I did not know that flutter depends not on indicated airspeed, or "q", but rather on true airspeed.

In case anyone is interested, a six page article on why it's bad to put too big an engine on the RV-9/10:

http://www.vansaircraft.com/pdf/hp_limts.pdf

BTW, I "borrowed" my signature from the article.

airspeed indicators are not pressure compensated for ambient pressure?
Better invent one ---

airspeed indicators do conpensate for ambiant pressure using the static port. The difference in true and indicated airspeed is caused by the air density.
Ben

Not sure I understand your question. Airspeed indicators measure the difference between ambient pressure and dynamic pressure.

Well , I just reviewed some pretty informative sites regarding error on instrument readings and actual ambient conditions . Looks like there are a number of compensating and correction setups. Temperature etc.. I can see how the device is sometimes not showing what we think it is reading. One site was " pitot- static systems and instruments".
I should not have said "pressure compensated" tho effectively that is what happens because if you change temp the pressure changes . A "true airspeed" system which is accurate in all conditions would be a nice bit of work.

An airspeed indicator works so well because it directly measures the difference between static and dynamic pressure. It uses this data to DISPLAY a measure of airspeed. Nonstandard temperature does not change the indicated airspeed used by the pilot for stall speed, takeoff speed, etc. Altitude causes changes in indicated airspeed for best rate and best angle of climb speeds, however. Best rate of climb speed and best angle of climb speed converge at the absolute ceiling.

True airspeed can be found easily by compensating for nonstandard pressure (temperature & sometimes humidity, if you really want to get picky), but true airspeed is generally only used to determine groundspeed, and also, as a flutter speed limit, as my original post points out. Something I hadn't thought about but it makes sense now that I have.

The difference between indicated airpseed and true airspeed, which is what you were reading about is not important for determining aircraft limit and performance speeds, except the aforementioned flutter speed limits that are factored into the design but usually not published.

So, despite the differences between indicated airpseed and true airspeed in nonstandard conditions, indicated airspeed is the single best and most reliable guage to use to show stall speed, best rate and angle of climb speed, gear and flap speed limits, etc., since dynamic pressure, not true airspeed alone, determines those speeds.

It would be useless to try to rely on true airspeed alone to determine the above speeds in all but standard atmospheric conditions. In fact, it's theoretically possible that the airspeed indicator could have been marked in pressure units instead of translated to speed units the way it currently is, and with proper training and annotation of performance data, it would work just as well as the current system. You could say, "My airplane indicates 200,000 Pascal at 55% power at sea level" (I just pulled that number out of the air, so to speak, since it's greater than 1 atm.).

yep-- it ain't a perfect world---------
Your third paragraph is a corker ----
Let's see
"you jes fly along and look at that speedometer thingy but if it feels like it is all coming apart - you done went too fast."

I'm glad it's not really like that .

What's a corker? Do you mean run-on? I would have to agree if so .

Not really, an ASI measure the difference between TOTAL pressure and the STATIC pressure, i.e, the DYNAMIC pressure.

/Red B.

.
Tbe ASI shows the Indicated Airspeed, including corrections for position and losses in the system.
Measuring total and static pressure is usually done upstream of the indicator, at the sensor, for these reasons.

You are correct in implying that total pressure is static plus dynamic pressure.

In a pitot-static system, the difference in pressure between the ram side of the diaphragm and the static side is what moves the diaphragm and in turn, the needle of the airspeed indicator. When the pressures on both sides of the diaphragm are equal, dynamic pressure is zero, and the airspeed is zero. While it’s true that dynamic pressure is added to static pressure on the ram side to create the difference, the diaphragm doesn’t know the total pressure (ambient pressure can be very high or very low – it makes no difference). It only moves in response to the difference in pressure between the two sides, so my wording was off.

From aerospace web (I don’t have my engineering books with me as I am at work):

http://www.aerospaceweb.org/question...ts/q0251.shtml

"The difference between the dynamic and static pressures is used to determine the indicated airspeed (IAS) that is displayed to the pilot on the airspeed indicator in the cockpit."

I suppose this statement isn't entirely correct, depending on how you look at it, since dynamic pressure is only the component of pressure resulting from motion. It must be added to static before static is subtracted again.

So we are left with the airspeed indicator measuring only dynamic pressure, and it doesn't have any way of knowing total pressure like the altimeter does.

<edit> added quotation marks, and wording.

