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jcomm

Why don't they quit? ( the reciprocating engines... )

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That nightmare of any prop pilot is not quite like so in X-Plane10 :-)

 

Is it acceptable for, say, the default C172 or the Stinson L5 (which I edited to set the critical altitude for the engine to 12.000') to keep their engines running, even without leaning, if we drop them ( by "Local Map" magic ) at, say, 65.000' ???

Problem is ( with all prop aircraft in X-Plane ) that if I place them ( not take them up to... flying...) no matter how high I want, their engines will still be running happily ... ????

 

What am I missing here?

 

Of course when I set a critical altitude for the reciprocating engine I know the engine will produce less power as I get past that altitude, and I wouldn't be able to get to those altitudes, but, shouldn't the engine model be consistent, and make the engine die at some thousand feet above the critical alt, even more when it is not leaned at all?

 

Sh###, thursday's ending.... Where are my pills?????

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Will you stop whining?
What the point of bringing a C172 at 65,000ft anyway. 

 

Plus not leaning at altitude will not kill your engine instantly. It may foul some plugs in the long run (days) but certainly not instantly.

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Good catch, but I think the actual discrepancy is less than it appears. If you display on screen the engine torque/power data, you'll see that the higher you go, the lower they are. At 50.000 ft, when leaned for max power, the engine outputs 20 hp.

 

Also, the higher the altitude, the slower becomes the engine to respond. Above 70.000 ft, in my case the engine is basically not responding to throttle commands.

 

Now, I agree that above a certain altitude the engine should quit, but I actually don't know how a C172 engine would behave at 60.000 ft. I know that an aircraft with a naturally aspirated (non turbocharged) piston engine reached 56.000 ft (Caproni CA.161).

 

What is important is not if the engine appears to be running or not, but how much power is it putting out.

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Ok, I accept both replies as good justifications for it not to happen...

 

After all, as I pointed in the OP, I couldn't have climbed to that altitude anyway because X-Plane10 would not allow me to - the power would decay dramatically above service ceiling or, in the case, 12.000' which is the altitude I set for "critical"...

 

So, while not really whining, I'm being to picky, I admit :-)

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Looks like there are problems, as you said, in the behaviour of the engine with regard to the mixture settings. The engine runs at 50.000 feet with mixture at max. Is that realistic? Mmm...

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Is that realistic?

 

I don't think it should, but again, I accept the thesis that I couldn't fly up to those altitudes... I was able because I placed the aircraft there :-)

 

Anyway, from a strictly perfectionist POV, I would like this sort of "simplifications" not to exist... A sound flight / engine model is prepared to cope with any situations, although that may well be asking way to much for a $80 sim...  and as someone pointed out at another forum, it can be done by programming and direct access to the datarefs ( like A2A and other do with their remarkable FSX add-ons.. )

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I agree, it could be reported as a bug, even if probably there are 100+ items with a higher priority than this, for Austin.

 

Do you know if the A2A or DCS P51 engine runs at 60.000+ feet?

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Plus not leaning at altitude will not kill your engine instantly. It may foul some plugs in the long run (days) but certainly not instantly.

It won't kill it instantly, but you'll start noticing the difference rather quickly, when doing the next mag check. At the point, rev up the engine to 1800-2000 rpm, and lean until it about wants to die. The plugs will then have the wet fuel burned off, and the mag check will be a good one. For reference, most of my flying began at 4600', and then 7500 to 13500'. At 4600' airport elevation, I'd lean right after engine start. If you don't, you can tell the difference just taxiing.

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I haven't tried with A2A's p51d, because I got bored at the power it still exhibited at around 50k, and other aspects that I find a lot more detailed in the DCS p51, although I know it's different models being represented...

 

Also, in DCS The P51d can't be put higher than around 42K using the mission map. From there you can still climb, but it's going to be really difficult. The higher I went was around 45k, and th engine was already showing some problems. If I enriched it would immediately stop!

 

Above all, the effects of the less denser air at that altitude are extremely well represented in DCS, as opposed to either FSX or XPX where you still get way to effective response from your control surfaces (it's like if you were cruising at 4000' ) In DCS there is a lot of lag, the controls feel sluggish, etc...

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For reference, most of my flying began at 4600', and then 7500 to 13500'.

 

Where did you fly out of Larry?  I'm always amazed at how different my perspective is having learned to fly and basing a plane out of an airport at similar elevation - just under 4700'.  On this warm summer evening, the DA out of my home airport is currently 8000'.  It does change the way you look at things.

 

 

 


I haven't tried with A2A's p51d, because I got bored at the power it still exhibited at around 50k

 

J.C., I have to admit - I enjoy reading and appreciate your posts and the knowledge they contain, but I do sometimes shake my head.  I guess I sometimes struggle to understand your, well, your lens on the world if you will.  I don't quite understand how something becomes "boring" (and I'll admit that's one of my least favorite words in the English language, so keep that in mind) because of its behavior at the margins.  In a perfect sim world, everything would be "book" in all flight regiments, but in the world we have I value most those planes that behave properly under normal, expected conditions, and get the least concerned as situations move towards the edges of the bell curve.  It seems sometimes that you live and die in 3 sigma territory, and from my perspective, that way lies unhappiness.

 

Please take these comments in the spirit they're offered, and note again - I do enjoy your knowledgeable comments.

 

Scott

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Above all, the effects of the less denser air at that altitude are extremely well represented in DCS, as opposed to either FSX or XPX where you still get way to effective response from your control surfaces (it's like if you were cruising at 4000' ) In DCS there is a lot of lag, the controls feel sluggish, etc...

