Yes, there would be less dense air the higher you go, so the props become less efficient. They may spool up more quickly but the resultant thrust is less due to the thinner/less dense air. Therefore more poser is needed to hover, climb or move forward, than for the same task at lower altitude. By using more power to fly the same course that was flown at lower altitude, more battery would be consumed during the same time frame at high altitude than that used at lower altitudes. You will still be able to fly of course, just not as efficiently at high altitudes, with a drone using electric motors.
Is This The Dumb Question Department?
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An airliner that is capable of say, 560mph cruise at 38,000 ft could not possibly get much above say, for arguments sake 380mph at sea level, at full throttle, straight and level, due to the density of the air (don't hold me to exact numbers, this is meant to give you an idea). That is why they climb up there, for efficiency, and the view is very nice too. But then, the sun's radiation is stronger too, so as always in aviation, there is always that trade off.
And that is where the beauty of an adjustable prop comes in handy. You can pitch up to take a bigger bite of air as your speed increases and the air thins.
Although you are correct on your temps, this should not be taken as gospel for all conditions. For those not familiar with Celsius and meters, you can say the temps drop by a fraction over 3.5 degrees F per 1,000 ft of altitude gained.
However, as I said this is a general rule and not the case in all weather conditions. Because if you are flying in rain for example, or snow, or possibly climbing through clouds in IMC flight, then those temps. Foxhall mentioned, are going to look a bit different. In those not so nice conditions I just mentioned (not a clear air day) the temperature decreases by about 3 and a third degrees F for every 1,000 feet you climb. Or going back into Celsius that would be a change of about 6 degrees C per 1,000 meters of altitude gained.
Not that any of this temp. difference is relative to us, when flying drones, it is none the less, interesting to understand how weather affects or can affect a flight, other than just meaning that we can or can't enjoy a fun day of flying.
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FoxhallGH
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Correct - which is why I put the caveat 'In clear skies ...' Not that I'm championing it, but there was a video posted on this forum not so long ago, of a drone in Russia that ascended to somewhere around 3,000 metres ... The temperature reading back from the drone at max height, got down to -40 degrees C!
FoxhallGH
Well-Known Member
There are several things going on in the 'airliner' scenario ... Mainly, the fact that when the airliner gets up to the thin air at its cruising altitude, it is flying at a fair rate of knots that's literally stuffing what little air there is into the engine intakes. I think it's safe to say, that if an airliner tried to take off from a standing start, in air as low density as at its crusing alt' - it probably wouldn't get off the ground! So airliners are designed to fly at an altitude where the balance is between the air being thin enough to reduce the airframe drag, but thick enough to provide the thrust as it's pushed by the fanjet turbines. Like most engineering problems, there will be a set of lines on a graph that will intersect around an area that will tell the pilot what the ideal mix of altitude and thrust will give best fuel consumption figures.
brett8883
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I don’t have much technical understanding of jet engines so don’t take it that way.
I always thought that a jet engine propelled its self forward by the expansion of gas caused when fuel is ignited similarly to how a rocket propels a space ship forward. However the difference being that a rocket fuel contains its own oxidizer much like a smokeless gun powder. On the other hand the fuel of a jet engine requires oxygen from its surrounding and that’s why they are referred to as “air breathers.” I thought the Turbofans are just a means to control the amount oxygen to the combustion chamber to ignite the fuel. The more fuel the more oxygen needed. So if the fuel could be ignited without an external source of oxygen then a jet engine would still provide thrust in even a vaccum. The wings are what provide lift being shaped to cause a higher pressure below as air is moved over them. Right?
So if that’s true and I’m not sure it is, then thrust isn’t really determined by air pressure or density for a jet engine neccesarly right? It’s more that the jet engine must only be able to extract enough oxygen from the atmosphere to the ignite the fuel. So if that’s correct wouldn’t a jet engine produce the same amount of thrust all the way up to the minimum oxygen density ceiling and then just stop working? If the drag is reduced with altitude then this would explain why a Jet is more efficient the higher you go?
