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About those wingtip vortices & engine smoke ...


Echo38

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Wingtip vortices happen because of pressure drop (already moist air suddenly cooles).

 

PS: relax, the effect is being worked on and its still WIP, and they already look better in tester's versions. Also you can very easily change the length later on if you want to, just by editing LUA file :)


Edited by Kuky

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I've been watching aircraft for my entire life, and I've never seen anything like those pictures. Indeed, I've never seen any trails at all coming from propellers or rotors. Is there a specific set of conditions under which visible trails on prop/rotor tips are present?

 

Here's one of our Herc's taking off at sea level.

 

C130_zps0f223d12.jpg

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Here's one of our Herc's taking off at sea level.

 

Again, I've seen countless C-130s taking off, and never any trails from the props. Have you seen this in person, or only in pictures?

In person, I've seen them take off in hot weather, cold weather, dry weather, foggy weather, and rain. No prop trails, and definitely not those crazy-long spirals.

Just now ran off to Youtube and watched a bunch take off in videos ... no trails.

Is it maybe a camera effect? Something to do with shutter speed or something?


Edited by Echo38
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I'm pretty sure, that Props have tip vapor because the tips of the prop are going supersonic. along with pressure changes,

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Prop tips don't go supersonic, that would create a massive thrust differential and is therefore one of the limiting factors in prop RPM and blade length.

 

I'm not 100% sure what causes this but I would imagine it is the same concept as wing tip votices, ie high pressure differential at high blade angles (ie take off). As the aircraft accelerates the blade angle will reduce and the pressure differential decreases, therefore it will be most pronounced when the engine is demanding the most additional thrust out of the prop.

 

Humidity obviously has an effect on making the vortices visible as water is condensed.

 

This effect certainly isn't common, but doing things like the air show max effort takeoffs (where pilots will have max power set on the brakes prior to rolling) with high humidity will be your best chance of seeing it.


Edited by Kaiza
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I would also hazard a guess that blade style would impact the chances of these being visible. A herc has a square blade tip for short field work which will probably make these more pronounced, while a p-3 (which has the same engines) has a rounded tip for higher cruise speeds and would probably have less pronounced vortices.

 

Although I have seen this effect on both aircraft types....

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Prop tips don't go supersonic, that would create a massive thrust differential and is therefore one of the limiting factors in prop RPM and blade length.

 

I'm not 100% sure what causes this but I would imagine it is the same concept as wing tip votices, ie high pressure differential at high blade angles (ie take off). As the aircraft accelerates the blade angle will reduce and the pressure differential decreases, therefore it will be most pronounced when the engine is demanding the most additional thrust out of the prop.

 

Humidity obviously has an effect on making the vortices visible as water is condensed.

 

This effect certainly isn't common, but doing things like the air show max effort takeoffs (where pilots will have max power set on the brakes prior to rolling) with high humidity will be your best chance of seeing it.

 

Dont remember where I read that, mighta been confusing it with helicopter speed limitations.

 

I've personally only seen Distinct Tip Vapor on the C-130s and some C-160s

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Prop tips don't go supersonic, that would create a massive thrust differential and is therefore one of the limiting factors in prop RPM and blade length.

The profile of the blades changes from the center to the outer tip. So that thrust differential is not necessarily a factor. The speed of the blade tip going supersonic does not mean that the airflow through the propeller has to be supersonic as well.

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The profile of the blades changes from the center to the outer tip. So that thrust differential is not necessarily a factor.

 

True, but that only works if the whole prop is subsonic. If any part of the prop goes supersonic it drastically reduces it's efficiency and of course the tips travel the fastest. So if the centre of the prop is subsonic and the outer is supersonic the centre of the prop will be creating more thrust than the outer. As you can imagine this will drastically reduce efficiency and for this reason props are normally designed so the whole blade will operate below the speed of sound.

 

That is what I was taught anyway, I've been wrong plenty of times before :)

 

The speed of the blade tip going supersonic does not mean that the airflow through the propeller has to be supersonic as well.

 

No sure what you mean by this. I dont think anyone is suggesting the air is supersonic?


