Airspeed indicator negative function on the Mk Ia

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Paughco
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Airspeed indicator negative function on the Mk Ia

Post Paughco »

Took off from Arlington Airport with no drama this afternoon. Headed west and let her climb. Oxygen on at 10,000. Got her up to 20,000 feet. At some point in the flight I hovered my cursor over the ASI and read 0 mph. Ground speed was about 240 according to the Shift+2 panel. OAT was about -15 degC. I turned on the pitot heat and waited for any ice to melt. I had only flown through one tiny little cloud layer on my way to 20,000, so I didn't think that the pitot would be frozen. Maybe it was, but the pitot heater didn't thaw it out very well. She handles really nice up there at 20,000. I turned her around and flew back towards Arlington, making sure I didn't fly through any more clouds. The OAT was about 15 degC on approach to the field, but still no indication on the IAS. Had to use the ground speed reading on the Shift+2 panel to make sure I wasn't lowering the gear or flaps at too high (or too low) airspeed.

Is this a "feature?" Or do they all do that? I checked the Shift+7 shop and there was no indication of any problem.

The Spitfire sure is a thoroughbred! She even parks gracefully!

Seeya
ATB
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Kilstorm
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Re: Airspeed indicator negative function on the Mk Ia

Post Kilstorm »

Did you remove the Pitot Cover? You never mentioned it was working before you noticed that it was reading 0.
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Paughco
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Re: Airspeed indicator negative function on the Mk Ia

Post Paughco »

Sorry - yes, it was working at the beginning of the flight. Does the Spitfire even have a pitot cover? I once flew my Comanche without removing the pitot cover and that was no fun. This was just like that, only it didn't happen until I got up past 10,000 ft.

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Jacques
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Airspeed indicator negative function on the Mk Ia

Post Jacques »

Did you have correct readings for amps/ volts?

I think you are correct, no pitot cover! :D

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Paughco
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Re: Airspeed indicator negative function on the Mk Ia

Post Paughco »

Took her out for a short hop around the pattern. The ASI worked! Must have been either a temporary glitch, or maybe the pitot tube got frozen, and the heater can only heat up the pitot enough to prevent icing, but is too anemic to melt it once it forms. I dunno.

It looks like the ammeter is set to show amps that are generated, but cannot show amps that are going out when the engine is not running. It would be great to have a ammeter that could show minus values, so you could check to make sure that everything is shut off and no current is flowing before you leave the airplane. I believe that the Spitfire exemplifies British logic. Classic.

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Jacques
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Airspeed indicator negative function on the Mk Ia

Post Jacques »

I guess what I'm wondering is, perhaps you forgot to switch on the generator ( on my FSX version "up" is "off")... and if the P3Dv4 version simulates loss of charge in the battery, it might be possible to lose your pitot heat and have a non-functioning airspeed indicator.

I tried this theory tonight in FSX and, although I flew around in this afternoon's conditions at Arlington and I had everything electric that I could locate set to "on" AND... I turned off the Generator, I could never run down the battery.
I never lost the airspeed indicator. Worth a try at the very least!


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Killratio
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Re: Airspeed indicator negative function on the Mk Ia

Post Killratio »

It was obviously well frozen up! Just remember you don't have to fly through cloud to ice up the pitot.. It is a small tube with a lot of air rushing over it. It collects moisture well and will generally freeze up on you at altitude (I spend most of my time at 20K odd....)

"Pressure Head Heat" is a compulsory item in your preflight checklist..as is turning it off on landing/shutdown.

And yes..the accumulator will run down over time. Remember though that it is a geometric progression, losing very little for a long time then falling off sharply. I've spent hours just testing that. The main consumer (in flight...starter motor of course is the biggest draw) is the Pitot Head Heat...next would be landing lights. Everything else is a relatively minor draw. I wish the ammetre would show negative draw too but it isn't a priority. Oskar and I spent many hours researching the electrical system back in '11 using a complete set of electrical drawings from original sources. It's as close as it can be, bar fuses.