Unlike a jet engine that uses the expansion of fuel to create the gases that provide thrust, a prop driven motor like on the Mavic produces lift by creating a higher pressure environment below it and a lower pressure environment above it thereby “floating” in its actual environment. Much like a ballon that is released underwater It’s not so much that it is expelling gas to provide thrust but rather the DIFFERENCE in density it creates. The thrust and the lift vectors are the same and indistinguishable.
So at lower air pressure wouldn’t it require the same amount of work to create this difference in pressure (lift/thrust) up to a certain point where there is no longer enough air for the difference in air pressure to be enough to float in anymore? Like how the balloon reaches the surface of the water and stops?
Again this may all be totally wrong but wanted to run it by you guys and figure out where I’ve gone wrong.
Thanks guys![/QUOTE]
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The drone does not actually create a higher and lower air pressure environment, like you may have in your minds eye, of how it is flying. It is the props that do the work. The prop blade is shaped in a similar fashion to a wing. Without going into too much detail, lets take too wing shapes, that is cross sections of them, if you were to cut a piece off and look at its thickness.
So, we shall look at a fat airfoil shape and a thin shape. The thin shape is a jet airliner, for argument sake and the fat one would be, say, an old Piper Cherokee 140. The fatter shaped wing cross section has more length of metal on the top surface, that the bottom surface. The jetliner does too but at a much lower ratio, so just a little bit more on top, compared the bottom surface. Because the top surface is curved and the lower surface is straight (keeping this shape thing simple, for explanation purposes) when air hits the leading edge and is forced to split itself up, the air going over the top has a greater distance to travel, than the air flowing underneath.
Air does not split itself up and stay divided or there would be a vacuum created at the trailing edge. Therefore the air travelling over the top surface of each wing must speed up in order to catch up to the air flowing under the wing sections. The faster the air travels on top, due to a thicker, or fatter shaped wing, the lower the air pressure created. The lower the pressure on top, the greater the lift will be on the wing. Now a fat shaped wing will be much more draggy too, thereby being less aerodynamic, so not as fast through the air. With that said, it does generate greater lift, so can get off the ground more quickly, but is less efficient, the faster it travels.
A Jetliner has thin wings, so are designed for speed but do not have as much lift generating design to them at low speeds, and will not get off the ground as quickly, meaning it would need an awful lot of runway to lift off, if left like that. To help with this, the jet wing will have slats and flaps built into them, which extend out the front and the rear of the wing on take off and landing. What these two pieces of metal do, is fool the air into thinking this is actually a fatter wing shape, so it creates greater lift, when configured this way at lower airspeeds. That way it gets off the ground sooner and can slow down more for landing.
Once the jet lifts off and continues to increase its airspeed, those slats and flaps are brought back into the wing, thereby changing the shape back into the fast and streamlined shape it was designed to be, becoming more efficient but requiring greater speed to work best. This being ideal for highspeed flight. A prop blade is the same thing as a wing, just tiny little thin wings. So when they spin, they also create the high and low pressure regions of air, right above and below each blade. As they spool up, the increased speed creates the increased lift, so it climbs or goes forward faster and faster.
If you want to see how high and low pressure works right in front of your eyes, just cut a strip of paper from a standard 8.5x11 inch sheet. Cut it so that it is about an inch or two wide and about 8 inches in length. Hold the thin end with two hands an place it just under your bottom lip, letting the rest of the length just droop down over your chin. Now take a deep breath and start to gently blow over the top of this piece of paper, increasing as you go. As you blow harder you will see the paper begin to rise in front of you, because your blowing of the air on top is creating the lower air pressure, compared to the air below that strip of paper. Therefor that whole piece of paper will act like a wing or a prop blade and rise up to a level position as you blow harder.
I hope that is all understandable for those who were not aware of these basic principles.
