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High or Low?
High. I like to play jet in my lawnmower. 19%  19%  [ 5 ]
Low. I like to cut grass and smash bugs. 81%  81%  [ 22 ]
Total votes : 27
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PostPosted: Wed Dec 26, 2018 8:34 am 
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Happy holiday season to all the participants of the best flight sim community on the internet! :)

Quite often a question pops up, asking after oxygen add-on to our fleet of high performance GA airplanes. This is sometimes in reference to their seemingly high ceilings, which are well above 10000 ft. I for one find such heights redundant in our current GA fleet, while others want to bring the thing high up there. It is simply a matter of our simming preferences. But this brings another question: what are the benefits and downsides of getting up there? What is the tradeoff?

The idea is very well demonstrated in the A2A Comanche's manual, in particular on page 66 from where the following image is adopted. Look it carefully and you get the idea. Note that this is clearly a calculated chart, not raw flight test results.

A2A Comanche:

        Image

What are the takeaways of this image? First, a naturally aspirated airplane reaches its top speed at sea level. This is a general truth, though the situation can be further complicated by things such as fixed pitch propeller, running into redline, etc. Second, if we held a constant power by carefully adjusting the settings, we increase our true airspeed with altitude almost linearly until we push our throttle to its stop.

You do find a similar graph in Bonanza's manual, it only does not plot the full throttle graphs down to the sea level. I picked this from Comanche's book because it illustrates the principle better.

How would an airplane with a turbo do? Well, if we take a turbo-normalizer as an example (it only takes a little imagination to figure out how one with some boost would do), it would tilt the full power graph to have a similar linear segment from its current sea level point all the way up to turbo's critical altitude. Thereafter it would plot a similar full throttle curve the naturally aspirated engine does. It would only happen at much higher altitude - and with massive increase in TAS with even relatively low critical altitude. Now, this is not strictly what happens because we are not usually holding precisely constant 100 % power up until critical altitude, but we shall discuss why if it sometimes comes relevant to our A2A GA fleet.

If, for any reason, we wanted to keep our cruise power setting at some given percent of maximum continuous power, our optimum altitude in terms of speed performance would be the altitude where the 'knee' occurs in the graph. It is not coincidence that this happens to be the full throttle altitude for that percent of power. For instance, using the graph above, doing 65 % of MCP, we would find our optimal airspeed at about 9000 ft. If we wanted to do faster, we'd come lower while keeping wide open throttle.

The speed is one thing, but what about economy? Let us pick one image from the Bonanzas manual, because in this case, it illustrates the principle better! (Please don't mind the red arrows illustrating the manual's example problem.)

A2A Bonanza:

        Image

It may be obvious that the range must increase linearly with altitude at constant power because our TAS did as well. However, what is different is what happens when the full throttle is used. That is, what happens after we reach the 'knee' in the graph. Unlike air speed which started to decrease, our economy will increase, and fairly dramatically so.

It takes a little bit of imagination to extrapolate the full throttle lines at various RPMs. They won't keep on rising eternally, but at some point fold together and reach their maximum at fairly close to the absolute ceiling of the airplane but not quite.

This means that if you want to reach your absolute maximum range, or reach your destination with very minimum amount of fuel used in cruise flight, flying high is preferred.



I prefer flying my A2A GAs at around altitudes where the 'knees' of the cruise power settings of around 75...65 % of MCP are, because this is the altitude range that gives me the most flexibility in going fast with wide open throttle, high RPM and proper mixture if I prefer, or doing slightly lower RPM and lean mixture if I want reasonable economy. I never fly such long sessions so that absolute maximum range would become a significant factor. Of course, other factors such as weather probably play the biggest part. I enjoy going under the clouds in between the peaks if those are around, smashing bugs and treetops. I get more from the scenery and it allows me more activity than following some straight lines.

So, I prefer relatively Low, that is, mostly under 10000 ft. :) I feel the naturally aspirated GAs are most home at there.

What is your preference, and why?

-Esa


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PostPosted: Wed Dec 26, 2018 9:03 am 
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See, this is hard for me to answer. What I do depends a lot on what I'm trying to accomplish. If I'm just on a short 1-2 hour hop, I'll stay low. But if I'm trying to go long, I'll often see what the winds are and go wherever I'm going to get the best trade-off. It also depends WHERE I'm flying. If I'm flying in a non-mountainous area, I tend to stay under 10,000, but if I'm heading across the Rockies, I obviously go significantly higher.

I just try to fly like I would if it were real life - the right altitude for the flight at hand. If I need to get above 10K, then I do. If not, then I don't.

