How will the ASI react in a forward slip.. for example lets say we are flying a 172 with the static port on the left side of the cowling and we enter a forward slip putting the static port downwind.. how will the ASI react.. Over read or under read? ...
Thanks for the help
How will the ASI react in a forward slip?
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How will the ASI react in a forward slip?
Last edited by Nanart on Thu Apr 15, 2010 7:36 pm, edited 1 time in total.
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Oor Wullie
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Re: How will the ASI react in a forward slip?
The ASI works on measuring the difference between static and ram air pressure (through pitot tube)
What would happen if the air flow changed around the static port?
Close your eyes and picture it. The answer will make perfect sense
Just remember, smaller pressure difference = lower A/S reading and vice versa
What would happen if the air flow changed around the static port?
Close your eyes and picture it. The answer will make perfect sense
Just remember, smaller pressure difference = lower A/S reading and vice versa
Everyone makes mistakes. The trick is to make mistakes when nobody is looking.
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Some days you're the dog, somedays you're the fire hydrant.
Re: How will the ASI react in a forward slip?
So would this statement be correct: If slipping with the static port into wind the ram air will cause the static pressure in the ASI to increase causing it to over read.
and slipping with the static port downwind will cause a vacuum over the static port causing the ASI to under read
Is that correct or would it be the other way around?
and slipping with the static port downwind will cause a vacuum over the static port causing the ASI to under read
Is that correct or would it be the other way around?
Re: How will the ASI react in a forward slip?
It would be the other way around.
As was mentioned the ASI reads the difference between static and dynamic pressure. If you were to increase dynamic pressure ASI increases. Decrease dynamic pressure ASI decreases.
If you put ram air into the static port you are decreasing the difference in pressure between Dynamic and Static (since you are artifically increasing the static pressure) therefore you ASI will decrease.
As your static pressure and dynamic pressure get closer together the ASI approaches zero. On the ground the static port and pitot tube both measure ambient pressure and the ASI reads 0. Assuming you are not moving.
BTD
As was mentioned the ASI reads the difference between static and dynamic pressure. If you were to increase dynamic pressure ASI increases. Decrease dynamic pressure ASI decreases.
If you put ram air into the static port you are decreasing the difference in pressure between Dynamic and Static (since you are artifically increasing the static pressure) therefore you ASI will decrease.
As your static pressure and dynamic pressure get closer together the ASI approaches zero. On the ground the static port and pitot tube both measure ambient pressure and the ASI reads 0. Assuming you are not moving.
BTD
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iflyforpie
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Re: How will the ASI react in a forward slip?
Another thing to consider for aircraft with static ports on both sides or on the bottom (like Piper's all-in-one pitot-static blade); a slip will cause the ASI to under read through the same process as position error at high angles of attack...
Geez did I say that....? Or just think it....?
Re: How will the ASI react in a forward slip?
Static pressure surrounds the can (aneroid capsule), dynamic pressure is inside the can.
A left slip forces some air into the static, increasing it's pressure and SHRINKING the can. The can also shrinks because dynamic pressure is less in a slip as well. A shrinking can reduces indicated airspeed.
A right slip causes slightly lower pressure on the static port side of a 172, decreasing the pressure around the can. Thus, the can EXPANDS causing a rise in indicated airspeed. But remember dynamic also is decreased in a slip. Thus, a right slip doesn't under-read as much.
I think it was 5 knot error on right slips, 10 knot error on left?
A left slip forces some air into the static, increasing it's pressure and SHRINKING the can. The can also shrinks because dynamic pressure is less in a slip as well. A shrinking can reduces indicated airspeed.
A right slip causes slightly lower pressure on the static port side of a 172, decreasing the pressure around the can. Thus, the can EXPANDS causing a rise in indicated airspeed. But remember dynamic also is decreased in a slip. Thus, a right slip doesn't under-read as much.
I think it was 5 knot error on right slips, 10 knot error on left?
Re: How will the ASI react in a forward slip?
OK, then to advance this thought a little bit, what happens to the stall characteristics in this case? (Left or Right probably doesn't make a huge difference). Drag is increased in the slip, but I think the stall characteristics won't change.C-GPFG wrote:Static pressure surrounds the can (aneroid capsule), dynamic pressure is inside the can.
A left slip forces some air into the static, increasing it's pressure and SHRINKING the can. The can also shrinks because dynamic pressure is less in a slip as well. A shrinking can reduces indicated airspeed.
A right slip causes slightly lower pressure on the static port side of a 172, decreasing the pressure around the can. Thus, the can EXPANDS causing a rise in indicated airspeed. But remember dynamic also is decreased in a slip. Thus, a right slip doesn't under-read as much.
I think it was 5 knot error on right slips, 10 knot error on left?
Re: How will the ASI react in a forward slip?
Actually it does change. Remember washout causes the stall to happen at the root first. But, in a slip, the downwind wing's wingtip will stall first. That wing takes on the characteristic of a swept wing as the relative airflow travels diagonally across it....
Re: How will the ASI react in a forward slip?
Just to add my two bits...
I think you will find it very hard to quantify, or perhaps to even generalize because of the number of factors involved.
A couple of more factors that I don't think have been mentioned is the placement of not only the static ports, but also the pitot tube. And relative to these two, what's the exact direction of the relative wind? As well what specific power setting is on the aircraft at that specific moment you want to know what is happening to the ASI. Another factor is the how and what role propwash flow around the aircraft will play.
I think its a good theoretical exercise to think this through, but you will generally see your speed drop when in a slip. Most of it caused by drag, with some resulting from instrument error.
But I wouldn't worry about how much is real and how much is instrument error. Just fly the plane by the numbers and don't make any allowance for 'potential' instrument errors. Because you really can't know if the instrument error at any point in time is significant. And at any point in time it will change. If your working yourself under Vs based on an assumption of instrument error in the reading, just to make the field, I would question your logic. Because flying outside of the numbers will probably eventually bite you.
I think you will find it very hard to quantify, or perhaps to even generalize because of the number of factors involved.
A couple of more factors that I don't think have been mentioned is the placement of not only the static ports, but also the pitot tube. And relative to these two, what's the exact direction of the relative wind? As well what specific power setting is on the aircraft at that specific moment you want to know what is happening to the ASI. Another factor is the how and what role propwash flow around the aircraft will play.
I think its a good theoretical exercise to think this through, but you will generally see your speed drop when in a slip. Most of it caused by drag, with some resulting from instrument error.
But I wouldn't worry about how much is real and how much is instrument error. Just fly the plane by the numbers and don't make any allowance for 'potential' instrument errors. Because you really can't know if the instrument error at any point in time is significant. And at any point in time it will change. If your working yourself under Vs based on an assumption of instrument error in the reading, just to make the field, I would question your logic. Because flying outside of the numbers will probably eventually bite you.