I've found that the ASI will sometimes come back with reduced altitude or head heat..but sometimes not. I guess if the freeze was slow and steady, it may freeze well up into the line, making the head heat of limited value. The ASI is a pressure instrument though, the status of the electrical system will not affetc it.

From memory:

Pressure/Vacuum:
ALT
VSI

Pressure/Pitot:
ASI

Gyro:
AH
T&S
DG

Electrical:
Volt
Amm
Fuel

Direct Drive:
RPM
Nose Trim
Flap Gauge
Oxy regulator "Height"

Themometre:
Oil T
Rad T


Capillary Pressure etc:
Fuel P
Oil P
TBG
Oxy regulator "Quantity"
Boost

No readily discernible sense or use (allegedly magnetic):
P8
:) :) :)


regards

Darryl
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Zacke
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Location: EDNY

Re: Airspeed indicator negative function on the Mk Ia

Post Zacke »

Hey guys,

today I flew her from EGHI to EDNY (my delivery flight after some weeks of flight school in Southampton).

Early during climb I realized the airspeed indicator is droping for some 10 mph. Pitot heat on for some minutes but no change. I continued (probably not good piloting) but the indictor didn't recover. Cruising FL250, 2100-2200 rpm, -4 psi, the indicator bounced around 120 mph. Pitot didn't help altough keeping it switched over for 10 minutes or so. All other systems running good.

Made it to EDNY. Even on the final (+14°C) indicated airspeed much to low.

I experienced the same issue some months ago with the T6 (IAS completely dead): viewtopic.php?f=129&t=57336

Any ideas?

Regards,
Zacke

alan CXA651
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Re: Airspeed indicator negative function on the Mk Ia

Post alan CXA651 »

Hi Darryl.
The VSI is static / delayed static , not pressure/vac .

paughco .
because the pitot was not turned on till indicator showed zero and at alt , not only the pitot , but pipes behind the heater would be frozen , which means pitot heater would have no chance of clearing it , and your engineers would if you made it back saftly , be very upset with you , as they would have to drain water out of the system once it thawed out , on the pitot / static systems they have drain traps , for when water gets in the pipes.
regards alan. 8)
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Zacke
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Location: EDNY

Re: Airspeed indicator negative function on the Mk Ia

Post Zacke »

Thanks for your reply.

What you explained is that simulated in FSX/A2A Accusim?

So if I turn on the pitot heater earlier that wouldn't happen at all?

alan CXA651
Senior Master Sergeant
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Re: Airspeed indicator negative function on the Mk Ia

Post alan CXA651 »

Hi Zache.
I dont know how A2A did the modelling of the pitot/static heating , as they need to work within the constraints of FSX/P3D , they may just have some extra long time delay on thaw rate before it works again , or some total failure thats only reset next time you use or respawn the aircraft , only A2A staff could answer that one , i was using a real aircraft issue if you forget the pitot heat .
regards alan. 8)
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Zacke
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Joined: 27 Sep 2018
Location: EDNY

Re: Airspeed indicator negative function on the Mk Ia

Post Zacke »

Ok. Maybe we'll get some more comments on that. I will keep a close eye on that!

Zacke
Staff Sergeant
Posts: 298
Joined: 27 Sep 2018
Location: EDNY

Re: Airspeed indicator negative function on the Mk Ia

Post Zacke »

Response from Lewis (A2A): viewtopic.php?f=129&t=57336

Zacke
Staff Sergeant
Posts: 298
Joined: 27 Sep 2018
Location: EDNY

Re: Airspeed indicator negative function on the Mk Ia

Post Zacke »

Did some testing.

Penetrating some clouds with OAT -5 - 2°C: ASI dropping to zero. Using pitot heat it comes back quite soon. Now letting it freeze for 30, 60, 300 sec. Always coming back within a few seconds with pitot heat. Everything seems to be quite normal.

Maybe it's a quirk as Lewis mentioned. Or it has to do wit AS16.

So don't know what's going on there. Will continue observing this one.

Zacke

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