Flying downwind?

[Bundy]

Guys, I am very confused so please help.

Scenario: I am taking off into a head wind of approx 15Kts. I get airborne much quicker because of the headwind increasing the speed of the air travelling over the wing. I climb to around 2000ft AGL and complete a 180 turn. Now on a downwind heading, wind at this altitude is usually stronger so why does the aircraft not stall? Surely there is less air travelling over the wing? Or am I still moving forward in relation to the air around the plane?

I recently did this during training and my ASI was reading 27MPH which is below my usual stall speed. If air is not entering the Pitot tube, then how can there be airflow over the wing?

Answers in plain english for dummies please guys!

[Hadams]

Hi Bundy,

Had the same question, but it is really simple. Remember that when you are on or close to the ground it seems that you are travelling slower with the head wind, but in fact the air flow over/under your wing on take off is the same as any other day, taking into account the temparature and elevation of the field, but the same as in flight. When you are airborne you are part of the air that you fly in, so if you take the ground away and say you are travelling with the wind (downwind) you might be traveling at 50MPH according to your ASI, but you are actually travelling at 65MPH as result of the 15MPH wind. Same into the wind, if your ASI indicates 50MPH, you are only travelling at 35MPH. The fact that your ASI is only reading 27MPH says to me that either your pitot tube is blocked or your static tube is obstructed in some way or another.

That is how i understand the situation. In flight you are still traveling at 50MPH whether you are with or against the wind!

[RV4ker (RIP)]

Correct. Ground speed and airspeed are 2 very different things. Ignoring effects of loading, CofG etc the plane will always stall at the same Indicated Air Speed for a given config at a given weight. Into wind it will be More than ground speed. Downwind it will be less than ground speed. Similarly it will always "fly" at the same speed.

Go fly at the coast. I have seen 19kts ground speed (30kts below fall out the sky speed) going 1 way indicating 60kts and 240kts (60kts over VNE) going the other way indicating 165kts. Indicated air speed is how fast the air is passing over the wing (and into pitot) in relation to the static air.

If your aerie does not fly at IAS 27mph then there is possibly a problem with either static or pitot or ASI.

[Morph]

my ASI was reading 27MPH which is below my usual stall speed. If air is not entering the Pitot tube, then how can there be airflow over the wing

correct. Your ASI shows you the wind spead relative to your aircraft. If it is showing 27mph, below stall speed, then either you are just about to fall out of the sky, or there is something seriously wrong with your setup

Based on the fact that you are still here, I would say the second option.

What do you fly? Can you send us pics of the pitot installation and ASI installation

[Bundy]

Hi morph, I fly an Aquilla 2, 582 BTop with an Aqilla 1 wing. The ASI showed 27mph IAS. that is approx 15MPH below stall with PAX in this aircraft. (to the best of my knowlege)

I dont think that there is a blockage in the pitot because when climbing out, the air speed is normal but as soon as we turn downwind, the air speed dropped way down. We were flying straight and level with the bar neutral as always. I'm just thinking that perhaps, we were crabbing more than I thought at the time. This would cause the tube to be out of line with the airflow and therefore underread?

Will check and see if the tube is not perhaps mounted skew. Just very strange as this is the first time I have noticed the airspeed just drop like that, therefore the question about the downwind airflow over the wing. I understand now that whether flying into the wind or away from it, the "flying speed" is always the same, its just the ground speed that varies.

It has to be an instrumentation problem though as I know we were not even close to stalling speed. What do you guys think of all this electronic flight data? are they reliable? I think I like the old fashioned "analog" dial instruments better.

[KFA]

Firsly you stall at a given angle of attack and is not necessarily related to airspeed.

The pitot error could be because of the turning angle as well as well as the crosswind component that caused the airflow to be momentarily hitting the pitot at an awkward angle causing it to under read.

[Bundy]

Thats a good point KFA, makes sense. Thanks for clearing this up for me a bit guys!

[Wargames]

Hi Bundy,

Has your ASI shown 27 from the start, or for how long has this problem been identified?

I would say that your problem is that your pitot tube is not properly attached to your ASI or it has a crack/hole in it that lets some of the pressure escape.

Nr 2 problem could also be that your static port getting a bit wind in the hair and therefor ASI measures the dynamic part as less.

[Asterix]

I read this article about a year ago, and I found it very enlighting, read it at: http://www.gbfs.co.za/images/Dragons.pdf

[zucac]

bottom line ' wind on your face ' what did you feel ?

[Dobbs]

Asterics, that link explains it very nicely.

I believe it is the reference to the ground which causes all the confusion - if you were above a layer of cloud where there was no reference to the ground, and you did not reference the GPS for ground speed, the sensation of flying differently into or with the wind would not exists.

At no time can the wind blow across the pitot (excluding on the ground of course), as mentioned previously, there is in all likelyhood a blockage somewhere. I have on more than one occasion found a mud nest of sorts in my pitot tube.

[KFA]

Just a correction

caused the airflow to be momentarily hitting the pitot at an awkward angle

This is possible in the trike because the cart can rock sideways and the nose can swerve at the same time because of turbulance or mishandling of the bar causing the above that is why I said "momentarily" because it will only be for an instant.

[Bundy]

Thank you for all your input guys, The AP in charge has found a problem (blocked pitot) and is busy rectifying the prob. Looks like a bit if sh#t got stuck in there as you all suggest.

Regards,

[Morph]

make sure you close your Pitot tube when not in flight. Insects and spiders tend to put sh#t in them when you ae not looking

[Bundy]

Thanks Greg, Unfortunately this trike belongs to the flight school and I have no control over keeping the pitot covered. Damn, I cant wait to buy my own aerie!!!

Thanks for all your input, I think we need to change your Username to "YODA". ha ha