Edited by Kaiza
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Again, I've seen countless C-130s taking off, and never any trails from the props. Have you seen this in person, or only in pictures?

In person, I've seen them take off in hot weather, cold weather, dry weather, foggy weather, and rain. No prop trails, and definitely not those crazy spiral ones.

Just now ran off to Youtube and watched a bunch take off in videos ... no trails.

Is it maybe a camera effect? Something to do with shutter speed or something?

 

It's actually not that rare, I have also seen it quite a few times in person. The video below is a good example of what it looks like in real life. I have even seen a Mustang produce them ;)

 

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I've been watching aircraft for my entire life, and I've never seen anything like those pictures. Indeed, I've never seen any trails at all coming from propellers or rotors. Is there a specific set of conditions under which visible trails on prop/rotor tips are present?

 

please be more specific, where have you been watching aircrafts most of your time ?

coz if you did it in neveda or in arizona (dry air), then im sure you have not seen much that efffect which is very common where the air is very humid like in florida or alabama (near ocean: 98-99% humidity)

im sure this effect is very common in britain and in canada as well, (after raining) moreover in hawaii or in guam where the air is not only humid but very hot too (hot air "can keep" more humidity in the air than cold air)

 

more vapor over the water

 

bit OT but cool and interesting videos (i like that KLM-747: SEX :D )


Edited by NRG-Vampire

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please be more specific, where have you been watching aircrafts most of your time ?

coz if you did it in neveda or in arizona (dry air), then im sure you have not seen to much that efffect which is very common where the air is very humid like in florida or alabama (near ocean: 98-99% humidity)

im sure this effect is very common in britain and in canada as well, (after raining) moreover in hawaii or in guam where the air is not only humid but very hot too

 

Very interesting! Thanks. Yeah, I live in a pretty low-humidity area. Now I have another reason to visit a state with an ocean shore!

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No sure what you mean by this. I dont think anyone is suggesting the air is supersonic?

Maybe I was inprecise with what I wrote. I will try it again: the air flow through a propeller depends only secondarily on the rotation speed. Primary factor is the blade profile. Imagine a propeller with no profile at all, totally flat. Even if the blades go supersonic, the air that goes through the propeller is not affected. The only effects of supersoning movement would be in direction of the rotation because of the shape (width) of the blade that moves through the air. That might cause difficulties, yes, vibrations and so on. But the thrust giving mechanism of the propeller can be influenced largely by the the blade profile and therefore don't have to cause thrust differencies along the blade.

 

How efficient that is (or if at all), is a different question tough.

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this is all about pressure

vapor "born" in low pressure spaces

higher humidity helps to born "easier" the vapor - becomes visible at lower pressure drop

higher temperature increasing it more besause hot air can keep more humidity

at lower temperature (and lower pressure too) the same humidity wont stay in the air: will condense: this is how the fog born

this can be visible at propeller-tips too: more speed on the tip, more pressure drop, more condensation visibility there - without superconic propeller-tip rotation


Edited by NRG-Vampire

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Maybe I was inprecise with what I wrote. I will try it again: the air flow through a propeller depends only secondarily on the rotation speed. Primary factor is the blade profile. Imagine a propeller with no profile at all, totally flat. Even if the blades go supersonic, the air that goes through the propeller is not affected. The only effects of supersoning movement would be in direction of the rotation because of the shape (width) of the blade that moves through the air. That might cause difficulties, yes, vibrations and so on. But the thrust giving mechanism of the propeller can be influenced largely by the the blade profile and therefore don't have to cause thrust differencies along the blade.

 

How efficient that is (or if at all), is a different question tough.

 

This does not accord with what I was taught. First off - a totally flat blade does have a profile i.e. totally flat. It will produce local lift forces if presented at an angle of attack to local airflow, as with a propeller. Secondly, airflow is relative. This means that any aerofoil moving (relatively) through a mass of air will have all the aerodynamic forces such as lift and drag present. The thrust produced is equivalent to lift on a wing section. Thus the actual thrust is proportional to the square of the local propeller section and only directly to the section of the aerofoil section (shape).

