View Full Version : IAS/TAS

03-26-2005, 09:39 AM
What is the IAS and what teh TAS

ohhh, and what is the prop. feathure option????(From the controls menu).

03-26-2005, 09:59 AM
IAS is Indicated Air Speed and that is what is indicated on the airspeed indicator in the cockpit. IAS varies depending upon air density, so at higher altitude your IAS is lower than at sea level even though you are actually covering the same linear distance at identical times.

TAS is True Air Speed. This is simply distance travelled divided by time, regardless of altitude.

Prop feathering allows you to turn the prop blades so that they are parallel with the direction of flight. This allows a damaged engine to be completely stopped and the prop will not spin wildly (called windmilling) and possibly break away from the engine and become an aircraft shredder.

03-26-2005, 10:02 AM
IAS=indacated air speed.
TAS= True air speed.

Feather the prop= That is when you disconect the prop from the motor and it just free wheels in the wind. It is not pulling or anything. Just spinning in the wind.

03-26-2005, 11:25 AM
Correct me if I'm wrong but a feathered prop actually changes the direction of the blades for least amount of wind resistance.

03-26-2005, 11:54 AM
It does. It turns the blades parallel to the flight direction so that the air rushing past them will not cause the prop to spin. If an engine dies and the prop can't be feathered it will spin very fast and can break the crank and go on a flight of it's own.

03-26-2005, 12:04 PM
In the 27 years of flying that I have done, I have had to feather the prop a few times. Not the most fun thing to do but it happens. And yes it does turn the blade into the wind, but the prop still spins. Although some what slowly at times.
You can still flatten out the pitch on the prop if you want to use it to slow down. Kinda like a wind break. That is on a single engine type, not the twin like I fly now.
Yes you can snap a crank shaft by letting the prop runaway, but that is rare.
Also I have had Ice build up on my prop to the point that it will cause a runaway. Now you say why did you not deice the prop?
Well I did , but you can get icing to the point and so fast that you cannot get it off the aircraft!
I have landed at runways and while taxing to the ramp had large sheets of ice slide off the wing and wham , smash onto the runway. Now that will make you wonder how you made it home.

And is also answers the question why the plane flew like it had a ton of bricks in it! http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

03-26-2005, 12:51 PM
Ice formation has brought down many a plane throughout aviation history.

I don't think flat or reverse pitch props were common during World War II, at least not on U.S. aircraft.

03-26-2005, 12:57 PM
A properly adjusted prop shouldn't windmill when feathered (I'm a mechanic; I can spell, but still.), although a slight movement wouldn't be detrimental. The classic runaway props are powered props at too low of a pitch (too high an RPM) due to a loss of prop control. Shut down engines with windmilling props are rarely more than airbrakes, although total loss of oil can create other problems and variations.


And . . . (to bring up one of MY old subjects) . . . the 109 and 190 (all variants and unusual for single engine aircraft) had full feather props. http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif

03-26-2005, 03:11 PM
Indicated airspeed is what the dial on the gauge reads.

True airspeed is the actual speed air is flowing around the aircraft.

I'll let someone else explain why they are different its hard to explain without drawing a diagram but just know that as you increase in altitude your IAS decreases and TAS increases. The rate at which this happens depends upon OAT, pressure and Rate of climb.

You can change the pitch of the propellor inorder to compensate for the change speed of of the aircraft not necessarily altitude. If you think of the prop as a spinning wing its simple. The faster you go the less bite prop is getting of the air. If you can increase the pitch of the prop with an increase in airspeed the prop is not only more fuel efficient but effectively uses more of the power the engine is generating.

For IL2 what you need to know is pretty much to increase the prop with altitude. Takeoff with 100% (the prop is at low angle of attack to the air). For each 500 meters increase the angle of attack 5%. Basically as you get higher that true airspeed is increasing because the ambient air is less dense at altitude. Watch your RPMS because you can easily overspeed the engine...

03-26-2005, 03:18 PM
Yes on the prop, But over the years I have seen them windmill at a very slow rate. No mater how it was setup.
And another cause for the windmill at a slow rate as I was told my a Mech was that the Ice tends to stick to the nicks in the blade, and that puts the blade off ballance.
I just fly them not fix them. And that is what I have seen over the years.
I'm sure that there are many other things that can affect this and does.
Like the last ntsb investigation on a crash in the mid west, a engine exploded on takeoff and the reasion was Ice on one blade of a 3 blade prop. Not cleaned off very well.
This caused the engine to exsplode and kill 5 people.
Now I would think that a pilot would be able to tell that he was getting that much vibration out of a engine. But maybe not. Or in time.