Naturally Aspirated GA Airplanes -- High or Low?

[Hook]

Based on books I've read by real pilots, I don't think freezing rain is overdone. Ten minutes sounds about right from various descriptions and it varies in the sim. I think there are 5 levels of freezing conditions.

I mean, someone talking about flying a DC-2/DC-3 and the ice building up on the air intakes far enough that the back of the prop was shaving it. Anti-ice boots that may or may not work, and sometimes making things worse. The inability to climb at all so you couldn't get above the ice. A race to climb through freezing rain far enough to get on top before you can't climb any more.

Ice in FSX is no joke for those who have experienced it and it is just like real life descriptions. And some people think icing isn't even modeled.

I don't know if we're getting ice simply from humidity, but on my last flight I was flying near a cloud layer but not in it and was getting ice. I didn't see any actual clouds of any kind where I was.

Part of why this is on my mind currently is because I'm flying in icing conditions a lot and I finally was able to make a gauge that I've been wanting for years: an outside air temperature gauge with an ice indicator, which I am currently testing. I already had an ice indicator in another gauge but it wasn't quite as convenient. So I might fly though ice more than I normally would.

Hook

[AKar]

My problem with current icing models is finding killer icing in places where none should exist, and not having any when doing a suicide passes through stuff that should practically be made of supercooled water. When the existence is got right the effect is not, as in FSX (by default) we get this stupid weight gain kind of effect while in reality, the effects are noted to be highly non-linear. Often, even severe icing produces fairly modest aerodynamic effects up to the point, and then the situation may already be beyond recovery. Depending on the aerodynamics of the airplane in question, of course. This is one of the golden rules of icing: speed is the key. That is, get out of there fast when you notice the accumulation, not when you start to feel it like in the sim where the dials start to move down like on a leaking compressor as a telltale.

-Esa

[Oracle427]

Freezing rain can kill very very quickly and it is important consider that this is the sort of icing that can take down airliners. 10 minutes is an eternity in those conditions.

Freezing rain rapidly contributes to serious increases weight and drag to the order of 50%, while lift is lost thanks to the altered shape of the airfoil. You're accumulating big fat droplets of water on the airframe and airfoils and performance drops off very rapidly. Unfortunately, with freezing rain the warmer air is generally above. Freezing rain encounters in these GA aircraft are essentially emergencies requiring prompt action by the PIC.

There are, unfortunately, several examples of aircraft encountering freezing rain and being downed within a couple of minutes.

Example: https://www.ntsb.gov/_layouts/ntsb.avia ... 115&akey=1

Edit: Agree that FSX icing simulation is not very good or realistic at all.

Edit2: Vote for low, I generally prefer to only go as high as required for the conditions of the trip and best performance accounting for winds. I'll also trade performance and range to get out of turbulence by climbing if needed.

[Dogsbody55]

The nature of naturally aspirated GA planes is such that you would generally stay below 9,000 or 10,000ft AMSL at most. I usually fly at altitudes of 4,000ft to 7,000ft AMSL, depending on the height of the terrain, so I can indulge in some sight seeing. However, at present I an crossing the Rockies so I need to be considerably higher to avoid "solid clouds". I do like to fly around Europe too which also means higher altitudes so for me, an oxygen system is something I would use. I've been as high as 14,000ft with no hypoxia issues, but the lack of an oxygen system is always a limiting factor if you want to fly in a realistic manner. Flying along valleys in mountain ranges is fraught with danger. I often find the end of that valley contains a mountain that I can't climb over, or do a U turn either. Pucker factor!!

"Luckily" for us, airframe freezing is not well modeled in FSX or P3D, which means that it is common for users to see how high their shiny new addon plane will go. This is probably at the heart of questions like this among simmers. However, this is quite unrealistic in the real world, and it's great that A2A model hypoxia which puts a limit on such cavalier behaviour. The AI planes you see at 18,000 feet and higher are a joke. I'm sure that one day, A2A will release a GA prop that is OK at such altitudes, and it will have an oxygen system, but I think it would be something of a learning curve if we were able to coax our A2A GA planes up to their maximum altitude too.

As to the issue of fuel consumption, I usually don't care as flight sim is paying the bill I rarely get a chance to do a long distance flight, so as long as I have enough in my tanks to last the distance, this is not an issue. I generally set mixture for best power for this reason.


Cheers,
Mike

[Hook]

...in FSX (by default) we get this stupid weight gain kind of effect

Can you prove that? I can't. I'd really like to know myself.

To me the icing effect acts like a weight gain although the gross weight of your aircraft is not affected. I cannot state that the actual effect is not a reduction in lift and an increase in drag with little actual weight effect at all. I don't think anyone can. But I am not sure it matters.

