IAS vs MPH - manual is confusing

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Gabe777
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IAS vs MPH - manual is confusing

Post by Gabe777 »

So, in the manual, the various speed limitations state an I AS and in brackets, MPH.

Surely the I AS is only equal to MPH at sea level...... this is highly confusing for my ageing brain !
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ratty
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Re: IAS vs MPH - manual is confusing

Post by ratty »

The airspeed indicator (ASI) is calibrated in mph, that is, STATUTE miles per hour, as opposed to knots, which are NAUTICAL miles per hour (5,280 feet and 6,080 feet approx, respectively). The INDICATED air speed for a given event or condition remains the same regardless of altitude. If an aeroplane stalls at 60 mph indicated at sea level it will stall at 60mph INDICATED at 10,000 feet. However, because the air is thinner up there the TRUE airspeed at 10,000 feet will be higher - you have to go faster to get the same amount of air into the ASI.
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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

Gabe777 wrote:So, in the manual, the various speed limitations state an I AS and in brackets, MPH.

Surely the I AS is only equal to MPH at sea level...... this is highly confusing for my ageing brain !
You are simply confusing MPH with TAS. MPH is simply the UNIT of speed NOT the type of speed. Speed can be expressed in MPH or in KTS. Airspeed on the other hand can be expressed in many different ways, indicated and true simply being two of the ways. There are others .
Dudley Henriques

Gabe777
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Re: IAS vs MPH - manual is confusing

Post by Gabe777 »

Yes I understand this.

But 180 IAS at 10,000ft is over 200 mph..... whether statute or nautical.

Every other aircraft I own has an IAS gauge as this is what matters, as you allude to in the stalling data you used.

So I ask again: why does it say in the manual that the IAS is essentially equal to the MPH ?

In the manual for the comanche, it states in the limitations section:

"Max cruising speed 180 IAS (MPH)"..... this implies they are the same.... but they are not unless you are at sea level at standard temp and air pressure blah blah.

Now forgive me, but on any level, this is confusing!

EDIT: just realised it shows both.... as the outer (white) dial adjusts for outside pressure and temp and shoes GAS as well... something I never noticed to be honest !

BUT, the manual is still confusing..... if it flies at 180 I AS at 10,000ft, then TAS (or ground speed) will be over 200.
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Gabe777
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Re: IAS vs MPH - manual is confusing

Post by Gabe777 »

It also suggests that many business men buying these GAs back in the 50s and 60s (and many killed themselves, especially in the bonanza !), were doing themselves out of bragging rights.

"Ooh look, it does 180 mph !".......... No, at 10,000ft it does closer to 220 mph..... That's the difference between a GT1 car and an LMP1 car.... which at Le Sarthe makes a huge difference to lap times !
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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

Gabe777 wrote:It also suggests that many business men buying these GAs back in the 50s and 60s (and many killed themselves, especially in the bonanza !), were doing themselves out of bragging rights.

"Ooh look, it does 180 mph !".......... No, at 10,000ft it does closer to 220 mph..... That's the difference between a GT1 car and an LMP1 car.... which at Le Sarthe makes a huge difference to lap times !
You need to research airspeed and how different speed affects an aircraft in flight.
From what you are telling me here there seems to be some confusion about how different speed affects a plane and how speed is presented and calculated.
Rather than bore you to tears with a tutorial on this can you simply research "airspeed"? All the answers will be found there.
I will say again that any of these speeds can be presented to you in print as either MPH or KNOTS. These are simply units of speed. It does get a bit confusing when you start converting them.
Research each different speed, how it is presented and used and you will have your answer.
Dudley Henriques

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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

Gabe777 wrote:Yes I understand this.