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PostPosted: Wed Dec 26, 2018 9:23 am 
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At the moment none of my A2A aircraft have oxygen. I did have to wonder about hitting 13000 feet for a while in the Cherokee to get through a mountain pass near Quito whether I'd need it though. :D Is hypoxia modeled in the Cherokee?

Typically I'm flying some bush plane and I prefer to fly between 3000 and 5000 feet, sometimes higher or lower to avoid clouds... because icing IS modeled. :D The DC-3 flies around 8000 to 12000 feet, whatever it takes to get above a cloud layer. Most of the time in mountains I'm flying below the peaks through a pass. On my last flight I had to fly somewhat lower to avoid headwinds, down to 1500 AGL at times.

If I had a GA airplane with oxygen, the only reason I'd get high enough to need it is to fly over weather or through a high mountain pass or possibly to take advantage of a tailwind or avoid a headwind.

Hook


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PostPosted: Wed Dec 26, 2018 11:16 am 
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Good points so far. :)

Of course, there are routes that simply force you to fly relatively high, but surprisingly few don't have a "low" option available, at least if speaking of the Rockies, the Alps etc. that I've done in the sim. South America I'm so far fairly unfamiliar with.

If I'm more specific, what I'm asking is that if you had a choice to fly this route from Spokane to Kalispell either like this (routed via Mullan Pass so that distance would not be a factor either ;)):

Image

...or like this:

Image

...would you have a clear preference in which flight you'd take with your A2A general aviation bird? :) Say for example, if another one took you under an overcast layer and the other above it.

-Esa


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PostPosted: Wed Dec 26, 2018 12:09 pm 
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Personally, I'd go above. Especially in terrain like that. But you're also looking at not needing to go above 11,500 on the "high" option, so that's not above what you can do without oxygen anyway. If there's any sort of weather, I'm going above it if at all possible when in mountainous terrain. I'm not becoming one of those guys who ends up having cumulus or stratus becoming cumulogranite.

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PostPosted: Wed Dec 26, 2018 12:17 pm 
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If the wind is with you go high. If not, go low.

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PostPosted: Wed Dec 26, 2018 12:31 pm 
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CAPFlyer wrote:
See, this is hard for me to answer. What I do depends a lot on what I'm trying to accomplish. If I'm just on a short 1-2 hour hop, I'll stay low. But if I'm trying to go long, I'll often see what the winds are and go wherever I'm going to get the best trade-off. It also depends WHERE I'm flying. If I'm flying in a non-mountainous area, I tend to stay under 10,000, but if I'm heading across the Rockies, I obviously go significantly higher.

I just try to fly like I would if it were real life - the right altitude for the flight at hand. If I need to get above 10K, then I do. If not, then I don't.


I agree. What we did in real life was to play the distance against the winds aloft against the time - to- climb vs the cost of avgas at the moment then pick an altitude.
Dudley Henriques


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PostPosted: Wed Dec 26, 2018 12:36 pm 
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CAPFlyer wrote:
If there's any sort of weather, I'm going above it if at all possible when in mountainous terrain. I'm not becoming one of those guys who ends up having cumulus or stratus becoming cumulogranite.
I love it, BTW, when the weather gets me off guard. I always check the weather if valleys are doable, but every now and then something unexpected happens. It makes really nice challenge to avoid the cheat of 'P'-key and figure a way out. I've never crashed an A2A bird while doing that, but on just few occasions it has been more luck than anything else, when pushing on when knowing that turning around would be the only sensible thing to do.

Obviously, I'm discussing purely the simulator flying here. In reality, 'Low' kind of flying in mountainous terrains seems to carry fairly high mishap risk with it, even if most folks doing it likely know their stuff.

-Esa


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PostPosted: Wed Dec 26, 2018 12:43 pm 
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AKar wrote:
CAPFlyer wrote:
If there's any sort of weather, I'm going above it if at all possible when in mountainous terrain. I'm not becoming one of those guys who ends up having cumulus or stratus becoming cumulogranite.
I love it, BTW, when the weather gets me off guard. I always check the weather if valleys are doable, but every now and then something unexpected happens. It makes really nice challenge to avoid the cheat of 'P'-key and figure a way out. I've never crashed an A2A bird while doing that, but on just few occasions it has been more luck than anything else, when pushing on when knowing that turning around would be the only sensible thing to do.

Obviously, I'm discussing purely the simulator flying here. In reality, 'Low' kind of flying in mountainous terrains seems to carry fairly high mishap risk with it, even if most folks doing it likely know their stuff.

