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pauljguy
02-27-2010, 08:33 AM
In an attempt to go "stealthy" above 7km (trying to turn off my contrail) I shut off my engine at 10km high, and coasted down to engage fighters at 7-8 km up. I was hoping that without a contrail I could sneak up on them.
It turns out that with engine off, 0% throttle, there is still the contrail that gives you away. This doesn't seem too realistic, since the engine would not be creating water vapour in the exhaust, or am I trying to exploit something that is just not simulated?

-Paul (dooffus)

pauljguy
02-27-2010, 08:33 AM
In an attempt to go "stealthy" above 7km (trying to turn off my contrail) I shut off my engine at 10km high, and coasted down to engage fighters at 7-8 km up. I was hoping that without a contrail I could sneak up on them.
It turns out that with engine off, 0% throttle, there is still the contrail that gives you away. This doesn't seem too realistic, since the engine would not be creating water vapour in the exhaust, or am I trying to exploit something that is just not simulated?

-Paul (dooffus)

general_kalle
02-27-2010, 08:46 AM
not sure but isn't it something to do with pressure difference between the overside of the wing and the underside?

VW-IceFire
02-27-2010, 08:57 AM
It is possible that the game engine wasn't programmed like that so the vapour trail is programmed to appear above no matter what the situation is.

On the other hand I thought a component of the trail was the hot engine and turning off the engine will not make it go away instantly because it is still hot.

JtD
02-27-2010, 08:57 AM
I suspect you're using a strong aftershave...

More seriously, you're certainly trying to exploit something that is just not simulated.

M_Gunz
02-27-2010, 09:26 AM
Contrail from engine exhaust/vapor? It comes from air being changed in pressure enough that
the water vapor in it condenses out into cloud droplets. Sometimes bomber pilots would change
the prop settings a bit and lose the contrails and sometimes it wasn't possible as I was told
by a former bomber crew member who "dropped below the radar" (no responses to email or web
postings) back in 2003.
It's not a difference the IL2 engine has the detail to model. We have propwash effects because
they're supposed to be there, not because air in motion is actually mapped.

arjisme
02-27-2010, 09:29 AM
Looks like it is just something not simulated in the game. OTOH, how realistic is it to turn off the engine near enemy fighters? First, the game does not simulate a realistic engine start sequence. And, second, is there even any record of a real pilot doing this? If not, you have to ask why didn't they? And, does the game simulate the situation so that you would be similarly compelled not to do this?

Choctaw111
02-27-2010, 09:30 AM
I have never tried this, but in reading about it here, I am lead to believe that Oleg does not have this programmed into the sim.

arjisme
02-27-2010, 09:35 AM
Here's the wikipedia explanation of contrails: http://en.wikipedia.org/wiki/Contrail. They make a distinction between contrails and wingtip, etc. generated vortices which, btw, they say are typically visible only at low altitude and/or in high humidity.

AndyJWest
02-27-2010, 09:42 AM
I'd have thought that expecting an engine to restart after being shut off for any length of time at the sort of temperatures you'd find at that height would be pretty risky.

If having a contrail with the engine off is a bug, it is a good one to have if it stops people exploiting other limitations of the sim.

voyager_663rd
02-27-2010, 09:44 AM
Maybe not because of engine heat but even with engine off and high speed friction with the air flowing over the wing, the same heat build up with the colder moist air causes the trail.

Go slower(less steep glide angle)-->less heat-->less to no condensation trail.

Think of the returning space shuttle and heat build up from friction with surrounding air.

I'm just guessing here but seems reasonable.

Sillius_Sodus
02-27-2010, 10:39 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AndyJWest:
I'd have thought that expecting an engine to restart after being shut off for any length of time at the sort of temperatures you'd find at that height would be pretty risky.

