Cambridge 302 Indicated Airspeed

[Ross Biggar]

I fly a Ventus CT with a Cambridge 302 and an Ipaq using Winpilot.
Flying yesterday I checked the IAS shown on the 302, I noticed that it is
giving
totally different readings to the mechanical ASI.
eg at 50kts on the mechanical, the 302 showed 40kts
at 70kts the 302 showed 63 kts.
I did not check at higher speeds to see if the difference reduced further.
I am reasonably sure the mechanical ASI is probably correct, (if it was
10kts out on finals
it would be an interesting approach) !!.
I checked the IAS as I have been doubting the wind shown on Winpilot, if it
is using the IAS fron the 302
and that is in error, then that would explain the wind problem.

Does anyone have any suggestions as to what can cause such a difference
betwen the mechanical
ASI and the 302??
Thanks
Ross

[Bob Fidler]

Start by checking the setup calibration numbers listed in the manual. Maybe
zero airspeed is wrong, therefore every speed is off by that amount.
"Ross Biggar" > wrote in message
...
>I fly a Ventus CT with a Cambridge 302 and an Ipaq using Winpilot.
> Flying yesterday I checked the IAS shown on the 302, I noticed that it is
> giving
> totally different readings to the mechanical ASI.
> eg at 50kts on the mechanical, the 302 showed 40kts
> at 70kts the 302 showed 63 kts.
> I did not check at higher speeds to see if the difference reduced further.
> I am reasonably sure the mechanical ASI is probably correct, (if it was
> 10kts out on finals
> it would be an interesting approach) !!.
> I checked the IAS as I have been doubting the wind shown on Winpilot, if
> it is using the IAS fron the 302
> and that is in error, then that would explain the wind problem.
>
> Does anyone have any suggestions as to what can cause such a difference
> betwen the mechanical
> ASI and the 302??
> Thanks
> Ross
>
>
>

[Eric Greenwell]

Ross Biggar wrote:

> I fly a Ventus CT with a Cambridge 302 and an Ipaq using Winpilot.
> Flying yesterday I checked the IAS shown on the 302, I noticed that it is
> giving
> totally different readings to the mechanical ASI.
> eg at 50kts on the mechanical, the 302 showed 40kts
> at 70kts the 302 showed 63 kts.
> I did not check at higher speeds to see if the difference reduced further.
> I am reasonably sure the mechanical ASI is probably correct, (if it was
> 10kts out on finals
> it would be an interesting approach) !!.
> I checked the IAS as I have been doubting the wind shown on Winpilot, if it
> is using the IAS fron the 302
> and that is in error, then that would explain the wind problem.
>
> Does anyone have any suggestions as to what can cause such a difference
> betwen the mechanical
> ASI and the 302??

Some obvious ones are:

* bad calibration on the 302
* 302 and ASI are on different pitot/static systems

But I think the problem is the 302 airspeed zero offset is incorrect.
The numbers you give can be explained completely by a zero offset in the
302. From the manual:

> Screen #10 (0) shows sensor readings as follows:
> · Indicated Airspeed (IAS) in knots or km/hr.
> · Airspeed Indicator (ASI) zero offset (should be less than 1000).

So, you can check it. Regardless of the outcome, I suggest a call to
Cambridge about the problem. With luck, it will be something you can
correct yourself with a calibration command of some sort.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[John Galloway]

The 302 has no facility for the user to perform airspeed
adjustment for static errors. On the gound in zero
wind my 302 airspeed often showed several knots of
positive reading. These 2 aspects perturbed me when
I had a 302 and as there was no information in the
manual I asked Cambridge and was told that it did some
self calibration thing when it flew and that in the
air the 302 airspeed reading should be accurate - which
turned out to be true. So checking what your 302 reads
on the ground is not going to give you much information
about your problem. On the other hand if you put the
302 and your ASI on the bench and give them some shared
pressure to the pitot inputs and compare readings you
might learn something.

Errors as large as described seem excessive for positional
static errors even if the ASI and 302 are on different
statics so that would suggest either a pneumatic tube
problem or a sensor problem. The bench test would
give you an idea of which and if it looks like the
302 sensor is duff you could contact Gary Kammerer
at Cambridge for advice.

)

John Galloway


At 00:12 15 August 2005, Bob Fidler wrote:
>Start by checking the setup calibration numbers listed
>in the manual. Maybe
>zero airspeed is wrong, therefore every speed is off
>by that amount.
>'Ross Biggar' wrote in message
...
>>I fly a Ventus CT with a Cambridge 302 and an Ipaq
>>using Winpilot.
>> Flying yesterday I checked the IAS shown on the 302,
>>I noticed that it is
>> giving
>> totally different readings to the mechanical ASI.
>> eg at 50kts on the mechanical, the 302 showed 40kts
>> at 70kts the 302 showed 63 kts.
>> I did not check at higher speeds to see if the difference
>>reduced further.
>> I am reasonably sure the mechanical ASI is probably
>>correct, (if it was
>> 10kts out on finals
>> it would be an interesting approach) !!.
>> I checked the IAS as I have been doubting the wind
>>shown on Winpilot, if
>> it is using the IAS fron the 302
>> and that is in error, then that would explain the
>>wind problem.
>>
>> Does anyone have any suggestions as to what can cause
>>such a difference
>> betwen the mechanical
>> ASI and the 302??
>> Thanks
>> Ross
>>
>>
>>
>
>
>