fuel efficiency.
Its a complex balance with loads of variables, I just checked one aircraft I used to fly, for the same MAP, lower RPM causes slightly lower TAS according POH. You can see for 19.2 MAP, lower RPM causes a slight penalty in TAS but reduces fuel consumption by a lot, so net you’ll have a better range but it would take you slightly longer. Actually you can see 2400 RPM is optimum, below that range starts to reduce.
This is why they (used to) keep you out of “complex” aircraft until you had 200 hours. Add in cowl flaps to help manage CHT, and retractable gear… you need to be comfortable with the basics of flying and staying ahead of the airplane before you start all the fun stuff! 
There is no propeller drag modelled in the sim. The devs have said they do, but they are thinking it is engine drag (to slow the propeller down), rather than the actual drag a windmilling propeller creates. That’s why shutting an engine down without feathering it on a twin has not a lot of effect. If anything is modelled, it would be the drag of a stationary feathered prop (extremely minimal).
Has anyone seen thrust/drag indicators on propellers/powerplants in the aircraft dev mode?