If having a contrail with the engine off is a bug, it is a good one to have if it stops people exploiting other limitations of the sim. </div></BLOCKQUOTE>

I don't think outside air temperature influence on engine starting is modelled. If it was, believe me, you wouldn't be shutting an engine down intentionally at those heights. http://forums.ubi.com/groupee_common/emoticons/icon_cool.gif

na85
02-27-2010, 11:02 AM
http://upload.wikimedia.org/wikipedia/commons/8/8b/B-17_Flying_Fortress.jpg

rnzoli
02-27-2010, 11:09 AM
Well, this may be related to the bug, where you run out of fuel at high altitude, and if you have your manual mixture set to 100%, you will still trail fuel vapor, despite engine not running at all http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

arjisme
02-27-2010, 11:34 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
http://upload.wikimedia.org/wikipedia/commons/8/8b/B-17_Flying_Fortress.jpg </div></BLOCKQUOTE>Would be interesting to know what altitude those B-17s are flying at. And where they are flying too.

Waldo.Pepper
02-27-2010, 12:04 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">In an attempt to go "stealthy" above 7km (trying to turn off my contrail) I shut off my engine at 10km high </div></BLOCKQUOTE>

That's pretty clever. Paul sounds like a crafty bugger. He bears close watching if you ask me.

M_Gunz
02-27-2010, 01:00 PM
The vapor condenses when the air it is in can no longer hold it as vapor.
That means less heat and/or less pressure. Air over the top of a wing is
at lower pressure and prop blades are very active wings. The higher the
AOA, the lower the pressure over the top. Look at the pictures of the
B-17's and that is what you are seeing. As Major Worsley explained, some
times he could adjust the prop just a bit and lose the contrail.

It's a matter of physical chemistry. How much salt can you dissolve in
cold water as opposed to hot water? I've never done that with differently
pressurized water but I bet in a pressure cooker you can make stronger
solutions.

Art-J
02-27-2010, 01:53 PM
Loosing pressure contrails from blade tips by adjusting the prop, sounds plausible. Never heard of it till now, you learn something every day!

However, we're still left with exhaust trails. Every engine running on hydrocarbon fuel works like this: fuel + oxygen = carbon dioxide + water (extremely simplified equation!). So I can't imagine loosing ALL the contrail high up there unless someone stops the engine completely. The water vapour present in the exhaust has to condense anyway in such freezing temps!

Looks like our sim doesn't model these aspects precisely, but what should we expect - it began its life as a low level attack planes sim, didn't it?

On a sidenote, somehow I presume I know what the original poster will check first when SoW eventually comes out! http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

Cheers - Art

M_Gunz
02-27-2010, 09:06 PM
Those are vortex from the whole blades same as wing vortex gathers behind the wing tips only much more
concentrated. Notice the thin contrail from the right wingtip of the B-17 on the right? And very little
trail from engine exhaust. I think it's a matter of how far away your trail can be seen. Some days here
I can look up and see the tracks of planes pointing towards the local airports, especially the big one.
From film of the BoB there were contrails twisting all over the sky, testaments of battle maneuvers.

How saturated the air is with water vapor is critical here. SOW -might- model that but it's a lot to ask!
There are already cloud levels so who knows how far they might take contrail formation?
I'd like to see how bumpy flight is just below the clouds first and things like thermals modeled. Du jour
contrails don't bother me much really but it would be cool to be able to change alt and see them stop
because the air you're in is more dry. We see clouds in layers, the vapor is not the same all the way up.

Anyway if we're going to get that then how about the wings icing? What fighters had de-icing of any kind?

BillSwagger
02-27-2010, 09:34 PM
http://www.youtube.com/watch?v=E8AvfXar9zs

stalkervision
02-27-2010, 09:56 PM
You guys expect an awful lot from a 40 dollar combat flight sim don't you? http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

AndyJWest
02-27-2010, 10:18 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">40 dollar </div></BLOCKQUOTE>
I'd ask for your money back. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

I wish that occasionally people would realise that 'not modelling the entire universe' isn't a bug, but a necessity with finite resources. The fact that condensation trails are modelled at all was a testament to the advances in hardware and software when IL-2 came out. SoW:BoB will provide the whiners with a whole new set of 'missing features' to complain about, but in terms of practical gameplay, it will really be of little significance. The OP was trying to exploit an inaccuracy in IL-2 (reliable engine starts at altitude), and it didn't work because of another inaccuracy. The net result was greater realism in the sim as a whole. Probably coincidence, but a good thing I'd say.

