REFLECTOR: Speeds

Scott Derrick scott at tnstaafl.net
Wed Jun 6 21:10:44 CDT 2012


Glenn,

I'd be glad to apologize if proved wrong, but...

There is no pitot-static inspection for VFR or IFR, just static. And all 
that verifies is that your altimeter and encoder report the correct 
"BENCH" test altitude.  I too had a current IFR static sign off when my 
static system was 250 ft off at 150 kias! Static sign off means 
absolutely nothing about the accuracy of it in flight.

You could have 10 GPS's on board and you still may not know your TAS! 
Not to belabor the point but I also fly with an IFR Approach Approved 
GPS.  Certainly doesn't mean any thing about the in flight accuracy of 
your static or pitot system, or your TAS.

Scott

-------- Original Message --------
Subject: Re: REFLECTOR: Speeds
From: Glenn Babcock <glennbabcock at roadrunner.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Date: 06/06/2012 06:04 PM

> Scott,
>
> I'd be glad to take you up and listen to your apology. I have an IFR bird with a current pitot-static inspection and 3 GPSs including an IFR certified unit.  I regularly fly 500+ mile trips using those airspeeds. My numbers are accurate.
>
> Regards,
> Glenn
>
> On Jun 6, 2012, at 6:48 PM, Scott Derrick<scott at tnstaafl.net>  wrote:
>
>> Richard,
>>
>> I think your doing very well at 170 KTAS..
>>
>> I haven't seen a Std RG any faster when comparing TAS.
>>
>> I don't have much confidence when I hear somebody reporting faster speeds than that with an STD RG. The longwing STD's are 5-10 knots slower than the orig STD.
>>
>> There are two main reasons for incorrect airspeed reporting in homebuilts.
>>
>> 1.) the pitot Static system has not been calibrated. Usually this is a result of a bad static reading. The only way I know of to check this is a very low(50 ft) pass over the runway at cruise speed. before the pass set your altimeter(on the ground) and verify the altitude on your altimeter, add 50 ft for comparison. During the low pass write down the altitude.  If it is off by more than then 50 ft, you probably should adjust your static port. If it reads higher than it should you have too much static pressure, if lower you have too little. I adjusted mine with a small piece of aluminum tape in front of my port, this reduced the pressure and now I read the correct altitude. Because the IAS instrument is a comparator, comparing the pitot pressure against the static pressure, if your static port is off your airspeed indicator will be too.
>>
>> 2.) The other reason is the confusion between GPS ground speed and TAS. Yes if you are in a no wind situation, very very rare, they are basically equal. but more often than not they are not and you will need to subtract a tailwind component or add a headwind component.
>>
>> Scott
>>
>> -------- Original Message --------
>> Subject: Re: REFLECTOR: Speeds
>> From: Richard J. Gentil<richard at naples-air-center.com>
>> To: Velocity Aircraft Owners and Builders list<reflector at tvbf.org>
>> Date: 06/06/2012 04:02 PM
>>
>>> Hi Scott,
>>>
>>> I am talking true airspeed. I am taking the alt, barometer, and OAT and using the KIAS I calculated my KTAS. I flew a triangular course and compared the GPS Ground speed to what the E6B was showing.
>>>
>>> I pushed the mixture, prop, and throttle full foward and let the speed stabilize on the two EFIS screens and wrote down my numbers. I tested several altitudes from on the deck up to 6,500 feet.
>>>
>>> Richard
>>>
>>> Sent from my iPhone 4 Classic
>>>
>>> On Jun 6, 2012, at 5:12 PM, Scott Derrick<scott at tnstaafl.net>   wrote:
>>>
>>>> Talking speeds kind be very frustrating.
>>>>
>>>> The only speed I like to talk about is KTAS,  Thats Knots True Air Speed
>>>>
>>>> not statute miles nautical miles, not indicated airspeed true airspeed, and certainly not ground speed(GPS).
>>>>
>>>> My IAS indicator has a TAS window, you select what your pressure altitude in reference to the OAT(upper window) and YOUR TAS will be indicated in the lower left window.
>>>>
>>>>
>>>> <jfaeejhi.jpeg>
>>>>
>>>> You can also fly a triangular course, using the GPS ground speed ,  and compute a fairly accurate TAS from that.
>>>>
>>>> And of course some of the higher end glass cockpits can compute it for you.
>>>>
>>>> And if flying very near to sea level on a std day temperature wise and assuming your IAS indicator is close to cal, it will show TAS.
>>>>
>>>> So Richard when you say 170 kts, what kind of speed are you talking about and how are you measuring it?'
>>>>
>>>> Scott
>>>> --
>>>> Talk sense to a fool and he calls you foolish.
>>>> Euripides
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>>
>> --
>> While I see many hoof marks going in, I see none coming out. It is easier to get into the enemy's toils than out again.
>> Aesop
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-- 
To live and let live, without clamour for distinction or recognition; to 
wait on divine love; to write truth first on the tablet of one's own 
heart - this is the sanity and perfection of living.
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