Propulsion of the Mars-based helicopter

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MetricRocketeer

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Hi everyone,

I am confused on a fundamental issue. I think that I am posting this question in the correct forum.

I just watched "Sixty Minutes," where JPL scientists were discussing the flight of the small helicopter launched from the surface of Mars. During the several interviews, the scientists and engineers focused much of the discussion on the difficulties of flying a helicopter in an atmosphere so much less dense than that found near the surface of Earth.

But I thought that lift emanated from the thrust produced by a helicopter's propellers.

As I understand the story, Robert Goddard was criticized for believing that a rocket could propel itself even in a vacuum. Goddard's critics incorrectly believed that a rocket required atmosphere against which the rocket's exhaust pushed. But Goddard correctly understood that the aft thrust itself produced the forward motion, in an application of Newton's Third Law of Motion.

So then why does the thinness of the Martian atmosphere affect the helicopter's flight? I am certainly not disputing JPL scientists -- not at all. I just want to understand what is going on.

To summarize my question, therefore, why is a rocket able to propel itself forward even in a vacuum, but the flight of a helicopter -- or a I guess an airplane for that matter -- depends upon atmosphere?

Thank you.

Stanley
 

neil_w

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A rocket motor carries its own fuel, and ejects it out the back at high velocity. So it works anywhere, even underwater (depending).

A propeller uses the atmosphere as its fuel, you might say. So when there's less of it, there's less thrust produced. Imagine a propeller in a complete vacuum. Would it do anything at all other than spin?
 

MetricRocketeer

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Thank you, neil_w, for your reply.

And that would also be the case with a jet plane? A jet plane in a complete vacuum would not be propelled forward -- is that correct?
 

neil_w

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And that would also be the case with a jet plane? A jet plane in a complete vacuum would not be propelled forward -- is that correct?
Yes, a jet requires atmosphere coming in the front in order to produce thrust (obviously in a much different way than a propeller).
 

MetricRocketeer

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OK, neil_w, I really appreciate your answering my question.
 

OverTheTop

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A helicopter blade is just a wing that rotates. It is the force generated by accelerating that mass of gas downwards that lifts the vehicle upwards. Equal and opposite. I seem to remember that even hovering a modest helicopter takes roughly 750kW of power to get sufficient mass flow.

On Mars the pressure is less than 1% of what we have here, so to accelerate enough mass to lift the chopper requires larger blades (diameters and width) and much higher RPM. Gravity there is only about 37% of what it is here, so that makes the task a little easier. If the vehicle has a mass of say 3kg, then on Mars it only needs a bit over 1kg of thrust (although it still has 3kg of inertia ;) ) to get it off the ground.

[edit] Remember I am talking about kg force in relation to the lift. I could talk about newtons of force but might confuzzle our imperial comrades in the USA. Technically to lift a 3kg heli on Earth you need 3x9.8=29.4 newtons of force. On Mars the same 3kg heli would require 3x9.8.0.37=10.9 newtons of force.
 
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AeroTech

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Thank you, neil_w, for your reply.

And that would also be the case with a jet plane? A jet plane in a complete vacuum would not be propelled forward -- is that correct?
A jet engine in a complete vacuum will not function whatsoever.
 

Steveclay

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The downwards thrust from a rotorblade is only effective near the ground. It's the aerofoil section of the blade that generates lift. The rotor blade is a wing thats spun through the air instead of being push/ pulled through it.

Steve
 

manixFan

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Maybe I've always misunderstood lift, but I thought it was the lower air pressure created over the top of the wing/rotor that provided the lift, not the downward force of the airflow. So based on that, a helicopter produces lift by creating an area of lower pressure above the blades which lifts the helicopter. Planes do the same thing - the air over the top of the wind flows faster and produces lower pressure, creating lift. So it seems it's not correct to think of the downward force of the air as thrust like a rocket engine. Rockets work by expelling mass, while helicopters work by altering air pressure. If rockets worked by altering air pressure, they could not operate in space, as has been pointed out.

Here's a quote from a NASA webpage on helicopters: "Wings are curved on top and flatter on the bottom. This shape is called an airfoil. That shape makes air flow over the top faster than under the bottom. As a result, there is less air pressure on top of the wing; this causes suction and makes the wing move up. A helicopter's rotor blades are wings and create lift."


Here's an article that talks about the Mars helicpoter specifically: "The Mars helicopter will need to achieve lift at more than twice the maximum height that helicopters have reached on Earth so the Mars helicopter's blades will spin at 2,300-2,900 revolutions per minute, which is approximately ten times faster than helicopters on our planet."


Note they use the word lift as opposed to thrust.

Maybe it's pedantic to differentiate between lift and thrust, but I've always thought of lift as pulling and thrust as pushing. I suppose from a practical standpoint they are basically the same, but hey, this is a rocketry forum!


Tony
 

Blast it Tom!

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Maybe it's pedantic to differentiate between lift and thrust, but I've always thought of lift as pulling and thrust as pushing. I suppose from a practical standpoint they are basically the same, but hey, this is a rocketry forum!