M_Gunz
02-27-2010, 10:24 PM
+1!

ROFLMFAO!

M_Gunz
02-27-2010, 10:28 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillSwagger:
http://www.youtube.com/watch?v=E8AvfXar9zs </div></BLOCKQUOTE>

In this demo the smoke particles get the water droplets started. If the plane gets the droplets started
and the conditions are right (air saturated with vapor for its temperature and pressure) then the cloud
droplets will stay. We've seen that the droplets can start without being seeded, or at least I don't
expect the prop blades or wing tips trail smoke.

AndyJWest
02-27-2010, 10:48 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillSwagger:
http://www.youtube.com/watch?v=E8AvfXar9zs </div></BLOCKQUOTE>

So the smoke from the exhaust, in the right conditions should help make cloud? </div></BLOCKQUOTE>

Speaking from practical experience (London in the early 1960s) I can attest to smoke particles being a good 'seed' for condensation - smog in this particular case. I think anyone who doubts the ability of human activity to influence the weather should be obliged to breath a few lungfulls of this crud - The smoke particles caused the smog to form, and when it did, it trapped more smoke particles. Just to add a little excitement, sulphur dioxide was a significant constituent of the smoke. Ask a chemist what you get when you add sulphur dioxide to water condensates... Then inhale it for an hour or two.

Sorry, going a little off-topic here.

PanzerAce
02-28-2010, 05:01 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
http://upload.wikimedia.org/wikipedia/commons/8/8b/B-17_Flying_Fortress.jpg </div></BLOCKQUOTE>

http://www.warbirdsresourcegroup.org/NARG/images/hellcat3.jpg


and before you guys ask, I don't think he's very high up yet http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Art-J
02-28-2010, 06:39 AM
Yep I've seen and recorded these last year during an airshow, when Yak fighter was taking off in high humidity conditions. Not as spectacular as on Hellcat shot, but clearly visible nevertheless. Cool effect!

This is why we should remember the difference between trails caused mostly by pressure drop (as seen, these can be spotted either low or very high), from trails caused mostly by temperature drop up there on 30k-something feet.

Cheers - Art

Eow_TK
02-28-2010, 11:43 AM
I'm not sure about the bigger engines they used, but with small aircraft, you have to worry about cooling the engine too fast. Do you think that the engines on warbirds would have to worry about sudden cooling by shutting off the engine up there?

Treetop64
02-28-2010, 12:36 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PanzerAce:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
http://upload.wikimedia.org/wikipedia/commons/8/8b/B-17_Flying_Fortress.jpg </div></BLOCKQUOTE>

http://www.warbirdsresourcegroup.org/NARG/images/hellcat3.jpg


and before you guys ask, I don't think he's very high up yet http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif </div></BLOCKQUOTE>

Well, how high up is the carrier...? http://forums.ubi.com/images/smilies/53.gif

AndyJWest
02-28-2010, 12:37 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Eow_TK:
I'm not sure about the bigger engines they used, but with small aircraft, you have to worry about cooling the engine too fast. Do you think that the engines on warbirds would have to worry about sudden cooling by shutting off the engine up there? </div></BLOCKQUOTE>

I doubt they would worry about it, because they wouldn't do it.

major_setback
02-28-2010, 01:54 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
The vapor condenses when the air it is in can no longer hold it as vapor.
That means less heat and/or less pressure. Air over the top of a wing is
at lower pressure and prop blades are very active wings. The higher the
AOA, the lower the pressure over the top.

.... </div></BLOCKQUOTE>

+1


The wing is curved at the top and flat at the bottom, this causes lift when moving forward: the air has to travel faster to go round the curve on the upper side. This means that air on top of the wing is at a lower pressure because it is spaced over the longer distance of the curve (and the lower pressure pulls the wing up). Low pressure = condensation of water vapour in the air.

