Air Speed Increases in Climb after RTB??

[Akilez]

Hey All,

 

Just recently picked up the DCS A-10C retail package and am working on weapons delivery. During two different runs through the 'Defend Camp' SP mission I ran low on fuel so RTB for supplies.

 

After returning to the AO I noticed a very odd change to the aircraft. When diving onto a target my airspeed actually decreased until it reached 50 KIAS where it stopped. Conversely during an escape maneuver incorporating a climb of no less than 10 degrees the airspeed continued to climb reaching speeds I've never seen in this aircraft. Both the HUD and mechanical air speed indications showed the same results.

 

Also the VSI stayed pegged at Zero regardless of whether I was climbing or diving.

 

I wouldn't have thought much of it but it happened twice with the exact same indications. Again this only occurred the two times I needed to RTB for fuel.

 

Anyone else notice this or do I just have gremlins running amok?

 

Cheers,

[EtherealN]

Sounds like nav system failures. Did you save a track?

[GGTharos]

Turn on your pitot heat ...

[Bvoiash]

That's very strange. Try posting the track file Akilez so that one of us here can have a look at it.

 

It appears that EtherialN and GGTharos beat me to it :).

Edited March 29, 2011 by Bvoiash

[Zenra]

Turn on your pitot heat ...

 

This is my bet - icing is usually the problem behind IAS and VVI issues, and most often occurs when ascending or descending through fog or cloud.

[Kaiza]

Very cool, pitot icing is modelled. However, shouldnt the VSI have I kept working as that is purely static? If the airspeed was changing it would suggest the static was not iced, just the pitot. I am not sure as I dont know where the static ports are on an A-10.

Edited March 29, 2011 by Kaiza

[Deadman]

PITOT-STATIC SYSTEM

The pitot-static system consists of a pitot-static

probe, located in a boom on the leading edge of the

right wing. The probe supplies impact air pressure to

the ai rspeed indicator and "Q" sensors. Static pressure

is supplied to the airspeed indicator, "Q" sensors,

altitude computer on ~, the CADC on ~,

and the VVI.

The pitot-static system functions automatically.

However, the pilOt-static probe contains a heating

device controlled by a switch in the cockpit.

Pitot Heater Switch

The pi tot heater switch (figure 1-:30), placarded

PITOT HEAT and OFF, is located on the environment

control panel. HEAT supplies power from the I left AC bus to the pitot heater and total temperature

probe and power from the right AC bus to the

AOA vane heater and the lift transducer vane

[EtherealN]

Very cool, pitot icing is modelled. However, shouldnt the VSI have I kept working as that is purely static? If the airspeed was changing it would suggest the static was not iced, just the pitot. I am not sure as I dont know where the static ports are on an A-10.

 

I'm not sure I understand the connection you are making. Airspeed changing does not say anything about the static.

 

 

AFAIK, the A-10 uses the third design.

[Le0kong]

Sounds similar to another user topic, with the airspeed reaching 50 KIAS on a dive.

 

http://forums.eagle.ru/showthread.php?t=69751

[Kaiza]

I'm not sure I understand the connection you are making. Airspeed changing does not say anything about the static.

 

 

AFAIK, the A-10 uses the third design.

 

I apologise if you already know this but, IAS is derived by comparing both pitot and static pressure to each other. If static pressure was not used in the equation then airpseed would decrease as the aircraft gets higher as environmental pressure decreases. By using static pressure the system measures the pressure created by the forward movement of the aircraft relative to the outside static pressure. This is why if you have a pitot port blocked the airspeed changes. The pitot pressure remains constant (due to being blocked) but the static pressure changes. The system is still comparing static and pitot so if you climb with high pressure that got stuck down at sea level, the system will compare it to the lower pressure outside the aircraft as you climb, causing the airspeed to over read, and vice versa decending.

 

If the A-10 uses the third design then a blocked pitot static port would cause a total blockage of the pitot static system. The VSI to remain still (as it sounds it does from the OP) but the airspeed would not change as the comparison between pitot and static remains constant as it is recieving no input.

 

The phenomenon of airspeed increasing with altitude is caused by a blocked pitot port but open static port in aircraft that have seperate static ports.

 

Poor explanation, I know, but hopefully you understand what I am getting at.

Edited March 29, 2011 by Kaiza

[rud]

The air speed indicator has several failure modes depending on which part of the of the pitot-static system is blocked (http://dcswiki.com/index.php?title=Airspeed_indicator_%28A-10%29) Icing can occur in the pitot or static tubes (or both). For the OP here, it sounds like the pitot tube is the culprit. When this happens the air speed indicator functions as a crude altimeter, diving makes the needle drop and gaining altitude makes it go up. It's confusing and it can be deadly. Turn on your pitot heat!