-Esa


For the sim I use our A2A planes EXACTLY as I would if they were real. That's the only way I can get a handle on how good or off base is our accuracy. So for me, each sim flight is a "real flight".
Dudley Henriques


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PostPosted: Wed Dec 26, 2018 12:50 pm 
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I totally agree with the earlier posters, regarding high or low altitude flight profile. Depends upon parameters such as: time of flight, terrain, weather, potential icing, etc. A few years back I rode my BMW R1200GS Adventure up to the top of Pike's Peak (14,115 feet). I did not enjoy the thin air at the top one bit! I didn't black out, but the combination of high temperature, high altitude, and major traffic on a Sunday afternoon totally blew the experience, and the heinous traffic on the way up sure didn't do my clutch any good. In the sim, we had to briefly pop up to 15,500 or so while crossing over a mountain range on our December 16th MP flight (https://a2asimulations.com/forum/viewtopic.php?f=79&t=66838). We all managed to get our Doctor Killers over the high ridge and safely to our destination, all except for one guy, who I'll call Icarus, who insisted on cruising at 16,500.

If I want to fly really high, I'll take the Civ Mustang.

Also, the fact that we are discussing this topic at all illustrates how realistic and immersive our flying experiences are today. Thank you, A2A!! (Also ORBX, Active Sky, …)

Seeya
ATB

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PostPosted: Wed Dec 26, 2018 1:15 pm 
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I think this time of year you fly at whatever altitude you need to in order to avoid ice. I assume none of my aircraft are equipped with oxygen.

Hook


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PostPosted: Wed Dec 26, 2018 1:22 pm 
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Paughco wrote:
In the sim, we had to briefly pop up to 15,500 or so while crossing over a mountain range on our December 16th MP flight (https://a2asimulations.com/forum/viewto ... 79&t=66838).
That route seems fun, need to try it some time. :)

DHenriquesA2A wrote:
For the sim I use our A2A planes EXACTLY as I would if they were real. That's the only way I can get a handle on how good or off base is our accuracy. So for me, each sim flight is a "real flight".
It would actually make an interesting topic of its own, how we use our simulators in comparison to how we would or do behave in reality. I'd expect we'd get almost as many variations as we do have simmers. :)

Personally, I use simulator "technically and tactically" how I'd expect things to be done in reality in given circumstances, but "strategically", I very often do flights that would make little sense to me in reality - or ones you couldn't even get me onboard in reality without fairly compelling reasons! I like it that I don't need to have a realistic reason to fly an Airbus to a 1500 m runway on a hillside, in the sim I simply can do that if I find the place interesting - but when I do, I take the challenge in serious way in that sense that I check the conditions, that the plane can stop and can get away from there in those conditions, plan my alternates etc... all the usual stuff. And if it wasn't plausible to achieve, I wouldn't attempt.

For me, it is an important part of the magic to be able to do... consider it like, unrealistic stuff in a realistic way, if you will. :)

Hook wrote:
I think this time of year you fly at whatever altitude you need to in order to avoid ice.
On the day we get Accu-Icing, I'm certain the game of chess is on the next level! Personally, I see non-FIKI GA pistons mostly VFR airplanes for the winter period.

-Esa


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PostPosted: Wed Dec 26, 2018 2:26 pm 
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Currently, icing in clouds isn't too much of a problem. You just don't want to fly in it very long. I haven't hit severe cloud icing in a long time.

Freezing rain is modeled with a vengeance. You might get 10 minutes of flight. You probably won't get 20. You only have to find yourself in freezing rain once to decide to avoid all icing conditions at all times.

As for real life flight, how much do you trust your anti-icing equipment? :)

Hook


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PostPosted: Wed Dec 26, 2018 2:52 pm 
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Hook wrote:
As for real life flight, how much do you trust your anti-icing equipment? :)
I'm not an active flyer, but from tech's point of view, in GA...not much! :mrgreen: In some specific circumstances, I even find them counterproductive (as they are sometimes poorly maintained). On the other hand, with pilots knowing their stuff and utilizing their gear to the extent it is worth it, why not. Around here, they sometimes combine ATRs with freezing fog with no drama at all. The photo don't show clearly the amount of accumulation on the tip of the radome, as it fell out of the flash pattern, but you may just get an impression. On the well swept-back areas it is still over a centimeter thick in places, and solid ice all over. Yet, critical areas had only some moderate traces of mostly shed ice where ice protected, and the tops and the bottoms of the surfaces were not too bad.

Image

In the sim, it is regularly either overdone or overlooked.

-Esa


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PostPosted: Wed Dec 26, 2018 3:03 pm 
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I try to fly between 4500' and 9500' depending on the terrain and winds. If the wind is from the tail I fly high if it is stronger. If there's headwind I fly lower if the wind is weaker (terrain permiting). I avoid flying above 10k and I try to use mountain passes if available.

In real life I use the same, but in my area de highest mountain is 8000' so no need to go above 10k, and there's passes at 6500'.


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