The propeller generates thrust, but what is the purpose of thrust of propeller? Is it to help the aircraft move forward or to produce lift to rise up?

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    $\begingroup$ You may read how it flies?, especially the sections about energy conversion. It is a good further reading to understand how an aircraft move and how pilot manage both altitude and speed variations. $\endgroup$
    – Manu H
    Oct 30 '19 at 14:55
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    $\begingroup$ I'm not trying to be rude at all, but what about the function of the propeller confuses you? $\endgroup$ Oct 30 '19 at 15:21
  • $\begingroup$ @quietflyer no ,i don't intend for purpose of thrust,obiviously it a force opposite to the resistive drag force,i am confused by the purpose of propeller? $\endgroup$
    – engr
    Oct 30 '19 at 16:53
  • $\begingroup$ @MichaelHall whether it/propeller is used to raise/lift airplane or to move it to forward direction or both functions? $\endgroup$
    – engr
    Oct 30 '19 at 16:55
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    $\begingroup$ The propeller's job is to keep the pilot cool;. If the propeller ever stops, the pilot will sweat profusely!! $\endgroup$
    – abelenky
    Oct 30 '19 at 18:11

The propeller generates thrust to counter drag.

Drag is created by both the forward velocity of the plane through air (parasite drag) and as a byproduct of lift (induced drag).

Lift is a also a consequence of movement through air and is used to counter weight.

The above is mostly true for conventional, heavier than air, fixed wing aircraft in un-accelerated flight. More complex cases like VTOL aircraft use thrust to directly counter weigh during some stages of flight.

From Wikipedia:

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A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed.

  • $\begingroup$ You mean that propeller help/supports to move aircraft in forward direction??and not to produce lift( for rising upward)?? $\endgroup$
    – engr
    Oct 30 '19 at 10:34
  • $\begingroup$ @engr yes, that is what usually happens, the propeller moves the airplane forward and keeps it at a speed at which the wings can produce enough lift. Only helicopters and other, more niche, aircraft use thrust to directly counter weight. $\endgroup$ Oct 30 '19 at 10:38
  • $\begingroup$ Isn't the airspeed over the wing higher directly behind the propeller? $\endgroup$
    – DJohnM
    Oct 31 '19 at 20:17

Here's a simple example. Without a propeller, a glider moves forwards and (in calm air) descends very gently. So lift is a result of moving forward. Then, moving forward is a result of pointing the nose slightly down -- or, if the glider has a sustainer engine, a propeller.

So the primary purpose of a propeller is to keep the airplane moving forward.

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    $\begingroup$ Could say something like -- "Then, moving forward is a result of pointing the nose slightly down -- or more precisely, of following a descending flight path through the airmass." Probably too much detail already for such a simple question, but helps avoid the apparent paradox of the plane or glider descending with nose pointing up as discussed here aviation.stackexchange.com/questions/70842/… . $\endgroup$ Oct 30 '19 at 15:13
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    $\begingroup$ Could say so, but the OP wants clarification of the basics, which justifies the simplification. $\endgroup$ Oct 30 '19 at 15:25
  • $\begingroup$ Fair enough, it is a very basic question. $\endgroup$ Oct 30 '19 at 15:27
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    $\begingroup$ Basic, ya think?! Trying to contain myself here, but the basic function of a prop is something most people understand pretty early on... $\endgroup$ Oct 30 '19 at 16:20
  • $\begingroup$ Lift is not necessarily the result of moving forwards, but of air moving over the wings. So it's possible (at least if you're a bird) to soar without moving relative to the ground. And with a helicopter, you can have the "wings" moving around in a circle while the rest of the machine stands still. $\endgroup$
    – jamesqf
    Oct 30 '19 at 16:57

The propeller generates thrust, but what is the purpose of thrust of propeller? Is it to help the aircraft move forward or to produce lift to rise up?

The most concise answer would be "Yes, both." Even though thrust and lift are different things, you can't sustain a climb without thrust, and if you try, you'll run out of airspeed, which means you'll also run out of lift. And you can't sustain a climb without lift, either.

Your question though very simple opens the door to thinking about what is meant by words like "purpose" and "cause" in the context of physical forces. All the forces at play in a given physical situation (e.g. a linear climb at constant airspeed) can in some sense said to be helping to "cause" that situation.