Tony
Well, that depends on whether you're looking from above the wing or below it! ;)
But you are quite right, it is the airfoil action of the rotating blades, not the mass flow downward, that lifts a hoppity-copter! (this is a rocketry forum!!!) :D
 

tsmith1315

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Maybe I've always misunderstood lift, but I thought it was the lower air pressure created over the top of the wing/rotor that provided the lift, not the downward force of the airflow.
...
So it seems it's not correct to think of the downward force of the air as thrust like a rocket engine.
Do they both not contribute?

Air forced downward due to angle of attack results in upward force on the wing.

Airfoil shape-induced pressure differential results in upward force on the wing.

Both result in lifting forces opposing gravity, and I don't think know if they are so easily separable in reality. The angle of attack influences airflow and the airfoil shape influences the downward deflection.
 

Blast it Tom!

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Do they both not contribute?

Air forced downward due to angle of attack results in upward force on the wing.

Airfoil shape-induced pressure differential results in upward force on the wing.

Both result in lifting forces opposing gravity, and I don't think know if they are so easily separable in reality. The angle of attack influences airflow and the airfoil shape influences the downward deflection.
Very good point; and I don't want us to get stalled on this issue, but I think if you had a look, the mass flow component would be small compared to the differential pressure part, though obviously at higher angles of attack, and, say, with flaps extended, you'll force more air downward...
 

davel

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It gets complicated. The lower pressure on the top of the wing compared to the higher pressure on the bottom of the wing is only a partial explaination of how lift is generated. The math is well beyond my understanding.
 

gtg738w

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Do they both not contribute?

Air forced downward due to angle of attack results in upward force on the wing.

Airfoil shape-induced pressure differential results in upward force on the wing.

Both result in lifting forces opposing gravity, and I don't think know if they are so easily separable in reality. The angle of attack influences airflow and the airfoil shape influences the downward deflection.
They are the same thing. The downward mass flow is caused by the pressure change across the fan/rotor. You can model a helicopter as a tube of air with a pump in the middle. The thrust is caused by the change in momentum in the flow, mass_flow*delta_v. This also explains the propulsive efficiency of different systems since the energy required is proportional to m_dot*v^2. You can make double thrust with double the velocity but it requires 4x the energy. Systems with big mass flows like propellers are efficient but limited in speed, a turbo fan is a reasonable compromise and rockets are the only choice out of the atmosphere.
 

BABAR

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Helicopters and airplane wings don’t create thrust, they create an imbalance in forces, optimally so force pushing UP is less than force pushing DOWN. both require a fluid medium (yes, from a physics standpoint gas/atmosphere is a fluid) in which to generate that force. The denser the medium, the easier it is for the rotor or wing to generate the force.

same principle works in the skies over your head. Airplanes and Helicopters can only operate up to a certain altitude, above that and the atmosphere (the medium) is too thin to generate aerodynamic force imbalance.



from a physics standpoint, there is no such thing as a “suction” force, what we CALL suction is really just creation of LESS force on one side (usually the side hooked to the vacuum pump) vs the other side (outside air pressure.)

From

  • Thrust is a force that moves an aircraft in the direction of the motion. It is created with a propeller, jet engine, or rocket. Air is pulled in and then pushed out in an opposite direction. One example is a household fan.
  • Lift is the force that holds an airplane in the air. The wings create most of the lift used by airplanes.
 
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manixFan

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...<snipped for brevity>....

  • Thrust is a force that moves an aircraft in the direction of the motion. It is created with a propeller, jet engine, or rocket. Air is pulled in and then pushed out in an opposite direction. One example is a household fan.
  • Lift is the force that holds an airplane in the air. The wings create most of the lift used by airplanes.
That's exactly the point I was trying to make, lift vs. thrust. Thanks for that quote.


Tony
 

G_T

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Well even in a rocket motor, it is the differential in pressure in the motor from the head end to the nozzle end which results in the mass flow moving down the core. The differential pressure across the nozzle causes the mass to accelerate out the nozzle. When the pressure outside the divergent portion of the nozzle is lowest (vacuum) then the resultant velocity of the exhaust is greatest. Not enough pressure differential, and you don't get supersonic flow out the nozzle, you get a road flare.

Oversimplifying on the expansion part of course; lower external pressure allows greater expansion to convert heat in the exhaust to velocity in the exhaust. You don't get much benefit of lower pressure on the outside if you don't have sufficient expansion in the nozzle to use it. And if you have too much expansion by more than a small bit, you get flow separation and goodby nice clean linear thrust. Still over-simplified, but that's enough for this post.

Gerald
 

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Always a compromise, eh? I recall reading something about the F1 nozzles on the Saturn V being over-expanded for the higher altitudes... I think it was cited as the reason for the large fanning out of the plume, and then the flame began recirculating and creeping up the low-pressure areas on the sides of teh 1st stage. I plan to look deeper into these issues when I retire. I'd like to see bells on model rockets but that may be a technical infeasibility.
 

kramer714

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keeping it simple F=MA F= force M=mass and A= acceleration

A rocket motor takes a small amount of mass and accelerates it a lot to create a force. small M X Large A = Force (thrust)

An airplane wing deflects air downward, it accelerates a LARGE amount of mass with a small acceleration to create a force. LARGE M X Small A = Force (Lift)

If you ever put your hand out of the window in the car, if your hand is at an angle to the wind your hand wants to move, you are accelerating the air in a direction and the resulting force acts on your hand. Airplane guys call the angle between the chord of the wing and the wind the angle of incidence. When you take off in a plane the pilot rotates the aircraft to increase the angle of incidence and the lift is increased - airplane takes off.

