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
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
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
10
votes
Accepted
What is the force to change the pitch of a variable pitch propeller?
In a well-designed propeller the hinge axis is very close to the line connecting the centers of pressure, so the aerodynamic pitch moment is rather small. The main force is friction; note that the ...
- 237k
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
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
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
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
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
5
votes
How will a variable pitch propeller move during takeoff?
What happens is the prop at idle will be at the maximum fine pitch limit. The prop control is set to Max RPM for take-off (it uses variation in pitch to regulate RPM) so the prop governor is set up ...
- 136k
4
votes
How will a variable pitch propeller move during takeoff?
As airspeed increases, it gets easier for the prop to spin, which would cause a fixed-pitch prop to speed up.
A constant-speed prop's governor, however, will alter the prop pitch to be coarser (which ...
- 27.9k
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
What would happen if the blades of a variable pitch prop did not all have the same pitch?
Yes the prop blades can be mis-indexed on older model turboprop systems. the Hamilton Sundstrand 54H60 system uses a hydro-mechanical prop control and a dome to change pitch. When reinstalling the ...
- 619
4
votes
Accepted
How do I get rated to fly complex aircraft that are still Microlights?
It appears that the British Microlight Aircraft Association is who you should refer to. They have a document on Differences Training. Details of all differences training can be found therein, but some ...
- 35.7k
3
votes
How will the Rolls Royce UltraFan provide reverse thrust?
The flow direction reverses only through the bypass duct pulling by blades with negative pitch. A portion of it will still be sucked by low pressure compressor making 180° turn into the core. Having ...
- 315
3
votes
How did the controls work for variable-pitch props?
Control by wire? As in electrically?
Many of the small planes like Cessna Cardinal use an engine like Lycoming O-360 (4 cylinder) use a spring to return the blades to a flat pitch, and engine oil ...
- 8,795
3
votes
Accepted
What is the relationship between speed, torque and pitch in a helicopter?
I'm afraid that a thorough answer requires the contents of chapters 2 and 3 of Leishman, but here is an outline. The relationship between power and true airspeed from this answer:
Power equals torque ...
- 63k
3
votes
What would happen if the blades of a variable pitch prop did not all have the same pitch?
I have a friend who owned a homebuilt aircraft with a ground adjustable prop. They had a terrible vibration problem that they could not diagnose. They spent months or years trying to get the bottom ...
- 16.9k
3
votes
Is the purpose of variable pitch propellers to maintain an optimal angle of attack?
The purpose of a variable pitch prop is to allow is to allow the pilot to select where on the engine's power curve (s)he wishes to operate the engine. When full power is needed, as for takeoff and ...
- 22.2k
3
votes
What other advantage(s) does an adjustable prop give you?
Propellers get more effective the faster they spin. The limiting factor is the speed of the blade tip, which must remain subsonic. So a propeller of a given size will offer maximum thrust when its RPM ...
- 7,102
3
votes
Variable pitch propellers: Why are they constant speed too?
For the same reason that your car has gears. Why can't you just put your car in first and drive at 70mph with your engine running at 50,000rpm? Or put your car in top gear and pull away from ...
- 2,424
3
votes
Variable pitch propellers: Why are they constant speed too?
The AoA of a prop's blades depends on both their pitch and the prop RPM, relative to true airspeed. You're carving a screw through the air some amount faster than the plane is actually going.
When ...
- 862
Only top scored, non community-wiki answers of a minimum length are eligible