26
votes
Accepted
Variable pitch propellers: Why are they constant speed too?
How did we switch from talking about a blade's angle, to caring about its RPMs?
Because of the engine. A piston engine will only run well in a narrow range of RPMs. Going too fast means
lubrication ...
- 237k
24
votes
Accepted
Why are there extra restrictions on constant-speed propellers?
The engine is the same, not simpler. There is one additional level of complication to the system -- the constant speed propeller has a number of extra parts.
The pilot needs to handle one more lever, ...
- 1,606
18
votes
Accepted
Why do pilots move the propeller lever full forward before landing?
It's both, but mostly the first one. You need maximum HP for go-around, which means you need the propeller governor regulating RPM at redline when you call for max power.
You could leave the prop ...
- 136k
17
votes
Accepted
Why, with a constant speed propeller, does moving the propeller control forward increase drag?
The prop control sets the engine RPM. Moving it forward allows the governor to flatten the blades, which results in a lower angle of attack. In fact, in certain circumstances the AoA goes so low that ...
- 5,580
16
votes
What other advantage(s) does an adjustable prop give you?
It's not just the operating RPM that is pertinent. It's also the pitch of the blades.
If your propeller blades' pitch is optimized for takeoff/climb performance, it's going to be aerodynamically ...
- 400
14
votes
During an engine failure in Diamond DA-40, should the prop lever be at fine pitch or coarse pitch?
If your engine fails in a DA-40, you've lost oil pressure, so there likely isn't enough to push the blade to the coarse position.
A DA-40 doesn't have an accumulator like a Seminole, for example. So ...
- 9,072
13
votes
Why are there extra restrictions on constant-speed propellers?
I recently moved from a fixed pitch propeller (FPP) (cessna 152) to a constant speed propeller (CSP) (Piper Dakota). In the FPP scenario the engine RPM is controlled by the throttle only. High ...
- 1,631
13
votes
Variable pitch propellers: Why are they constant speed too?
Because engine RPM is the variable in the feedback loop.
If you did have a nice big knob to directly set blade angle, how would you know when to adjust it, to what setting? You would need to have a ...
- 63k
12
votes
What other advantage(s) does an adjustable prop give you?
Next to the aerodynamic advantages (both engine and propeller can run at their optimum speed for best efficiency) already mentioned in the other answers, there is an important safety advantage: a ...
- 58.3k
11
votes
Accepted
Why does a low pitch propeller have better acceleration/take-off and climb characteristics than a high pitch propeller?
Low pitch advantage
Power
A lower pitch allows the blade to spin faster ($\omega$) for the same torque ($T$). The relationship between power ($P$) and speed and torque is $P = T \omega$, so this means ...
- 5,580
10
votes
In a constant speed propeller, why does the RPM drop with a prop pitch increase?
The propeller blades indeed have least resistance when aligned with the inflowing airstream, however the inflowing airstream is not the free stream at infinity, but the vector sum of free stream ...
- 63k
9
votes
Did WW2-era aircraft have constant-speed or variable-pitch props?
Most WW2-era military aircraft had constant speed propellers.
People often confuse variable pitch, controllable pitch, and constant speed. In general practice they are all the same thing, as all ...
- 37.9k
8
votes
In a constant speed propeller, why does the RPM drop with a prop pitch increase?
Koyovis explained it with maths and cool diagrams, here is my layman's version, if it helps.
You're confusing aircraft drag with the drag on the prop. Since the prop is powered, it typically produces ...
- 11.8k
8
votes
Did WW2-era aircraft have constant-speed or variable-pitch props?
Variable pitch (with feathering) was virtually universal on (at least American) multi-engine fighters, bombers, and transport aircraft even early in the war: the DC-2 had feathering variable pitch, ...
- 17.3k
8
votes
Accepted
At what point does the throttle control RPM on a constant speed prop?
What happens is when you're on final and you move the prop to MAX RPM, the prop governor moves the blades finer until the RPM is at red line, then governs RPM there by adjusting blade angle as ...
- 136k
8
votes
Accepted
What is prop RPM control logic on PC-12?
You effectively have a two speed prop (kinda like the 2 speed Power Glide automatic transmission on my first car, a 1964 Pontiac - well, not really). It's controlled by software, and there is a 1700 ...
- 136k
8
votes
Why, with a constant speed propeller, does moving the propeller control forward increase drag?
Let’s take the engine out of the equation to simplify things. In fact, let’s take the prop out too, and consider a very elementary comparison – the old “hand out the window” example...
Put your hand ...
- 27.3k
8
votes
Why do pilots move the propeller lever full forward before landing?
It's the former.
Having your props taking smaller bites of air means that you get faster response from increased throttle, allowing you to reconfigure from landing to missed with less flailing about. ...
- 4,372
7
votes
Does a constant speed prop maintain the optimal angle of attack at all time?
You may be overthinking it. The governor will set the blade angle that provides the load on the engine that keeps it from speeding up or slowing down from the setting determined by the spring preload ...
- 136k
7
votes
Why does a low pitch propeller have better acceleration/take-off and climb characteristics than a high pitch propeller?
A low-pitch or "climb" propeller takes smaller "bites" of the air and allows the engine to run all the way up to its maximum speed, and therefore produce its maximum rated power ...
- 22.2k
7
votes
Variable pitch propellers: Why are they constant speed too?
Controlling a blade's pitch by its RPMs and Manifold Pressure, to my very uneducated mind, sounds indeed like a very counterintuitive way of solving a problem.
It depends on the problem. And here the ...
- 56.5k
6
votes
Accepted
What other advantage(s) does an adjustable prop give you?
The comparison with a car's gearbox is unfortunate, and leads to misunderstandings...
At the small airspeeds typical of takeoff, the inflow at the rotor plane is small too, and the prop needs a small ...
- 11.2k
6
votes
"Mixture, Pitch then Power" - why?
This is mostly about when going balls to the wall, but is generally applied to more gentle power applications as well to make it an automatic SOP, and just to go easy on the equipment. Also it's not ...
- 136k
5
votes
Accepted
On a constant speed propeller, how is blade pitch related to airspeed and efficiency?
A propeller has an optimum AoA, where the thrust/drag ratio is highest. A variable prop can maintain this optimum AoA at a wide range of speeds, thereby reducing fuel consumption. That's all it does.
...
- 63k
5
votes
Do constant speed propellers actually increase fuel efficiency?
With a constant speed prop you are paying for maximum performance across the operating spectrum.
A constant speed prop will have only a marginal efficiency improvement over a fixed pitch prop ...
- 136k
5
votes
How can I find the minimum and maximum airspeeds for efficiency with a specific prop/engine combination?
What you really need to do this analysis is a detailed propeller map and a detailed engine map. Then you can do some analysis to match them together and make your own combined map.
Piston engine maps ...
- 16.9k
4
votes
Why did the propellers not fail to the feathered position in the Atlantic Southeast Airlines Flight 2311 Crash?
You can find the full text of the NTSB investigation here and according to that report (bolded for emphasis),
Examinations of the left propeller components indicated a propeller
blade angle of ...
- 102k
4
votes
Is the purpose of variable pitch propellers to maintain an optimal angle of attack?
What you know about variable pitch propellers is generally correct. If you make a slight change, your flight instructor would not have had any reason to object: They keep propeller blades near their ...
- 237k
4
votes
In constant speed propeller, if i increase RPM lever from 2300 to 2500, will the Blade AOA decrease?
When you adjust a constant speed propeller to a higher rpm, this will be done by making the propeller blade AoA smaller.
Contrary to your question though, manifold pressure and power output will not ...
- 30.5k
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