What happens is that you start to notice your altitude dropping, so you add trim and power. Before long your nose is elevated well above normal and you are flying slower. The elevated AoA adds drag which slows you down and it is more difficult to climb and the lower speed causes the aerodynamics to change. Is this a weight gain or a reduction in lift and an increase in drag? I don't know.

Unless you are staring at a gauge that displays the value for (A:Structural ice pct, percent) then you don't know when icing starts or how severe it actually is.

As far as I'm concerned the effect is Close Enough especially as this is not a normal flight condition and when it starts getting bad it gets bad very quickly.

We may be able to create a "gauge" as of Prepar3D 4.4 to display actual ice accumulation on the wings.

The published ice gauge does odd things that affect how icing works in FSX. I decided not to use it as my own indicator worked better for my purposes and I preferred the stock ice effects.

Hook

[AKar]

Just some further words on icing and the FSX/P3D, even if somewhat a spinoff of the original topic idea.

To me, the most fundamental problem that still exists is that in reality, the ice accretion is generally pretty localized phenomenon, and FSX/P3D even with the latest and greatest weather engines do these local phenomena poorly. Because the clouds are not truly volumetric, you can't really see by looks where the 'edge' of the cloud is for instance (at least with my addons and settings), and end up going in and out of the clouds unpredictably. Further, the weather resolution is not good enough. This results in overdoing of the weather effects, like the thunderstorms are, in their overall effect on flying. And no, I don't mean that flying into a thunderstorm cell would not potentially kill you in reality, but in the sim flying in general area of such storm cell produces unflyable, severe turbulence and other effects specifically tailored to kill you - that is not right. The same can happen with icing if not simulated with care: if icing conditions are made equal to ice accretion in the simulator, which is not the case in reality, it will render unrealistically large areas essentially unflyable. Obviously, as of now, it is somewhat a question mark to me what triggers the icing in the FSX/P3D/Active Sky and what does not.

Moreover, as described, the effects from the icing themselves are wrong where they do exist. If speaking of reality, where this phenomenon of in-flight icing is quite a bit researched for obvious reasons, up to a point in-flight icing really does not reduce the lift significantly at cruise speeds. Instead, the maximum achievable lift coefficient is reduced, so the airplane stalls earlier, at smaller angle of attack.

• • • 
      Image source: Getting to grips with cold weather operations, Airbus Industrie

This is why in some occasions, the ice accretion compromising the aircraft's performance has gone unnoticed right until control has been lost, sometimes with fatal consequences. In FSX/P3D, the effect is this stupid 'weight gain' as I put it: "Oh I'm getting linearly heavier and draggier - I must be picking ice!" No, in reality, at relatively low CL's, the lift is not significantly affected at first - only the stall margin is. This is why airplanes are certified to fly in icing conditions where they pick up some ice on their leading edges and other protruding parts. Where necessary, this is taken into account with appropriately increased speed margins used after the airplane has picked some ice. Obviously, depending on the wing profile in particular, drag is increased some by in-flight icing as well, more so at high angles of attack. Laminar flow profiles are usually particularly affected. This increased drag may or may not be clearly noticeable, depending on the airplane in question and the severity of icing.

Of course, extreme and relatively rare stuff, such as freezing rain, where one should not fly in the first place, are another story. Likewise is on-ground icing that usually contaminates the upper surfaces of the airfoils - this is way more dangerous than simple in-flight icing is.

-Esa

[Oracle427]

Interesting point about the way that lift vs AoA curve is affected by icing. I guess it makes sense to a point. As you noted with ground ice on the upper surface of the airfoil the location and type of ice has a significant affect. I believe this is one of the many reasons why freezing rain aloft is so dangerous.

I also couldn't help but notice the shape of those airfoils and wonder when my Cessna will be reaching transonic speeds.

[AKar]

Interesting point about the way that lift vs AoA curve is affected by icing. I guess it makes sense to a point.

How Airbus (the source of the image) kind of puts it, the icing in this case messes the boundary layer provoking its easier separation and therefore makes the wing stall sooner, however, it does not affect the circulation much at some given angle of attack as long as the flow stays attached. This can be visualized quite easily, and links to why the lift remains rather unaffected.

An important consideration, by the way, is that if we extended flaps of critically iced wing - unslotted flaps in particular - we may effectively try to force more circulation or CL than can be produced by the compromised airfoil, thereby stalling it.

-Esa

[Oracle427]

Yes, I agree it makes sense under the circumstances presented.

[Hook]

When I first published my findings on FSX icing more than 10 years ago, I described it as acting as if it added weight to your aircraft, and even posted a formula to determine how much weight would be added. Of course, it didn't actually add any weight at all, but this was an easy way for people to visualize the effect and they could even test it by going into the loading screen and making their aircraft overweight to see what the effect would be. I can't be sure but I may have been the first to describe icing as having the same effect as adding weight, as at the time some people claimed icing was not modeled at all and I needed a way to illustrate the effect.

