OK...It's dumb question time...again

[engguy]

The manifold pressure gauge on my setup reads atmospheric when engine is not running. Meaning around 30" or so depending on what it is at the time.

[JDK]

Now, why do we use "indicated airspeed" instead of real airspeed?

What do you mean by "real airspeed"? Are you referring to ground speed?

Hey, I know this one! No, Bill's not referring to groundspeed.

[b29flteng]

Think of the MP gage as a throttle position indicator.

[Bill Walker]

Now, why do we use "indicated airspeed" instead of real airspeed?

What do you mean by "real airspeed"? Are you referring to ground speed?

Hey, I know this one! No, Bill's not referring to groundspeed.

You know, airspeed. The speed of the air. It's nothing like Indicated Airspeed.

Just trying to start another long, rambling technical discussion.

[Vessbot]

I think this thread's gone in enough tangents that that topic deserves a new one

[Bill Greenwood]

Indicated airspeed is what a pilot uses for handling the plane, because that is the airspeed that the wings of the airplane feels, in other words it is how much air is flowing over the wings so that determines stall speeds etc. Let's say I was going to fly an unfamiliar plane. How do I know what speed to use for landing approach? Maybe there is a pilots manual and it gives such speeds? First, it may not be detailed enough, also it may be based on a different plane, at a heavier combat weight with different equiptment, such as underwing racks. So the speeds given may be a little on the high side. Then you can asks another pilot experienced in type. If I was going to fly a P-38, I'd sure want to know what Steve Hinton had to say, or Howard for a Bearcat. It is possible that the other pilot may have a different style, though. Perhaps the best method is to have a ballpark figure in mind, but when you go up for the first flight, do a few stalls, both clean and gear, flaps down. This way not only gives you an accurate stall speed, for THAT AIRPLANE, THAT DAY, but also shows the pilot how the plane handles, what the warning is and the recovery mode. Lets say you find the VSO speed to be 62 knots, as my MK IX, then you can have normal approach at 1.3 or 81k or 1.2 or 76k for short field.

There is also a calibrated ASI, corrected for error in the instrument system usually a few knots, but the pilot doesn't see it in the plane, nor need it to fly.

The next is true airspeed, this means the IAS is corrected for non standard pressure, and temperature. Thus, on a hot day up high the true airspeed will be much higher than indicated. The pilot uses true airspeed, plus or minus tailwind or headwind, so that the result gives you your groundspeed. This is vital in flight planning, in finding what your range is and how long it will take to get there.

[engguy]

Think of the MP gage as a throttle position indicator.

I would say its more speed related. Remember you can have the highest attanable manifold pressure possible in an NA engine, when it is stopped with the throttle closed.
What gives you X manifold pressure in a climb with all settings at a certain constant, and then dive the manifold pressure will continue to drop as speed increases. Especially if there is an airfilter or bends and other restrictions that act as a choke point.
Also you can have a given throttle setting, and add or remove prop pitch and change the pressure. In that case what is changing is the engine speed.
Manifold pressure is a load indicator.

[Bill Greenwood]

I think of mp as a power indicator. For example for takeoff power I set 3000 rpm, full, and 7 lbs or 44 inches of manifold pressure. Climb would be 7 lbs and 2650 rpm, power climb 12 lbs or 54in and 2850 rpm. I use 0 boost and 1800 rpm for economy cruise. You could also say a throttle position indicator since throttle opening, along with rpm controls boost or mp.

[Mudge]

OK youse troops...have fun...you lost me about 3 pages ago.

Mudge the dense

[Bill Walker]

Indicated airspeed is what a pilot uses for handling the plane, because that is the airspeed that the wings of the airplane feels...

All good stuff Bill, but as I expected you fell into the old trap of thinking of IAS as speed. It is actually a measure of dynamic pressure, converted to the true speed that produces this dynamic pressure at sea level on a standard day. It is pressure (not speed) that keeps the airplane up, determines control effectiveness, etc., so it is a very useful tool for operating an aircraft. It is also dead easy to measure, unlike real speed through the air, so that is why the earliest speed indicators were pressure gauges calibrated in speed.

[Bill Greenwood]

Bill Waker, hey, if I wanted to get confused I'd sign up for a Katy Couric interview. You may be correct about pressure on the wings, not speed. But in order to fly, I as a pilot use the airspeed indicator and is seems to work, and it even worked in Canada. I would like to try a lift reserve, angle of attack indicator also.

[Vessbot]

so that is why the earliest speed indicators were pressure gauges calibrated in speed.

So are the current ones.

[Bill Walker]

so that is why the earliest speed indicators were pressure gauges calibrated in speed.

So are the current ones.

Unless it is a TAS indicator, or a GPS based ground speed indicator, or a digital display in a glass cockpit.

[Vessbot]

Unless it is a TAS indicator, or a GPS based ground speed indicator, or a digital display in a glass cockpit.

Fair 'nuff, but all that digital doodly ding dong just confuses me so I turn it off. Ignorance is bliss!

I think of mp as a power indicator.

I like to think of it as an indicator of not how much power you're necessarily getting, but how much power you're asking from the engine.

[sdennison]

OK youse troops...have fun...you lost me about 3 pages ago.

Mudge the dense

Hey Mudge, look at it this way. In a normally aspirated (non-turbo, non-supercharged) engine. the manifold pressure is just like a vacuum gauge on a 51 Chevy. If the throttle is closed, you have high vacuum, when wide open you have low vacuum. The highest MP you can ever have in a normally aspirated engine is "atmospheric" pressure wherever you are located. Barometric pressure, if you will.

In a turbo-charged engine, the exhaust gas pressure turns a turbine wheel that drives a compressor wheel that pumps more inlet air into the engine to make more power. Fuel has to be adjusted accordingly. This inlet pressure, manifold pressure is controlled by a waste gate that is simply a dump valve on the exhaust side that controls how fast the exhaust turbine spins, thus the intake compressor. The more exhaust that is dumped, the lower the intake pressure and visa versa. In the good old days of Indy, we used to qualify with 120" of manifold pressure in the Offy's. That's 45 pound per square inch of pressure in the intake manifold. Lot's o'power, not much reliability (4 laps hopefully)

Supercharged is much the same but is a mechanically driven pump, not exhaust driven.

These engines are the only ones that will actually have manifold "pressure" in the sense of greater than normal atmospheric pressure. The general premise being to maintain sea level performance at altitude.

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