Yet in another sense, though lift is always present in a climb (other than a purely vertical one), it is not really the the "cause" of the climb. Read on for more--

A propeller's thrust exerts a forward force on the aircraft, acting mainly in the opposite direction as the drag vector. Yet even if no thrust is present, an aircraft can maintain airspeed just fine (at least up to its terminal velocity) by using gravity to offset drag. This means that the flight path must be descending, at least in relation to the airmass. An aircraft flying without any thrust can't maintain altitude or climb, at least not without trading off some airspeed, assuming no updraft is present in the airmass. So thrust is always needed for sustained horizontal or climbing flight in non-rising air.

One misconception that may be embedded in your question is a hint of an idea that we may need more lift to climb than to fly level or descend. Actually, lift is important regardless of whether we are gliding downward, flying horizontally, or climbing upward. A climb does not require more lift than level flight. It's just more complicated to generate lift when we are climbing or flying level than when we are descending, because it's harder to sustain the required forward motion, because we aren't getting any help from gravity. Hence the need for thrust. So your intuition that a propeller has something to do with generating lift to allow climbing flight has some truth to it, but not because we need any more lift to climb than to fly horizontally or to descend.

Some of these things were said in a simpler way in another good answer to your question: What is the purpose of the propeller? .

If you want to ask "what happens if the propeller falls off", (ignoring the resulting CG shift), then the answer depends on what constraint we apply.

Is the flight path constrained to be horizontal or climbing? Then the airspeed will decrease until the stall speed (stall angle-of-attack) is reached.

Or is the airspeed constrained to be constant? Then the aircraft will start to descend.

Or is the wing's angle-of-attack constrained to be constant? Then the aircraft will start to lose airspeed, the flight path will curve downward, the aircraft will go through several cycles of a pitch "phugoid" oscillation, and the aircraft will finally stabilize in a descending glide at a slightly lower airspeed than it had in powered level flight.

The pilot's control inputs will determine which of these cases apply, or some other case that is a hybrid of two or more of them.

The ultimate answer to your question, which appears to be essentially "what is the effect of thrust", ultimately lies in an understanding of the vector diagrams showing the forces acting on an aircraft in various situations. Such a diagram will give a better answer than any number of words.

So you may want to learn how to draw the force vector diagrams that apply for unpowered gliding flight, powered horizontal flight, and powered climbing flight. These related answers may help:

What produces Thrust along the line of flight in a glider?

'Gravitational' power vs. engine power

Does lift equal weight in a climb?

John S Denker's "See How It Flies" website ( https://www.av8n.com/how/ ) is great resource for anyone wanting to gain a detailed understand of the physics of flight. However it is aimed at people that already have some understanding of flying on the practical level, so it may not be the best resource for a complete beginner. This is the section that addresses the balance of forces in level flight, gliding flight, and climbing flight: https://www.av8n.com/how/htm/4forces.html#sec-four-forces . Note that Denker has not made the simplifying assumption used in the other links and comments above, that thrust acts parallel to the flight path through the airmass.

(It appears possible that you may simply be asking a question about which direction the propeller's force vector points in various modes of flight. If so, that could be better addressed with a different and more specific question. While thrust is not always defined as acting purely opposite to drag, as a rule of thumb the direction of the force from the propeller does act mainly parallel (and opposite) to the drag vector, with only a small component acting parallel to the lift vector. Bear in mind that sometimes engines are mounted with a small amount of down-thrust, so in high-speed cruising flight a propeller's force might actually include a very small component acting opposite to the direction of the wing's lift vector.)

  • $\begingroup$ Other links could be added to this answer that would be more appropriate for an elementary, beginner-level approach to the question-- $\endgroup$ Oct 30 '19 at 14:39
  • $\begingroup$ Nothing wrong with your answer, but it may be a bit much for a basic question like this. You know... What's a CG shift? What's a stall speed/angle-of-attack? etc. Maybe those will lead to new questions from the OP who seems to be starting from near zero on the phenomenon of flight. (Not that there's anything wrong with that, either.) $\endgroup$
    – FreeMan
    Oct 30 '19 at 16:24
  • $\begingroup$ I know, can't help myself. $\endgroup$ Oct 30 '19 at 16:25
  • $\begingroup$ future edit: change "lift is important" to "lift is essential" $\endgroup$ Oct 31 '19 at 20:57

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