Some of that was later posted here as well.

There are at least two other ways to get approximately the same effect which you can test by editing the aircraft.cfg file and binding a keystroke to reload the user aircraft so you can see changes during a flight. There is another way as well which is already modeled in the sim.

The bottom line is, we don't know how FSX models icing, and it is likely a combination of factors. It is probably more sophisticated than people realize. Or it might be very simple, but does it really matter? No one knows.

One problem is that you generally don't know when icing starts or how bad it is. Leaving pitot heat off will give you an early indication, but doesn't tell you the severity. An early test I ran was to time how long it took the pitot tube to freeze to determine severity and I'd toggle the pitot heat on and off to check. Later I had a gauge I made to display the actual ice percent and my best guess as to the equivalent weight it seemed to add to the aircraft. I've spent a lot of time flying in FSX icing.

How many people here can describe the actual effect of icing in a GA aircraft from personal experience? If we can get a good description, we might be able to find a way to implement it. A theoretical description isn't good enough for this purpose.

And just how accurate does the sim have to be? FSX icing probably isn't any less accurate than FSX stall behavior. It just has to be good enough to teach you to avoid icing conditions. It is definitely something outside the normal flight envelope.

Hook

[Hook]

I just realized exactly what you posted. I had read it but hadn't put it together.

This is why in some occasions, the ice accretion compromising the aircraft's performance has gone unnoticed right until control has been lost, sometimes with fatal consequences.

This is an Airbus you're talking about here. For those who understand the Airbus and problems associated with its automation, this is sufficient information... just that it was an Airbus. I wouldn't be at all surprised that a problem like this would go unnoticed by an Airbus driver. Not every time, of course, but often enough to be remarkable, especially when it leads to a crash.

For those not familiar with Airbus, it is highly automated. You aren't really flying the plane, you are flying the automation. It is fly by wire, with a computer interpreting the pilot's instructions from a joystick and attempting to do what has been ordered. You can get into a situation called Alternate Law where the normal protections provided by the computer are not in effect, and this has lead to crashes. I did not get this from a video game, I read accounts by Airbus pilots. I'm not sure if anything about an Airbus is applicable to GA flying.

Just as an aside, is that a real airfoil? It almost looks like someone was trying to disprove Bernoulli. This is a serious question; I really don't know.

Hook

[AmazonChitlin]

If I'm IFR, I'll typically fly the MEA unless it's low and the winds/weather above are better. Sadly in the game, a lot of times you wont pick up a VOR, or you'll lose a VOR at the incorrect distances, so a little bit of imagination is required

VFR, best winds/weather.

Sent from my SM-N960U using Tapatalk

[Hook]

If I'm IFR, I'll typically fly the MEA unless it's low and the winds/weather above are better. Sadly in the game, a lot of times you wont pick up a VOR, or you'll lose a VOR at the incorrect distances, so a little bit of imagination is required

This brings up an interesting point about how high to fly.

VORs (and NDBs) in FSX have a fixed range, 195 nautical miles for most VORs, but not all. If you fly too low you can lose the signal especially if the terrain isn't flat. This probably gets interesting if you're on autopilot slaved to the VOR. Real life pilot accounts talk about flying dead reckoning until they pick up the signal again. If you are flying VOR you have to stay high enough to hold the signal.

This happened to me a couple of times on my last flights in Scandinavia. I wasn't using the autopilot.

Hook

[AKar]

I just realized exactly what you posted. I had read it but hadn't put it together.

This is why in some occasions, the ice accretion compromising the aircraft's performance has gone unnoticed right until control has been lost, sometimes with fatal consequences.

This is an Airbus you're talking about here. For those who understand the Airbus and problems associated with its automation, this is sufficient information... just that it was an Airbus. I wouldn't be at all surprised that a problem like this would go unnoticed by an Airbus driver. Not every time, of course, but often enough to be remarkable, especially when it leads to a crash.

For those not familiar with Airbus, it is highly automated. You aren't really flying the plane, you are flying the automation. It is fly by wire, with a computer interpreting the pilot's instructions from a joystick and attempting to do what has been ordered. You can get into a situation called Alternate Law where the normal protections provided by the computer are not in effect, and this has lead to crashes. I did not get this from a video game, I read accounts by Airbus pilots. I'm not sure if anything about an Airbus is applicable to GA flying.

No. I can't think of single accident or serious LOC incident related to structural icing in an Airbus aircraft. There could have been some, but I don't recall any. What I was referring into were completely different airplane types, mostly turboprops that tend to collect most of the icing hazards due to way they are operated, and their related accident/incident accounts. In cases, the airplane has been flown with autopilot, against manufacturer's (current) recommendations. That effectively hides the deteriorating handling and this is why manual flight is recommended under serious icing encounters.