But 180 IAS at 10,000ft is over 200 mph..... whether statute or nautical.
This is what is confusing you. This is NOT correct ! 180mph IAS at 10,000 ft, is NOT 200mph. It is 180mph and is what you are reading on the ASI. The TAS however, is higher at 10,000 ft. TAS is IAS corrected for pressure altitude and temperature. TAS will be higher, NOT the IAS.
Dudley Henriques

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AKar
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Re: IAS vs MPH - manual is confusing

Post by AKar »

If it was up to me, we would measure 'IAS' in Pascals for precisely this reason! :) :wink:

-Esa

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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

AKar wrote:If it was up to me, we would measure 'IAS' in Pascals for precisely this reason! :) :wink:

-Esa
And of course we all just LOVE figuring pressure in kilo pascals/cm2. LOL

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AKar
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Re: IAS vs MPH - manual is confusing

Post by AKar »

Pascal of course being newtons per m².

Kidding of course about using pascals, but IAS being principally a pressure measure helped me at least. I think rocket guys used 'Q' as well.

Happy new year Dudley & Gabe! :)

-Esa

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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

AKar wrote:Pascal of course being newtons per m².

Kidding of course about using pascals, but IAS being principally a pressure measure helped me at least. I think rocket guys used 'Q' as well.

Happy new year Dudley & Gabe! :)

-Esa
The little L39 is very popular here in the US. We love the jet and a lot have been sold here. It's a source of humor for me anyway that the first thing our pilots want to do is retrofit the panels with American instrumentation they are used to reading. :-))

Yes, have a great and safe New Year.
DH

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AKar
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Re: IAS vs MPH - manual is confusing

Post by AKar »

DHenriquesA2A wrote:It's a source of humor for me anyway that the first thing our pilots want to do is retrofit the panels with American instrumentation they are used to reading. :-)
Yeah...I'm certain I've got less than 50 hours at controls total if I could gather that all up together, but I've had a chance to land airplanes with airspeed indicators in knots, mph, and km/h. I never found any issue with that. They tell you to fly seventy, and you try to do that. How hard can it be, quoting Mr. Clarkson! :mrgreen: Not really any question of arithmetics, really, for there is very little absolute meaning in the IAS.

-Esa

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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

AKar wrote:
DHenriquesA2A wrote:It's a source of humor for me anyway that the first thing our pilots want to do is retrofit the panels with American instrumentation they are used to reading. :-)
Yeah...I'm certain I've got less than 50 hours at controls total if I could gather that all up together, but I've had a chance to land airplanes with airspeed indicators in knots, mph, and km/h. I never found any issue with that. They tell you to fly seventy, and you try to do that. How hard can it be, quoting Mr. Clarkson! :mrgreen: Not really any question of arithmetics, really, for there is very little absolute meaning in the IAS.

-Esa
I was joking. LOL

DH

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AKar
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Re: IAS vs MPH - manual is confusing

Post by AKar »

DHenriquesA2A wrote:I was joking. LOL
Me as well. :) I got your joke, which has been particularly entertaining for some around here as well, for all the mix of units we've got around here. Damn, even the Hornets used stand-by indicators in km/h back when I had something to do with them. :) Actually, I understand when the primaries were in km/h with older generation airplanes, there actually were some issues with ATC, mainly with speeds to fly stuff.

-Esa

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DHenriques_
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Re: IAS vs MPH - manual is confusing

Post by DHenriques_ »

AKar wrote:
DHenriquesA2A wrote:I was joking. LOL
Me as well. :) I got your joke, which has been particularly entertaining for some around here as well, for all the mix of units we've got around here. Damn, even the Hornets used stand-by indicators in km/h back when I had something to do with them. :) Actually, I understand when the primaries were in km/h with older generation airplanes, there actually were some issues with ATC, mainly with speeds to fly stuff.

-Esa
The only mental magic our pilots might be faced with in say one of our locally owned L39's is with time and distance with the panel in metric and ATC throwing out non-metric units. Pressures and temps are what they are. You know the numbers in metric and keep it all between the lines. But ATC can be a chore cross country.
Most of our guys use the standard conversion "shortcuts" when dealing with ATC. It's usually close enough for government work.
LOL

DH

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