 

That's strange, since it's an area where the Blade Element Theory should shine, since it models natively the basic geometry of the airflow for the different aerodynamic parts, and hence natively account for the differences between low and high altitude, given the air density is correctly modeled (and it seems to be, in ISA conditions).

 

I know that in XPX, aircrafts are indeed more sluggish at high altitude than at low altitude, but I don't know if it's underdone, correct, or overdone. Actually I don't know how sluggish would a P51 feel at 40.000 feet. :)

 

EDIT: Should I ever make an aircraft add-on, I'd certainly ask you to do the flight model tester eheh. :)

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Where did you fly out of Larry? I'm always amazed at how different my perspective is having learned to fly and basing a plane out of an airport at similar elevation - just under 4700'. On this warm summer evening, the DA out of my home airport is currently 8000'. It does change the way you look at things.

U42, 10 miles south of KSLC (Salt Lake City). Lived next door.

Last evening it was 104 degrees. At the same time tonight, it's 69 F.

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Last evening it was 104 degrees. At the same time tonight, it's 69 F.

 

Still 91 here at 8:30 PM local (near KGXY). Send some of that 69 this way!

 

Scott

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@ttocs:

 

Hehe ttocs, good post :-) You are so right, although I chose that word - "bored" - late at night, before going to bed and completely out of inspiration :-)

 

And, please keep in mind my native language is Portuguese, and all the english I learned comes from high school time, more than 31 yrs ago :-/

 

I meant I was not happy with the kind of power I was getting out of it at such a high altitude... My pilot perished soon after, because A2A models at least hypoxia, and I had forgotten to fill my oxygen bottle :-/  Of course I don't know if the kind of performance I was getting isn't exactly the correct one! I'm exploring a territory where I was never in RL - aircraft with an engine, and such high altitudes...

 

@Murmur

 

Yes, X-Plane correctly calculates air density, but I still find the aircraft to have their control efficiency higher than I might expect at such an altitude

 

Finally, at another forum Andy posted exactly what I was about to test - operating from a high airfield in X-Plane10. He chose KLXV, at 10,000', and the engine worked poorly around 30'' until dying, so, I am satisfied :-)  Probably the windmill when we place the aircraft "by Map magic" at such altitudes and it starts to fall, explains the engine keeping alive (?)

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jcomm, on 12 Jul 2013 - 07:22 AM, said:

Yes, X-Plane correctly calculates air density, but I still find the aircraft to have their control efficiency higher than I might expect at such an altitude

I don't see why the controls should behave very differently for a given IAS, at different altitudes. Keep in mind true airspeed is higher at high altitude, and compensates for the lack of static pressure. Aviation uses IAS as primary speed indication because the aerodynamic effects are more or less constant for a given indicated airspeed. But I'm sure you know that.

 

Pascal

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And, please keep in mind my native language is Portuguese, and all the english I learned comes from high school time, more than 31 yrs ago :-/

 

I'll also keep in mind that your English is waaay better than my Portuguese! :-)  Sorry, as I freely admit, I overreact to that word.

 

Scott

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I don't see why the controls should behave very differently for a given IAS, at different altitudes. Keep in mind true airspeed is higher at high altitude, and compensates for the lack of static pressure. Aviation uses IAS as primary speed indication because the aerodynamic effects are more or less constant for a given indicated airspeed. But I'm sure you know that.

 

You're almost right...  regarding static pressure, but the problem has to do with dynamic pressure, as in q = 1/2 p V2 which is really the one playing the role when it comes to the lift generated by the control surfaces... and, since given a constant TAS ( the V in the expression for q ), and assuming an ISA atmosphere, the dynamic pressure will be greatest at sea level... you can easily see why flying higher, even if you keep your TAS (instead of IAS) constant, will turn your control surfaces less responsive ;-)

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@Pascal_LSGC:

 

Pascal, of course if the IAS / CAS remain constant, dynamic pressure will too.... Now I understand your point ;-)

 

And... it it remains constant, because TAS is actually increasing, and thus compensating for the decrease in density, then there should be no mushiness...

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I don't see why the controls should behave very differently for a given IAS, at different altitudes. Keep in mind true airspeed is higher at high altitude, and compensates for the lack of static pressure. Aviation uses IAS as primary speed indication because the aerodynamic effects are more or less constant for a given indicated airspeed. But I'm sure you know that.

 

Pascal

 

Actually the behaviour of the aircraft changes with density (and hence with altitude), even if you keep a given IAS. At first this may seem counter-intuitive, but let's consider, as an example, roll dynamics.

 

Suppose you're keeping a steady IAS and a steady roll rate, at either Sea Level or 40.000 ft. Now suppose you instantaneously center the ailerons. What happens?

 

 

 

As you can see from the image, the AoA the wing is seeing when you center the ailerons, depends on the ratio between the horizontal velocity of the aircraft and the relative velocity induced by the roll rate.

 

At 40.000 ft, for a given IAS, the actual horizontal velocity is doubled compared to sea level, so the AoA the wing is seeing is approximately (for low AoA's) halved.

 

So, when you center the ailerons at 40.000 ft, the wing is seeing the same IAS, but (about) half AoA. It's this AoA that dampens the roll rate, hence the damping effect on the roll rate is approximately half at 40.000 ft than at sea level. This explain why the aircraft feels more sluggish the higher the altitude.

 

I didn't focus on the effects of altitude on other aspects of flight dynamics (pitch, yaw, control response, etc.) but I anticipate the effect should be similar.

 

Marco

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Superb description Murmur!

 

The closest I found to this was the excellent explanation (also geometrical) of how sideslip forces tend to bring an aircraft to equilibrium, in my many times used "Principles of Flight" JAR-FCL manual :-)

 

Thank you very very much for your excellent explanation!!!!! Austin needs to contract you ;-)

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