[Kaiza]

The air speed indicator has several failure modes depending on which part of the of the pitot-static system is blocked (http://dcswiki.com/index.php?title=Airspeed_indicator_%28A-10%29) Icing can occur in the pitot or static tubes (or both). For the OP here, it sounds like the pitot tube is the culprit. When this happens the air speed indicator functions as a crude altimeter, diving makes the needle drop and gaining altitude makes it go up. It's confusing and it can be deadly. Turn on your pitot heat!

 

Concur, but if this is the case, the VSI should still function, which it sound like it does not. If the OP is correct I believe the VSI is incorrectly modelled

Edited March 29, 2011 by Kaiza

[WarriorX]

Where is the AOA vane on the airframe?

[Temphage]

All things considered, an iced up pitot/static probe on the A-10 isn't that huge a deal... On the F-15, the input of various probes dictate how the ramps move which control how much air goes into the engines, and how the air is shaped... it does more than just tilt up or down, there's more ramps inside that bypass or further shape it. A malfunctioning ramp probe can cause a mach wave to be fed into the engine at supersonic speeds, pretty much instantly causing a blowout...

 

WarriorX - if you're wondering about the AOA probes, curious about the icing of those... many probes on aircraft are automatically heated when the aircraft goes weight-off-wheels. Again, on the F-15, the AOAs, TOT, and duct probes are automatically activated, whereas the pitots have a cockpit switch.

[EtherealN]

The phenomenon of airspeed increasing with altitude is caused by a blocked pitot port but open static port in aircraft that have seperate static ports.

 

Poor explanation, I know, but hopefully you understand what I am getting at.

 

Yeah, but my point is that I'm a bit confused how, given the third design, we'd find a situation where this happens. Not impossible of course, and I don't know exactly how the model handles this. But IRL I've never encountered a frozen static OR dynamic (though I once saw someone get perilously close to taking off while having forgotten to remove blockage - lots of hand-waving happened to make him stop :P ), so I don't have any first-hand experience to rely on. Pitot heat does seem a likely culprit though - a track would have been golden to be sure. :)

[Bucic]

Where is the AOA vane on the airframe?

http://i414.photobucket.com/albums/pp226/BloodnGuts/DCS%20A10C/A10-34-lo.jpg

About 0.4 m in front of and 0.2 m lower to the US Air Force star insignia.

 

http://www.aircraftresourcecenter.com/AWA1/001-100/walk003_A-10/images%20Burkhard%20Domke/2-02.jpg

http://www.aircraftresourcecenter.com/AWA1/001-100/walk003_A-10/images%20Burkhard%20Domke/4-94.jpg

Edited March 29, 2011 by Bucic

[MTFDarkEagle]

Aaaah that's where it is :D

[atledreier]

OT: What happens to the measured AOA if you are in a high yaw, high AOA attitude?

[sobek]

OT: What happens to the measured AOA if you are in a high yaw, high AOA attitude?

 

IIRC there are vanes on both sides, connected with each other. At least that is how it is on the Draken.

[Bucic]

OT: What happens to the measured AOA if you are in a high yaw, high AOA attitude?

Two separate questions should be asked - what happens to AoA indication and what happens to the sensor vane itself. In both cases no one on the forums will give you the answer :) To answer the first question you'd have to know the measures of AoA indication errors compensation used on A-10C. In some AC manuals indication error charts are included.

 

To answer the second question one would have to know the exact airflow (direction of stream lines) over the A-10C airframe in the particular flight configuration.

[WarriorX]

http://i414.photobucket.com/albums/pp226/BloodnGuts/DCS%20A10C/A10-34-lo.jpg

About 0.4 m in front of and 0.2 m lower to the US Air Force star insignia.

 

http://www.aircraftresourcecenter.com/AWA1/001-100/walk003_A-10/images%20Burkhard%20Domke/2-02.jpg

http://www.aircraftresourcecenter.com/AWA1/001-100/walk003_A-10/images%20Burkhard%20Domke/4-94.jpg

 

I have been looking for that little S.O.B. for a while....Thank you!

[Bucic]

I have been looking for that little S.O.B. for a while....Thank you!

You're welcome. I've learn its location on ED forums myself.

[Yo-Yo]

Just to clarify: we presumed that icing starts from the pitot-tube end, i.e. dynamic port. If the icing continues the static port can be blocked and your gauges readings will correspond pressure values inside the tubings.

By the way - before ice blocks the ports fully there will be "variometer-like" behaviour (lags) because of little orifices instead of normal ports.

[Akilez]

Hello...

 

Thank you for the replies and I must apologize for not having the foresight to save a track after experiencing this phenomena.

 

I've been out since my original post so I have yet to fly the A-10C again but hope to get some air time NLT tomorrow.

 

I will of course make sure my pitot heat is active and if I see this 'reversal' of indicators (and a stuck at zero VSI) I'll be sure to save a track and post it in this thread.

 

Thanks again for the detailed responses.....great community.

 

Cheers,