Stingray333
02-28-2010, 01:57 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The wing is curved at the top and flat at the bottom, this causes lift when moving forward: the air has to travel faster to go round the curve on the upper side. This means that air on top of the wing is at a lower pressure because it is spaced over the longer distance of the curve (and the lower pressure pulls the wing up). Low pressure = condensation of water vapour in the air. </div></BLOCKQUOTE>

http://www.austincollins.com/myths.pdf

most of the lift comes from Newtons laws of motion: for every action there is an equal and opposite reaction. Wings need a slight angle of attack and the wind gets deflected down and that gives the majority of the lift, not the curvature. See equal-time fallacy http://en.wikipedia.org/wiki/Lift_%28force%29

stalkervision
02-28-2010, 04:04 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AndyJWest:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">40 dollar </div></BLOCKQUOTE>
I'd ask for your money back. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

I wish that occasionally people would realise that 'not modelling the entire universe' isn't a bug, but a necessity with finite resources. The fact that condensation trails are modelled at all was a testament to the advances in hardware and software when IL-2 came out. SoW:BoB will provide the whiners with a whole new set of 'missing features' to complain about, but in terms of practical gameplay, it will really be of little significance. The OP was trying to exploit an inaccuracy in IL-2 (reliable engine starts at altitude), and it didn't work because of another inaccuracy. The net result was greater realism in the sim as a whole. Probably coincidence, but a good thing I'd say. </div></BLOCKQUOTE>

FYI..that was what it originally cost at release noodle noggin.

I didn't pay that. http://forums.ubi.com/groupee_common/emoticons/icon_razz.gif http://forums.ubi.com/images/smilies/metal.gif

ROXunreal
02-28-2010, 04:21 PM
Lots of nice prop vortice pictures in the wingtip vortices wikipedia article

http://en.wikipedia.org/wiki/Wingtip_vortices#Gallery

M_Gunz
02-28-2010, 04:50 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by major_setback:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
The vapor condenses when the air it is in can no longer hold it as vapor.
That means less heat and/or less pressure. Air over the top of a wing is
at lower pressure and prop blades are very active wings. The higher the
AOA, the lower the pressure over the top.

.... </div></BLOCKQUOTE>

+1


The wing is curved at the top and flat at the bottom, this causes lift when moving forward: the air has to travel faster to go round the curve on the upper side. This means that air on top of the wing is at a lower pressure because it is spaced over the longer distance of the curve (and the lower pressure pulls the wing up). Low pressure = condensation of water vapour in the air. </div></BLOCKQUOTE>

There is actually a good bit more than that. If that were even most of it you could get lift without any AOA
and would not need a sharp trailing edge. The air over the top moves faster than the difference in distance
too, when you have the sharp trailing edge and some AOA. The big part is how the wing forces air mass down.
Different speed air from over the wing mixing with air from under the wing does that as vortex which also
puts extra pull on the air going over the top. Or in the case of the horizontal stab, just the opposite way
and with the prop blades, front and back. But the pressure differential does contribute.

If it wasn't for that then the plane would not fly.

BillSwagger
02-28-2010, 10:50 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillSwagger:
http://www.youtube.com/watch?v=E8AvfXar9zs </div></BLOCKQUOTE>

In this demo the smoke particles get the water droplets started. If the plane gets the droplets started
and the conditions are right (air saturated with vapor for its temperature and pressure) then the cloud
droplets will stay. We've seen that the droplets can start without being seeded, or at least I don't
expect the prop blades or wing tips trail smoke. </div></BLOCKQUOTE>

I thought the video summarizes several principals that cause the condensation.

Squeezing the bottle probably simulates a similar effect of the prop blades, pressure/release adiabatic cooling. i think the added smoke helps give the water droplets something to condense to in a table top experiment. The pressure of the prop blades is probably more extreme. Also at high altitudes you see the effects of exhaust fumes meeting the cooler air. It really is a cloud.