That is what I was trying to say-- Having seen these little buggers up close -I could not see how they could handle CHANGING conditions as the plane moved up n down n thru cold and hot air etc..
I always thot the airspeed indicators in state of the art glass cockpit commercial carriers and "spare no expense" militaty craft, was a few steps up from the simple airspeed indicator in an old Mooney.
I wonder what they have in that new Airbust - the 500 passenger thing - I bet it is reading three satellites all the time it is out of a hanger --The news said the thing has private showers etc..
Maybe a handball court?

Pilots of aircraft of all sizes need to know "indicated" airspeed. Some aircraft also display true airspeed, but not without indicated airspeed. We cannot do away with indicated airspeed--it's too important. The fact that it doesn't match true airspeed is unimportant, since we can determine TAS easily when needed, which isn't all that frequently compared to the number of times we check indicated airspeed.

.
This was "state of the art" in 1941 (when the p-38 was born), 1971 when the photo was taken (a NASa U-2C), and just became obsolete last year... When the U-2 Schanged to a glass cockpit.
Steam gages all.
The chart on the left console lists IAS for differing altitudes.

There should be a distinction made here about what the plane "sees" and what the pilot sees.

The plane's performance is governed by dynamic pressure (a function of true airspeed and density, or Mach and static pressure).

However, true airspeed is not the most effective measure of airspeed for a pilot, at least with respect to performance data (the traditional V-speeds). Calibrated airspeed, and at higher Mach and altitudes - equivalent airspeed, are much more convenient measures for aircraft performance because the relationship with dynamic pressure. Unfortunately all instruments have errors inherent to the device itself and its implementation (position error), and the result of these errors is reflected in the indicated airspeed. Luckily for LOW Mach, LOW altitude applications, indicated airspeed is a pretty useful measure, as compressibility is not a major concern (indicated airspeed is not too far off from calibrated or equivalent airspeed). The colored arcs on an airspeed indicator are computed conservatively (heavy airplane for stall and light airplane for V_A and V_NE), and placed on the face of the airspeed indicator in terms of indicated airspeed for pilot reference, and they work quite well.

I alluded to this in an earlier post. I quote myself:

"It would be useless to try to rely on true airspeed alone to determine the above speeds in all but standard atmospheric conditions. In fact, it's theoretically possible that the airspeed indicator could have been marked in pressure units instead of translated to speed units the way it currently is, and with proper training and annotation of performance data, it would work just as well as the current system. You could say, "My airplane indicates 200,000 Pascal at 55% power at sea level" (I just pulled that number out of the air, so to speak, since it's greater than 1 atm.)."

The SR-71 had the usual airspeed indicator.

That is amazing, given issues such as heating (of the pitot), shocks, etc... The pitot was mounted directly in front of the nose wasn't it? I am always amazed at what the Engineers of the day were able to do with relatively little technology (compared to today anyway). The SR-71 in particular is an amazing engineering feat, then again Kelly Jonhson was no average engineer

Yeah! Compressibility is a bit of an issue at Mach3+ and 1100°F!

That thing still inspires me. I usually pause for a moment whenever I see a photo of one. It reminds me of a stylized dagger. The only one I saw up close is at DMN. Those J58 engines are gigantic. Of course lots of people have seen it by now.

There is one a few miles from me at Hill Air Force Museum-also an engine on display - that is the most spaghetti I have ever seen on an engine -miles of tubing all over it .
The plane looks like there was no budget limit on it when it was dreamed up..
It is fun to wander around the place-see what the boys did thru the years -
The latest technologies tho -really ring my bell- -If you can think of it - you can build it -- from fully proportional one ounce models to that new Air Bust .

We had those IAS conversion stickers in the helicopters I flew in the service. Never really flew high enough to need them.

Speaking of static ports, the OH-58 has them on the left and right sides of the fuselage, right in front of the pilots doors. We flew doors off a lot. When giving instrument checkrides, I would slide my foot out in the slipstream and carefully deflect the air going past the static port vanes. If you do it right you can slowly screw up the VSI, altimeter, and AS indicators. A nervous pilot under the hood usually didn't crosscheck enough to catch it right away and the fun began.

editted for spelling

The Compass Cope C-130 mod put an APU in the left main wheel fairing.. its inlet was -ahead- of the static port for the airspeed system.
On the first flight, at liftoff, airspeed went berserk... had there not been the test instrumentation system with a seperate static port available. that flight might have been very short!