 

Taking the case of a propeller tip rotating so fast that the local airflow is moving supersonically in relation to the tip, then you will get shock waves being produced. These may have visual product through condensation, with visible vapour being produced, and which usually converts back to invisible water vapour almost immediately as it exits the shock zone. These shock waves interfere with the airflow in the immediate vicinity and cause localized loss of "lift", which, in the case of a propeller, is thrust. Vibration is caused by the effect of the shock waves produced giving continually variable local "lift" as the air meets the tips at slightly varying angles.

 

Thus you can see that at the propeller tip the air is moving supersonically, but in relation to the tip itself. Think of an aircraft going supersonic: the air is not moving over the ground at supersonic speed, but the aircraft is moving through the air at supersonic speed. It is the relative speed between the air and the aerofoil which produces the supersonic effects. :thumbup:


Edited by roadrabbit

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This does not accord with what I was taught. First off - a totally flat blade does have a profile i.e. totally flat. It will produce local lift forces if presented at an angle of attack to local airflow, as with a propeller. Secondly, airflow is relative. This means that any aerofoil moving (relatively) through a mass of air will have all the aerodynamic forces such as lift and drag present. The thrust produced is equivalent to lift on a wing section. Thus the actual thrust is proportional to the square of the local propeller section and only directly to the section of the aerofoil section (shape).

With "flat" I meant "no profile and not angled". Sorry for being inprecise. Again.:doh: This type of propeller blade will not produce thrust, no matter how fast it spins, as it does not move through the air horizontally (along it's rotation axis). It does move through the air vertically - as it is rotating, yes. But I can not imagine how any supersonic effects at the blade tips could possibly produce thrust or drag in horizontal direction (or at least, both cancel each other out) - if this is what you meant here.

 

Taking the case of a propeller tip rotating so fast that the local airflow is moving supersonically in relation to the tip, then you will get shock waves being produced. These may have visual product through condensation, with visible vapour being produced, and which usually converts back to invisible water vapour almost immediately as it exits the shock zone. These shock waves interfere with the airflow in the immediate vicinity and cause localized loss of "lift", which, in the case of a propeller, is thrust. Vibration is caused by the effect of the shock waves produced giving continually variable local "lift" as the air meets the tips at slightly varying angles.

As far as it concerncs vapor production, I am with you there. :o) But, allright, beyond that ... we really leave the area of my layman knowledge. For an actual propeller with a working profile, I can imagine that these effects do affect the airflow. Somehow.

 

But for my theoretical "flat" propeller (or almost flat at the tip, only minimal profile/AoA, this could probably ignored. Taken my example to the extreme: shape the profile so that the subsonic part produced the thrust and the supersonic part has only so much profile that it does not only produce drag. (Inefficient, yeah, therefore it is only a theoretical example ;P )

 

Thus you can see that at the propeller tip the air is moving supersonically, but in relation to the tip itself. Think of an aircraft going supersonic: the air is not moving over the ground at supersonic speed, but the aircraft is moving through the air at supersonic speed. It is the relative speed between the air and the aerofoil which produces the supersonic effects. :thumbup:

A flying airplane with our propeller, the propeller blade tip is moving on two axes: horizontally (direction of movement of the whole plane) and vertically, 90 deg. angled to the horizontal axis (rotating propeller). The horizontal movement is relevant for the thrust, but here are no air masses moving with supersonic speed in relation to the blade. At the other axis there is such movment: blade tip vs. air - but this movement is not producing thrust.

That was my point so far. But that movement can cause disturbances that can influence the horizontal air flow ... alright, I got that. :o)


Edited by Flagrum
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A flying airplane with our propeller, the propeller blade tip is moving on two axes: horizontally (direction of movement of the whole plane) and vertically, 90 deg. angled to the horizontal axis (rotating propeller). The horizontal movement is relevant for the thrust, but here are no air masses moving with supersonic speed in relation to the blade. At the other axis there is such movment: blade tip vs. air - but this movement is not producing thrust.