Bill

julian265
02-28-2010, 11:04 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Stingray333:
http://www.austincollins.com/myths.pdf

most of the lift comes from Newtons laws of motion: for every action there is an equal and opposite reaction. Wings need a slight angle of attack and the wind gets deflected down and that gives the majority of the lift, not the curvature. See equal-time fallacy http://en.wikipedia.org/wiki/Lift_%28force%29 </div></BLOCKQUOTE>

It's good to read that... I've thought the same thing for a while now (but never bothered to look into it!)

I do remember some physics teachers regurgitating counter-intuitive explanations to their classes, and this is a good example of it. Ironically it seems the intuitive answers that I remember students giving were more correct than the 'official' explanation. It's more commonly the other way around of course!

M_Gunz
03-01-2010, 07:52 AM
There were still new pilots being taught the Bernoulli explanation for lift less than 20 years ago.
Get one to say that and you have an instant anecdote from a "highly reliable source" since the pilot
does fly actual airplanes. http://forums.ubi.com/groupee_common/emoticons/icon_frown.gif

Oh yeah, and 50 cals bouncing off stone roads have been witnessed to kill Tiger tanks.

major_setback
03-01-2010, 08:17 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Stingray333:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The wing is curved at the top and flat at the bottom, this causes lift when moving forward: the air has to travel faster to go round the curve on the upper side. This means that air on top of the wing is at a lower pressure because it is spaced over the longer distance of the curve (and the lower pressure pulls the wing up). Low pressure = condensation of water vapour in the air. </div></BLOCKQUOTE>

http://www.austincollins.com/myths.pdf

most of the lift comes from Newtons laws of motion: for every action there is an equal and opposite reaction. Wings need a slight angle of attack and the wind gets deflected down and that gives the majority of the lift, not the curvature. See equal-time fallacy http://en.wikipedia.org/wiki/Lift_%28force%29 </div></BLOCKQUOTE>

I'm not trying to argue this, because I don't know what I'm talking about. But...

The diagram in that second link clearly shows that the air above the top of the wing is moving faster. It is the assumtion that it has to arrive at the trailing edge at the same time as the air below which is referred to as the equal-time fallacy, as far as I can gather from reading that article. I merely stated that it moved faster.

From the article:

"The assertion that the air must arrive simultaneously at the trailing edge is sometimes referred to as the "Equal Transit-Time Fallacy".

http://upload.wikimedia.org/wikipedia/commons/9/99/Karman_trefftz.gif


Of course angle of attack plays a big part in generating lift. But the question was about vapour trails.

Art-J
03-01-2010, 09:44 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Stingray333:
most of the lift comes from Newtons laws of motion: for every action there is an equal and opposite reaction. Wings need a slight angle of attack and the wind gets deflected down and that gives the majority of the lift, not the curvature. See equal-time fallacy </div></BLOCKQUOTE>

Looks like we're hijacking the thread a bit, but let me play devil's advocate http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif: wings don't always need a slight angle of attack - look what happens when they have assymetrical foil (and that aplies to most, if not all planes we "fly" in Il-2), in this case even at attack angle of 0 degrees, the lift force is very much generated. Plus, increasing the foil camber does cause increase of lift coefficient as well, so the curvature is still quite important - after all, this is how the flaps work.

Generally, many factors influence generating lift force (and importace of each of them vary with different airfoil design), this is why each of the "wrong" theories of the past included a little bit of reasonable observations and seemed to be "good" to people who once invented them. This is nicely shown on NASA site, guiding us through all these historical theories, with their "pros" and "cons". I haven't yet read the whole wiki article You provided, but I can see the NASA link being used as one of the refereces, so I'll surely finish reading it later!

Cheers - Art

Bremspropeller
03-01-2010, 12:32 PM
Some more pictures...
http://www.air-and-space.com/20020516%20SBA/2%2021%20C-130A%20N531BA%20Tanker%2067%20left%20rear%20take%2 0off%20prop%20vapor%20l.jpg
http://jazzroc.files.wordpress.com/2008/11/prop-kear-cobra.jpg
http://www.kiwithunder.com/images/Pacific%20Wings%20-%20Rob%20Neil/Prop-vortex%20-%20Rob%20Neil1.jpg
http://photoblog.deviantdeadlock.de/images/atlantique.jpg

Bremspropeller
03-01-2010, 12:52 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Do you think that the engines on warbirds would have to worry about sudden cooling by shutting off the engine up there? </div></BLOCKQUOTE>

Depends on the engine. Turbocharged engines generelly want to be treated well - requiring to reduce power over extended amounts of time (high-performance-pistons require something like a 1'' HG reduce per minute - descend planning thus gets to be a real issue!).


