Ahh, could be an error on my part.  Standard Cessna fuel tanks hold 21 gallons each, so 42 gallons total.  I'm not sure if that's with or without the unusable amount.  My data is for the 172 equipped with long range tanks (the 54 gallon tanks), and I'm not sure which one FS models.  It sounds like FS only has the standard tanks?

Also, with the airspeed, an important thing to consider is the different types of airspeed:

Indicated Airspeed - this is the airspeed that is read directly off the airspeed indicator from the pitot-static system.

Calibrated Airspeed -  this is indicated airspeed corrected for position, instrument, and installation errors.  There is a table of calibrated airspeeds for different indicated airspeeds in the 172 POH.  I didn't include this in the performance data file, but I can scan it and add it this weekend.  It usually doesn't vary by much more than a couple knots.

True Airspeed - this is calibrated airspeed corrected for non-standard temperature and altitude.  This usually takes a little math to figure out, but it can usually be estimated relatively accurately with a TAS calculator, like this one:  http://www.paragonair.com/public/aircraft/calc_TAS.html

So, when you say you are supposed to be flying at 126 kias, but you can only go 108 kias, you're reading your indicated airspeed off the AI, but your true airspeed is somewhere around 125 kts (using the handy calculator, 6000 ft altitude, 28.95 altimeter setting, and 80°F OAT).

This is because as you increase in altitude, the air is less dense, so there are fewer molecules of air going into the pitot tube.  What the airpseed indicator does is it compares the dynamic pressure (proportional to the air density and velocity) measured by the pitot tube to the static pressure (ambient pressure) measured by the static ports.  So, by decreasing the density of the air, the dynamic pressure decreases, which will show up as a decrease in airspeed on the AI, while the actual air velocity stays the same - if you can follow that, because I barely can.

If that didn't make sense, I'll try to clear it up.

And for the fuel in the MSFS C172...  Here's the paragraph from aircraft.cfg file:


***************
[fuel]
LeftMain  = -3.9, -2.1, 2.5, 26.5, 0.0       //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightMain = -3.9,  2.1, 2.5, 26.5, 0.0       //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
fuel_type = 1                                //Fuel type: 1 = Avgas, 2 = JetA
number_of_tank_selectors = 1
electric_pump=1
engine_driven_pump=0
**************

It shows tanks that hold 26.5 gallons each, with all of it usable (the '0.0' after each '26.5' is the unusable amount). I'm going to research the actual unusable amount, and change mine accordingly.

** ALWAYS make a backup of these files before editing them***

This is great discussion...

As far as icing and a C172 goes (especially in a Gulf climate). It's not really a concern. Before anyone jumps on me, let me elaborate. Pitot ice, and airframe ice don't form under the same conditions as carb ice.

Your concern about realtive humidity and cooler temperatures (even well above freezing), show that you're DEFINATELY getting the pilot's frame of mind working (the whole reason I started these threads)


Carb ice is a product of the super-cooling that happens when you compress and then decompress a gas (even plain old air). It's the same theory behind air-conditioning and refrigeration (except the gas lends itself better to the effect). When air enters a carb venturi, it gets accelerated and hence compressed, because the same volume of air is forced through the carb and then it gets decompressed as it enters the realtive lower pressure of the intake manifold. Remember the last time you blew dust out of your computer with a can of compressed air ? As you DEcompressed the air, the can got very cold, very fast. That's the same phenomenon that causes carb ice.. and why carb heat is needed ESPECIALLY when you reduce power... because you're increasing the compress/decompress ratio as the carb's throat gets restricted by the throttle "butterfly".

In order for airframe ice to occur, the actual outside air temperature has to be below freezing.. AND the realtive humidity must be high enough that water vapor is visible. You won't pick up airframe ice flying in clear, mist/cloud-free air.. no matter how cold it is.

Pitot ice is a little different.. Kind of in between, because there can be a venturi effect .. but it's almost insignificant. Heck, if you wanna split physics hairs, there's a compress/decompress effect created by an airfoil, as it moves through the air. Again though... far from a concern for C172 pilots.

The main use for pitot heat in small, GA planes is to avoid that surprise pitot/static failure. And why it's always used when flying by instrument. Scary, creepy things can happen when pitot tubes and static ports ice over. And they'll likely ice over before you can get a sense of any airframe ice forming.

Consider this:  You're flying along on instruments, and one of the key indicators during your scan that you might be climbing or descending, is a change in airspeed... right ?  If you're airspeed is increasing, you're likely descending (and visa-versa). Since the airspeed indicator gauges speed by the difference between air pressure at the static port, and air pressure being created in the pitot tube by the air being forced "into" it, a freezing over of the pitot tube (and it's drain hole) will lock the current pressure into the tube... effectively locking the airspeed indicator on whatever reading it had at the time. Here's where it gets spooky....

Since the static port will likely ice over AFTER the pitot tube (if at all)... a decrease in altitude will give a higher static pressure, and since the pitot tube's pressure is now constant ('cause of the ice), the airspeed indicator will now see a realtive DECREASE in the difference between the two... SooOOoooOOooo.. as you inadvertantly descend, your scan will see a DECREASE in airspeed .. and just as bad... if you inadvertantly climb, the airspeed indicator will show you an INCREASE in airspeed. NOT the kind of confusion you need, when you can't see the wing-tips...

As for the economy of GA flight... you're getting a good idea of why people who travel crave faster planes with higher payloads.. The break-even point for a no-stop flight in a C172 is about 100nm... and you can pretty much carry that ratio out. If you need to re-fuel sooner than traveling 100nm, you're better off driving.

However... if it's just you and a buddy and you can load the tanks full. You're looking at 400+nm legs, and flying becomes rather attractive.  


Edit:  The MSFS C172 is fuel injected.. so carb-ice is not a concern...

I've never experienced carb ice. There was one time however, when I went to add a little power on final, and the engine wasn't as responsive as I expected. I haven't landed a carb-equipped airplane since, without carb-heat applied, no matter what the temperature/humidity.  

Picking up carb-ice in level, constant power-setting flight would be pretty un-nerving.

I  HAVE experienced airframe ice. During my instrument training (on a genuine, IMC day), we ended up having to hold while a pilot at a small airport (where we intended to shoot an approach) who had got his clearance by phone, was taking off. Rather than diverting for some IMC navigation.. my instructor decided that entering and flying a hold in true IMC was good experience (good debate/test of the "double the correction vs triple it"). We ended up spending nearly 30 minutes in that hold, and my instructor tapped me on the shoulder pointed to the left wing (we were in a Cardinal... you can see the leading edge from inside the cockpit).. and SURE enough..  I C E was forming around the opening for cockpit air    He said that in another 10 minutes or so, we'd have to descend to thaw out. We were cleared for the approach right after that... the ice vanished quickly as we descended... and what was planned as a low approach, ended up being a full-stop. I neede to get out of that airplane for a bit...

I'm glad you're enjoying them 

VOR navigation is fun stuff.. and represented very well in MSFS.

I'm gearing up for Sim Instrument Ground School... I just want to take the time to do it properly...

And just for good measure:

Celcius to Fahrenheit     = 1.8 x Celcius + 32.
Fahrenheit to Celcius     = Fahrenheit -32 / 1.8

markag wrote on Mar 25^th, 2008 at 12:22am:

That means the wind is blowing from 260°.  So, you would want to land on runway 26 or the runway closest to 26 (runways numbered more than 17 or less than 35).  Feel free to ask any more questions you have.