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[/QUOTE]
Your understanding of a jet engine is pretty much spot on. Thrust does fall off with altitude however, because the engine cannot ingest enough oxygen at higher altitudes to burn as much fuel as it does at lower altitudes.
Both jet and propeller engines fundamentally produce thrust by the same mechanism - by accelerating a working fluid backwards. The change in momentum of the working fluid, whether it is the surrounding air (propeller) or combustion products (jet), is equal to the propulsive force exerted on the aircraft, by Newton's Second Law.
FoxhallGH
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You are sort-of right, if you are talking about a 'Turbo-jet' like you have in a jet-fighter. They have a large air-intake, that pushes air into a small pipe, where it is compressed down by a series of fans, then mixed with fuel and ignited to create thrust from the gases escaping the rear of the pipe at a very much higher speed than they went in!
turbojet ...However, the average airliner uses a 'Fan-jet' engine (also known as a 'high-by-pass' engine), that has a small core that's similar to the turbojet, but uses the escaping gas not only as thrust, but also to turn a shaft connected to 'large' fan blades at the front, which are driven and 'shrouded' in such a way that a very large proportion of the thrust that comes from this type of engine is due to the big turbine blades at the front of the engine turning through the air, just as a propeller does. You can almost think of this type of jet engine as being a compex way to spin a highly designed propeller in a shroud - which removes all the issues about tip vorticies that cause 'open' prop's so much trouble! In fact, with most modern Fanjet engines, the amount of by-pass thrust is greater than that produced by the actual 'hot' jet, because a lot of the energy the jet produces, goes into turning a turbine to drive the large fan ... This type of engine burns less fuel, and is quieter due to the column of fast moving exhaust in the middle, being surrounded by slower moving air from the large turbine fan.
fanjet ...You won't see many military jets using fanjet motors, as they accelerate slowly, don't have an afterburner, and have a lot of drag at low level due to the frontal area. However, a notable exception is the A10 Warthog, which is renown for being quiet, reliable, and having a very long loiter time over the target.
Lastly, don't forget that there is a big & important difference between full size aircraft (prop' and jet) and the Mavic, and that's about the motors being electric. People-carrying aircraft are mostly (yes - I know that there's some electric a/c out there now), using air-breathing engines that mix fuel with air to get some sort of reaction to spin something that keeps them airborne. The Mavic does not need air at all to spin its prop's! It helps to have air there for the prop's to produce lift from, but that's all - so that's why a Mavic 2 Pro can fly up to 6,000 metres (19,685 ft), before it can't find enough air to claw its way any higher - but you could take it into space and it would still be spinning it's prop's - 'till the battery died.
Some general comments, if all of you will allow:
There are three types of jet turbine powered aircraft: turbo-jet, turbo-prop, and turbo-fan. The turbo-jet produces all of its thrust from the combustion gases exiting the tail pipe. Only aircraft that need to fly at very high airspeeds, like military fighters, use these engines. The turbo-prop uses the turbine to power the propeller and all thrust comes from the propeller. The turbo-fan is a hybrid of the other two. It uses a turbine to power a ducted propeller (the fan) for 80% of the thrust with only about 20% of the thrust due to the exhaust gases of the turbine.
Now to the question of does one of our multirotors use more power to fly at higher altitudes. It is true that, if one uses the same propeller, the motor will turn faster at higher altitudes. But, that by itself, does not mean that the motor will use more power when turning faster. Electric motors, like we use in our multirotors, do not work that way. Electric motors produce the amount of power based on the load placed on them and thus the power they consume is the load plus the losses in the motor. The only way to really answer this question would be to hover the multirotor at low altitude and high and measure the motor current and voltage for both and compare them. Anything else is just uninformed speculation.
Nick
There are three types of jet turbine powered aircraft: turbo-jet, turbo-prop, and turbo-fan. The turbo-jet produces all of its thrust from the combustion gases exiting the tail pipe. Only aircraft that need to fly at very high airspeeds, like military fighters, use these engines. The turbo-prop uses the turbine to power the propeller and all thrust comes from the propeller. The turbo-fan is a hybrid of the other two. It uses a turbine to power a ducted propeller (the fan) for 80% of the thrust with only about 20% of the thrust due to the exhaust gases of the turbine.