 

:) Hi Flagrum - I have just shortened your quoted reply in order to answer a specific problem. What you have tried to do is resolve the propeller motion of an aircraft in flight into two seperate motions. This can be instructive on occasion, but could be confusing the issue here. Notice I have not even introduced supersonics here!

 

As a propeller rotates it meets the air. With an aeroplane stationary on the ground the relative speed of the air to each propeller blade increases with distance from the centre of rotation. This means that with a constant section airfoil (one with no twist in it) the angle of attack of each blade increases towards the tip. If the physical blade angle is more than a small amount, at some point along the blade's length towards the tip the blade will stall at normal rotation speed. To prevent this a propeller blade is twisted and has "wash-out" from the spinner to the tip. Thus at a typical rotational speed of 2,000 +/- rpm the blade will produce thrust (the 'lift' force spoken of previously) evenly along its length.

 

Now the tricky bit :D. As an aircraft gathers speed the forward motion of the plane comes into the picture. There is, as you say, now a longitudinal component to the airflow meeting the blade which is along the propeller's axis to the free airflow. (Try though here to picture the 'total' airflow meeting the blade - the propeller cuts a helical path through the air like a screw going into wood) This has the effect of decreasing the thrust of the propeller if at constant rpm. At some point in the aircraft's acceleration the thrust will reduce until it equals the total drag of the aircraft - this will be the maximum speed of the aircraft at that propeller rpm. Increasing power can increase rpm and thus speed, but eventually the propeller drag will equal the engine power - the aircraft speed reached will then be the maximum in level flight.

 

There is much more theory and explanation about variable pitch and 'constant speed' propellers, but it would be better for anyone interested to consult one of the many textbooks available. In the UK I started off with "Flight Briefing for Pilots" written by Birch and Bramson. I am not sure if it still in print, but I am sure there have been replacements! :book:


Edited by roadrabbit

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hey guys...a couple of days ago, i flew a bit in the a10c again after many months....i immediately noticed those really good looking wingtip voritces when i made a couple of tighter turns....they indeed look great in my view..

but ive never noticed them in the p51...from inside the cockpit...though i did notice them when watching p51ai flying around, as well as on fw190s.

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So the positive angle of attack on the A-10A passes critical when flying as fast as 0.7M with only 10%fuel and no weapons, and the obtained G won't reach 6 at least? I bet you were at a low enough altitude, let's say below 10000', when you did it, otherwise there should be no aerodynamic difference between the A and C model, except for the weight (if there's any).

 

i am not talking about speed in fact, im talking about G load

i tried both low and high altitude, but was the same - differ from A-10C

something wrong with that A-10A flight model - far from A-10C FM as i wrote before

rudder are ok like on A-10C but elevators ? ? ?

AOA and G load are very strange after the A-10C

maybe because of some SFM limitation which remained in PlaneConst.lua

only 5.9 MaxG allowed in lua file so i guess remained A-10A SFM is always quarrels with the not full AFM when i try pull more than 5.9 Gs


Edited by NRG-Vampire

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Going supersonic on propeller blades isn't a good thing (although the effects of vapor and condensation will increase), even if only the tips go above M=1 locally somewhere along their airfoil's sections, which is more a transonic region and not fully supersonically developed along the whole tip section as we might think when we all say "supersonic", but even in those conditions, the effects of local lift/thrust reduction and blade vibrations will occur, therefore no blade should reach sonic speed values in normal operations.

 

Precisely! :D

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Wingtip vortices and engine smoke are very common in modern fighters, usually at full military power engines are smokiest, as when the reheat kicks in it burns much cleaner and hotter

 

Another element in the equation is altitude. Jet engines are most efficient at higher altitudes with reference to specific fuel burn per distance travelled. Therefore the inlet guide vanes ae designed to be most efficient at these higher altitudes. With fixed IGV's they are not as efficient at lower altitudes and one result is less efficient fuel-burn leading to visible black smoke in the exhaust. Many military (and civil) jet engines of the past had fixed IGV's and so produced black smoky exhausts when at high power and lower altitudes.

 

A comparison is an off-tune diesel engine in a car - if you floor the accelerator you get clouds of black exhaust smoke.

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