A helicopter isn't a propeller it is a rotary wing, this also how helicopters can land without the engine running, think of it like landing a glider, as long as I go forward I can create lift.

Mars Vs Earth. think of the helicopter blades making one revolution, at sea level (highest density atmosphere ) the blades pass by many air molecules plenty of Mass to Accelerate. At 30,000 feet the air density is lower so I pass by less molecules in each revolution of the blades, less M to A. On Mars the atmosphere is very thin, I barely pass by any molecules of atmosphere in a single revolution. So to create the same lift (in lbs) on Mars I need a longer wing span /rotor diameter (touch more molecules per turn) and / or turn it faster (touch more molecules per second) than a helicopter that flies on earth..

Bernoulli (air faster lower pressure) isn't really how airplanes fly, but the simplest way to think of it is Newton F=MA

Pressure differential in a motor and expansion ratios, yup, ok, the simplest way to think of it is F=MA

Mike K
 

G_T

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Always a compromise, eh? I recall reading something about the F1 nozzles on the Saturn V being over-expanded for the higher altitudes... I think it was cited as the reason for the large fanning out of the plume, and then the flame began recirculating and creeping up the low-pressure areas on the sides of teh 1st stage. I plan to look deeper into these issues when I retire. I'd like to see bells on model rockets but that may be a technical infeasibility.
Bell nozzles are a poor choice on most solid propellant motors because of the two-phase flow problem.

Gerald
 

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Bell nozzles are a poor choice on most solid propellant motors because of the two-phase flow problem.

Gerald
I figured there was a good reason, because us rocketeers like AUTHENTIC! Seeing a model Saturn V going with no F-1 nozzles just kinda harshes my vibe , ya know?! ;)
 

manixFan

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...<snipped for brevity>....

Bernoulli (air faster lower pressure) isn't really how airplanes fly, but the simplest way to think of it is Newton F=MA

...<snipped for brevity>....
Hmm, so the NASA page I linked to above is wrong? We should let them know!

Seriously, NASA provides at least two explanations on how airplanes fly. In the first link it's the downward force of the air, as kramer714 describes:

What makes a plane go up?

"In response to the force of moving the air down, the air pushes the airplane upward."

But on this page it says it's lift created by Bernoulli's principal:

"When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing."

So apparently not even NASA is sure how planes work!


Tony
 
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kramer714

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Hmm, so the NASA page I linked to above is wrong? We should let them know.


Tony
not trying to be 'that guy' but yup it is wrong (and they know).

Understanding how a wing generates lift isn't simple. The Bernoulli description is simple to show students or to use or in a website but kind of does a disservice because it isn't correct. Bernoulli is important for stall speed, laminar flow and drag, all important to aircraft designers but you don't create lift by it alone.

Airplanes fly with symmetrical airfoils, airplanes fly inverted, look at a 747, the inboard section of the wing is flat on the top and curved on the bottom, missiles fly with symmetrical wings all of these are opposite of how 'Bernoulli lift' would work. Lots of other examples out there.

better NASA page for lift https://www.grc.nasa.gov/www/k-12/airplane/lift1.html

Mike K

P.S. now figure out why the shower curtain pulls in not out.....
 

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>>>>>----->

Are you referring to an African or European Swallow?

Zeke
 

G_T

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Mike, when a symmetrical foil is not at zero incidence, the forces are not symmetric any more. Having a non-flat camber line is not a requirement for generating lift. And FWIW, because of flow instability one could have a symmetric foil at zero incidence with flow attached on one side and detached on the other (use hysteresis of flow detachment to get to that state) and actually get a lift vector from a zero incidence symmetric foil. Not that anybody would want to...

BTW, experienced foil series and wing designer here, for low Reynolds numbers.

For those interested, this might be worth looking at: https://www.amazon.com/Flight-Vehicle-Aerodynamics-MIT-Press/dp/0262526441

Gerald

PS - Shower curtain is probably due to lower density rising humid air on the inside resulting in a pressure differential across the curtain barrier. After all, (1) it is warmer inside than out (2) and H2O as a gas is less dense then N2 as a gas at the same pressure and temperature (18 molecular weight vs 28) with N2 being the majority component of air.
 

neil_w

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PS - Shower curtain is probably due to lower density rising humid air on the inside resulting in a pressure differential across the curtain barrier. After all, (1) it is warmer inside than out (2) and H2O as a gas is less dense then N2 as a gas at the same pressure and temperature (18 molecular weight vs 28) with N2 being the majority component of air.
Not so simple. A cold shower sucks in the curtain as well.

Here's some reading: https://www.scientificamerican.com/article/why-does-the-shower-curta/
 
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