Generally, flying at high power/ speed at high altitude and then suddenly chopping the throttle won't please your engine.

But then again, fighters from back then had a much more thorough interflight-inspection than today's high-performance pistons (generally only thoroughly checked every couple of 100 hours).

Engines and airframes also had much shorter lifetimes.
A turbocharged engine of today (much less HP-output, though) will go into overhaul after about 2000 hours.
Usually, fighters didn't grow that old back then.
Engines were pulled off the airframe after 20-30hrs (depending on the engine/ air-force).

Therefore, safety-standards and circumstances are quite different.
Replacing a mistreated engine wasn't much of an issue back then - there were many engines and spares around.

Changing the engine of your Arrow/ Saratoga / Malibu because of fatigue-cracks every couple of hours ain't that feasible http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif


Would be interested to get a source on engine-related crashes (non-combat, of course) from the 40s compared to today's high-performance SEPs.
Maybe Kettenhunde can help here?

M_Gunz
03-01-2010, 03:18 PM
There are still a number of those old airframes flying today. They aren't being stressed fantastically though.

M_Gunz
03-01-2010, 03:27 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Art-J:
Looks like we're hijacking the thread a bit, but let me play devil's advocate http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif: wings don't always need a slight angle of attack - look what happens when they have assymetrical foil (and that aplies to most, if not all planes we "fly" in Il-2), in this case even at attack angle of 0 degrees, the lift force is very much generated. Plus, increasing the foil camber does cause increase of lift coefficient as well, so the curvature is still quite important - after all, this is how the flaps work. </div></BLOCKQUOTE>

Source on the zero-AOA lift please?
Flaps change the AOA as well as the chord when they go down.

Really, find out those planes have wings at Angle of Incidence to they can hold alt while the fuselage is level at cruise.
It is not as with supersonic jets "only a degree or so" though the actual numbers are not easy to find.

M_Gunz
03-01-2010, 03:30 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:
Some more pictures...
http://www.air-and-space.com/20020516%20SBA/2%2021%20C-130A%20N531BA%20Tanker%2067%20left%20rear%20take%2 0off%20prop%20vapor%20l.jpg
http://jazzroc.files.wordpress.com/2008/11/prop-kear-cobra.jpg
http://www.kiwithunder.com/images/Pacific%20Wings%20-%20Rob%20Neil/Prop-vortex%20-%20Rob%20Neil1.jpg
http://photoblog.deviantdeadlock.de/images/atlantique.jpg </div></BLOCKQUOTE>

If nothing else these pictures really show the angles that propwash at low speeds has.

Stingray333
03-01-2010, 06:22 PM
Sure, the pressure difference between the top and the bottom is obviously so large that planes can fly with an asymmetric airfoil with perfectly zero AoA. This is why we see contrails generated across the entire wing surface and not just the tips right?

Flaps work the same way that wings generate lift as with AoA: putting a bigger paddle into the air = more deflection = more lift by newtons 2nd law.

AndyJWest
03-01-2010, 06:53 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Source on the zero-AOA lift please? </div></BLOCKQUOTE>
Sorry, M_Gunz, but NASA seems to think differently:
http://i958.photobucket.com/albums/ae65/ajv00987k/FoliSim1.jpg
http://i958.photobucket.com/albums/ae65/ajv00987k/FoliSim2.jpg
From here (http://www.grc.nasa.gov/WWW/K-12/FoilSim/index.html).
OK, it's just a simplistic model, but I doubt they'd get something that fundamental wrong. It is an interesting little applet, worth downloading just to play around with.