Airbus is yet another airplane, there is nothing mysterious about it. It has wings and traditional control surfaces and it flies by common aerodynamic principles. They are hardly alone in that their handling involves automated control principles. The only way Airbus relates here is that their booklet had the nicest picture of ones I quickly had at hand.

Just as an aside, is that a real airfoil? It almost looks like someone was trying to disprove Bernoulli. This is a serious question; I really don't know.

It is a rough look-alike drawing of a classical supercritical airfoil shape used in majority of modern airliners. Somewhat similar ones are used in some gliders, btw, in portions of their wings but for different reasons.

How many people here can describe the actual effect of icing in a GA aircraft from personal experience? If we can get a good description, we might be able to find a way to implement it. A theoretical description isn't good enough for this purpose.

Theoretical description is based on experimental data. Also, there are numerous icing encounters of various levels of accretion in various GA types - even in gliders. Only the most serious ones that have more dramatic consequences get all the attention. When properly dealt with, and flying airplanes properly equipped to the conditions, encounters with light, or moderate icing even, are routine with no drama at all. I haven't spend much time in the sky at all compared to many folks, but even I have experience of light-to-moderate icing when taking rides in airplanes that would be considered general aviation.

It just has to be good enough to teach you to avoid icing conditions.

Except that it isn't! I can't find any appreciable icing in conditions in which flying was simply asking for it. Though I do prefer 'not at all' over 'done badly', so in that sense I don't mind it missing that much even if it would be nice to have done properly. Meeting some icing conditions every now and then is all but non-avoidable in big part of the world in IFR flying. That's why it would be nice to have it done right some time in the future.

VORs (and NDBs) in FSX have a fixed range, 195 nautical miles for most VORs, but not all.

This can be changed, and is affected by sim navdata updates. Some of such do shorten the reception ranges of the navaids somewhat. I find terrain masking model surprisingly ok, though it is rather hard on/off.

This probably gets interesting if you're on autopilot slaved to the VOR.

I've not checked if it is a certification requirement (I suppose it probably is), but autopilot should not do anything too wild when coupled to a nav source going invalid. VOR signals are sometimes prone to "scalloping" and other nuisance errors, so the autopilots should be relatively smooth in tracking anyway.

-Esa

[Hook]

Good post.

What we need to be able to model it is descriptions from GA pilots who have experienced icing to any degree while actually flying the aircraft. If you don't see actual ice, for example at night, how do you know you're getting airframe icing? What are the symptoms? From the most minor to the severe but survivable. What is the very first thing you notice, not from pitot icing or carb icing, but airframe icing. What happens as it gets worse. It would help if they were here to answer questions.

There is something about FSX that you can get ice when you are not expecting it, and you might not get it when you are expecting it. Icing from clouds builds up slowly and you might never notice it before it dissipates. You might not even notice in freezing rain if you fly through it quickly. Unless you are watching the actual variable used by the sim to calculate ice effects, it is going to seem very random.

Up to about 5% ice is difficult to detect. You notice somewhere between 5% and 10% and at 10% it is obvious. By 15% you know you are in trouble.

I've got a couple of recent screen shots taken to document icing in freezing rain. At 1% my KTAS was 115 and my angle of attack was 1.4 degrees. At 15% my KTAS was 89 and my AoA was 4.6 degrees. Pitch went from 1.0 degrees to 5.0 degrees. Power was unchanged. I do not know to what extent prop icing was involved. I can't even say that the symptoms were linear as this happened over a period of 13 minutes and I don't think anyone is quite that perceptive to be able to tell. With 5 levels of icing severity it won't be linear anyway. I cannot guarantee that I was getting freezing rain the entire time and I probably wasn't. I was not on autopilot.

So what was happening? Drag, obviously. Lift, maybe. Weight, none at all. Power, who knows. I did not try testing stall speed, my control surfaces did not freeze, and I didn't have a chance to test if my flaps could freeze in a down position which makes attempting a landing in freezing rain interesting as this can happen in real life, don't know about the sim. My windscreen did not obscure. Ask A2A if they model any of this in their AccuSim aircraft.

I have been saying all along that people who don't think ice is modeled in FSX haven't experienced it. If you want to see what ice is really like, go find some freezing rain. You might get some in clouds, but you may not be able to tell before it dissipates. You will know for sure after a few minutes in freezing rain. Stall speed may not matter as there will come a point where you can't maintain it anyway and I got close to that once long ago in the Goose. I got the plane on the runway, then spent 2.5 hours in time compression waiting for the ice to dissipate to zero (I was curious).

Is the ice model mathematically precise? No more so than the rest of FSX, and probably no less so either. And it doesn't have to be. You only have to sweat a landing once to gain a healthy respect for even small amounts of ice.

Hook