Now to the question of does one of our multirotors use more power to fly at higher altitudes. It is true that, if one uses the same propeller, the motor will turn faster at higher altitudes. But, that by itself, does not mean that the motor will use more power when turning faster. Electric motors, like we use in our multirotors, do not work that way. Electric motors produce the amount of power based on the load placed on them and thus the power they consume is the load plus the losses in the motor. The only way to really answer this question would be to hover the multirotor at low altitude and high and measure the motor current and voltage for both and compare them. Anything else is just uninformed speculation.
Nick
FoxhallGH
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That's my line of thought Nick ... An electric motor draws power based on the amount of work it's doing, and not on its rpm. A graphic way to test this would be to take the prop's off a Mavic, and see how long the battery lasts when the motors are efffectively doing minimal work on maximum rpm. In very broad terms, the motor will try to do the same amount of 'work' to do the same hover or manouvre at all altitudes. Since the amount of 'work' is constant (as the weight of the Mavic doesn't change), the propellor is a constant shape & weight etc., and the air density is decreasing as you go up ... the thing that has to change is the rpm ... The higher you go, the more rpm you'll see to do the equivalent e.g. hover.
Interesting you mention the high / low test above, as @sar104 and I were going to try this. Only thing is I have a Mavic Pro Platinum, and he has a Mavic 2 Pro, and a Mavic Pro ... If you have a MP or M2P and want to do a short hover run at or near sea level, @sar104 can do the high altitude equivalent and compare log files ... Are you able to do that??
brett8883
Well-Known Member
I think we should do that and I have a mavic Pro and I’m at high altitude so I can do the high altitude test. We’d have to agree on a set of props though. I have both the master airscrew and the standard mp1 props.
I also think that we also need to have a horizontal flight test as well because I do a lot of extreme(for a drone) elevation changes and what I think I notice is that it’s not so much it uses less power to hover but that it uses a lot less energy to maintain a speed at higher elevations than at lower elevations and that’s where the benefit to battery life comes from in my original hypothesis.
As I write this, I'm sitting at sea level. But, my Mavic Pro Platinum is at home at 2500ft. I'd only be able to get much higher after the snow melts off the mountains. Getting much lower at home would involve some digging.
For what it's worth, if one wants to fly a Mavic at high altitude, the limit with standard props is due to the limit of the battery voltage and not the power required to spin the prop. The proper way to increase the max operation altitude is to increase the pitch of the props as appropriate for the altitude. An approximation is to increase the prop pitch so that the hover rpm is about the same as it would be at low altitude with stock props.
Nick
For what it's worth, if one wants to fly a Mavic at high altitude, the limit with standard props is due to the limit of the battery voltage and not the power required to spin the prop. The proper way to increase the max operation altitude is to increase the pitch of the props as appropriate for the altitude. An approximation is to increase the prop pitch so that the hover rpm is about the same as it would be at low altitude with stock props.
Nick
brett8883
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I have a Mavic Pro and Mavic 2 Pro, both at high altitude. Surely someone in this discussion has one of those at sea level. @BudWalker.
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That works fine up to a certain point, after which it obviously fails due to insufficient density of working fluid.
Well, I do have both and routinely fly at about 1500 feet. But, I have flown them both at sea level.
Is there a question? Do I really have to read all 97 posts?
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Well, I do have both and routinely fly at about 1500 feet. But, I have flown them both at sea level.
Is there a question? Do I really have to read all 97 posts?
Of course you do - don't be lazy. Actually the request is for a flight DAT file for the same aircraft and props hovering at different elevations. I have data for the M2P and MP at 10,000 ft. If you have a DAT file for (preferably) your M2P with stock props at sea level or 1500 ft that's what I need.
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