Kettenhunde
03-01-2010, 07:06 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">OK, it's just a simplistic model, but I doubt they'd get something that fundamental wrong. </div></BLOCKQUOTE>

No they did not get it wrong but remember that zero lift is not zero angle of attack in all airfoils.

Camber greatly effects the zero lift angle of attack.

I think M_Gunz is guilty of a typo...He probably meant zero lift angle of attack.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Would be interested to get a source on engine-related crashes (non-combat, of course) from the 40s compared to today's high-performance SEPs.
Maybe Kettenhunde can help here?
</div></BLOCKQUOTE>

I might have some data. Let me look.

M_Gunz
03-01-2010, 07:15 PM
No, I did not expect lift or at least useful lift at zero AOA.

You get enough lift to fly an airplane at zero AOA? It must have awfully big wings to take off?
Why else do planes that cruise over 300 mph have AOI anyway? Because they need it even at speed!

AndyJWest
03-01-2010, 07:27 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">You get enough lift to fly an airplane at zero AOA? It must have awfully big wings to take off? </div></BLOCKQUOTE>

I doubt there are many planes that will take off at zero AOA, but that doesn't mean they can't generate sufficient lift at higher airspeeds at zero AOA to remain airborne.

Kettenhunde
03-01-2010, 07:30 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">No, I did not expect lift or at least useful lift at zero AOA. </div></BLOCKQUOTE>

Yeah you are wrong in that one. Lots of airfoils generate useful lift coefficients at zero angle of attack. Helps reduced angle of incidence.

julian265
03-01-2010, 07:59 PM
Is the AoA for those airfoils measured from the chord line, or another line?

M_Gunz
03-01-2010, 08:22 PM
I think it's the chord but hey I've already learned something new today.

AndyJWest
03-01-2010, 08:59 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Is the AoA for those airfoils measured from the chord line, or another line? </div></BLOCKQUOTE>
From what I understand (not a great deal...), it's normally measured from the chord line, but if you go into the detail of 'real' airfoils, as opposed to the theoretical ones with perfectly-formed ideal surfaces, this gets a bit arbitrary. Nevertheless, it certainly seems to be the case that a conventional cambered airfoil can generate significant amounts of lift around the 'zero' angle - the NASA simulation suggests the zero-lift angle for that particular airfoil is about -4 degrees. More than enough to discount minor doubts about where the exact chord line is.

Stingray333
03-01-2010, 09:21 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AndyJWest:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Source on the zero-AOA lift please? </div></BLOCKQUOTE>
Sorry, M_Gunz, but NASA seems to think differently:
http://i958.photobucket.com/albums/ae65/ajv00987k/FoliSim1.jpg
http://i958.photobucket.com/albums/ae65/ajv00987k/FoliSim2.jpg
From here (http://www.grc.nasa.gov/WWW/K-12/FoilSim/index.html).
OK, it's just a simplistic model, but I doubt they'd get something that fundamental wrong. It is an interesting little applet, worth downloading just to play around with. </div></BLOCKQUOTE>

Pretty darn cool, that is a lot more lift than I expected.

In the second shot, what does the plot of "Press PSI vs chord %" indicate?

AndyJWest
03-01-2010, 09:34 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">In the second shot, what does the plot of "Press PSI vs chord %" indicate? </div></BLOCKQUOTE>
I think it's the air pressure measured at the surface, upper and lower. Note that contrary to what you might intuitively expect, it is lower than the static pressure on both surfaces for most of the chord - our friend Bernoulli wasn't entirely wrong. The graph would be easier to understand if better colours and label positioning had been used - the pressure is lower on the upper surface, hence lift.

EDIT-
Same airfoil at 5 degree AOA:
http://i958.photobucket.com/albums/ae65/ajv00987k/FoliSim3.jpg
Though air pressure is marginally higher than static over the lower surface, most of the lift is due to the drop over the upper surface.

I'd really recommend anyone interested in the subject to download this, and play around for a bit. It isn't perfect, but it gives you a feel for what is going on in a more intuitive manner than maths and dry theory ever can. (But then, I'm useless at maths, so this is all I have to go on http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif )

EDIT 2---
Ignore this:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> most of the lift is due to the drop over the upper surface </div></BLOCKQUOTE>
A bad case of brain fade. The airfoil neither 'knows' nor cares about static air pressure. The only lifting forces acting on it are due to differences between upper and lower surface air pressure.

Incidentally, notice how the centre of pressure has moved forwards as the AOA has increased - one reason why a horizontal stabiliser is necessary.

JtD
03-01-2010, 11:21 PM
Lift coefficient with 0 AoA would be in the region of 0.1 - 0.2 for many airfoils as used in WW2. This often enabled the aircraft to fly near 0 AoA in cruise condition.

Stingray333
03-02-2010, 01:01 AM
I was wrong about contrails not appearing over the wing surface too, I thought that only happened in very high AoA maneuvers on modern fighters:

http://www.flightlevel350.com/...ion_Video-15825.html (http://www.flightlevel350.com/Aircraft_Boeing_747-400-Airline_Virgin_Atlantic_Airways_Aviation_Video-15825.html)

M_Gunz
03-02-2010, 01:55 AM
Andy, it can't give pounds of lift without wing area and speed, right?

AndyJWest
03-02-2010, 07:43 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Andy, it can't give pounds of lift without wing area and speed, right? </div></BLOCKQUOTE>

No, of course not - the applet allows these factors to be adjusted too. The figure given is for this particular example:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Joukowski Airfoil
Camber = 3.0 % chord , Thickness = 12.5 % chord ,
Chord = 5.0 ft , Span = 20.0 ft ,
Surface Area = 100.0 sq ft ,
Angle of attack = 0.0 degrees ,
Standard Earth Atmosphere
Altitude = 0 ft , Density = 0.00237slug/cu ft
Pressure = 14.695lb/sq in, Temperature = 58F,
Airspeed = 100 mph ,
Lift = 949 lbs
</div></BLOCKQUOTE>

(I'd done the test with 'aspect ratio correction' off. With it on, the result is 922 lbs of lift)

Given the simplistic modelling involved, I don't know how accurate this is, but it gives a good idea of how changing different parameters affects results.

Bremspropeller
03-02-2010, 03:09 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Why else do planes that cruise over 300 mph have AOI anyway? Because they need it even at speed! </div></BLOCKQUOTE>

Because that drecreases drag.

You certainly know that net lift is a function of dynamic-pressure as well.
Dynamic pressure decreases with altitude, requiring greater AoAs as opposed to flying the same TAS at lower altitudes.

Thus the need for a cruise-AoA of modern airliners in the region of 3-5, depending on speed, temperature and altitude.

julian265
03-02-2010, 05:17 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AndyJWest:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Is the AoA for those airfoils measured from the chord line, or another line? </div></BLOCKQUOTE>
From what I understand (not a great deal...), it's normally measured from the chord line, but if you go into the detail of 'real' airfoils, as opposed to the theoretical ones with perfectly-formed ideal surfaces, this gets a bit arbitrary. Nevertheless, it certainly seems to be the case that a conventional cambered airfoil can generate significant amounts of lift around the 'zero' angle - the NASA simulation suggests the zero-lift angle for that particular airfoil is about -4 degrees. More than enough to discount minor doubts about where the exact chord line is. </div></BLOCKQUOTE>

I wouldn't be surprised if some wings' reference lines varied by this much or more from the chord line.

However I'm not doubting now that zero AoA lift is significant, but I still don't like the usual explanation, of the two particles meeting at the end of the wing (unless a good reason is provided with it!)

Art-J
03-06-2010, 03:05 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Stingray333:
I was wrong about contrails not appearing over the wing surface too, I thought that only happened in very high AoA maneuvers on modern fighters:

http://www.flightlevel350.com/...ion_Video-15825.html (http://www.flightlevel350.com/Aircraft_Boeing_747-400-Airline_Virgin_Atlantic_Airways_Aviation_Video-15825.html) </div></BLOCKQUOTE>

I thought the same as you, but looks like in very humid air, even not so high angles can produce spectacular effects! That's some great footage! Thanks for the link, mate.

Cheers - Art