Airspeed measuring
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Airspeed measuring
Hi all,
When you see the behaviour of an analog airspeed needle when the airplane is facing variable wind strenght its seems difficult to assess a medium airspeed as the needle moves quickly between high and low values. Do speedtapes act the same way ie moving fast trough values or is it compensated to a medium reading? I'm asking this because on several parts of the flight you need to have a speed value to do stuff, like approaches. If the speedtape is not compensated how do you find the right speed?
Thanks
GD&L
When you see the behaviour of an analog airspeed needle when the airplane is facing variable wind strenght its seems difficult to assess a medium airspeed as the needle moves quickly between high and low values. Do speedtapes act the same way ie moving fast trough values or is it compensated to a medium reading? I'm asking this because on several parts of the flight you need to have a speed value to do stuff, like approaches. If the speedtape is not compensated how do you find the right speed?
Thanks
GD&L
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I put a note in flight test to link back to this thread so that you have the best chance of the answer you want.
Generally, though, fluctuating values can be dampened by various means to slow down the apparent fluctuation and assist interpretation. At the end of the day, the intent of the ASI for the pilot is to achieve an alpha appropriate to the circumstances.
Generally, though, fluctuating values can be dampened by various means to slow down the apparent fluctuation and assist interpretation. At the end of the day, the intent of the ASI for the pilot is to achieve an alpha appropriate to the circumstances.
G,D&L,
Looking at your profile, I'm guessing that you are a student in a reasonably lightweight type such as a C172 or PA28 - if I'm incorrect in that please tell me and I'll moderate the tone of what I'm saying.
Yes, an ASI (either speedtape or dial gauge) can over or undershoot, but it's small - the devices are well damped and so long as you aren't close to the stall or making sudden airspeed changes will read consistently.
However, the aeroplane itself may well be less well damped and can unless you treat it appropriately tend to hunt airspeed. The reasons for this are two different negative feedback loops - one is the aeroplanes attempts to settle on a specific airspeed / AoA / attitude in the short period, the other is the combination of your brain and arm(s) reading the airspeed indications and trying to achieve a particular airspeed. If you get the two out of sync, you end up with the two negative feedback loops combining into a positive feedback loop and the aeroplane starts oscillating - often quite wildly. This can happen in any axis, but is most common in airspeed or pitch attitude. The technical term for this is PIO or "Pilot Induced Oscillation".
Now the important question, how do you deal with this? Essentially what you need to do is start at a speed above or below the speed you are seeking (if it's on finals, above is best or you're in danger of getting too close to the stall, if you're trying to achieve a particular cruise speed, it doesn't really matter) - get the aeroplane roughly trimmed there. Then very slowly, with no large pitch or throttle inputs (different aeroplanes and pilots favour the two controls for this, but the odds are your light aircraft instructor will favour the yoke), creep the speed down to what you're looking for making very small and relaxed inputs. That way, you shouldn't have a problem with the stick.
If you've got into a PIO, you need to do what a Test Pilot would call "opening the loop", which means stop making active inputs. So, clamp the stick or yoke in the middle and let the aeroplane settle down, or make a single (gentle!!!) input to pitch up or down below or above the required airspeed then start sneaking up on it again.
The other thing is, as JT says, the ASI is only really an approximation to AoA - which is what is really important. So, just as a matter of flying technique in a small aeroplane - learn the attitude for various standard manoeuvres (that is the relationship between the horizon and the aeroplane references - such as the top of the instrument panel), fly to that, then make small changes with the occasional glance at the ASI. Chasing airspeed does you no favours in this context anyway.
G
Looking at your profile, I'm guessing that you are a student in a reasonably lightweight type such as a C172 or PA28 - if I'm incorrect in that please tell me and I'll moderate the tone of what I'm saying.
Yes, an ASI (either speedtape or dial gauge) can over or undershoot, but it's small - the devices are well damped and so long as you aren't close to the stall or making sudden airspeed changes will read consistently.
However, the aeroplane itself may well be less well damped and can unless you treat it appropriately tend to hunt airspeed. The reasons for this are two different negative feedback loops - one is the aeroplanes attempts to settle on a specific airspeed / AoA / attitude in the short period, the other is the combination of your brain and arm(s) reading the airspeed indications and trying to achieve a particular airspeed. If you get the two out of sync, you end up with the two negative feedback loops combining into a positive feedback loop and the aeroplane starts oscillating - often quite wildly. This can happen in any axis, but is most common in airspeed or pitch attitude. The technical term for this is PIO or "Pilot Induced Oscillation".
Now the important question, how do you deal with this? Essentially what you need to do is start at a speed above or below the speed you are seeking (if it's on finals, above is best or you're in danger of getting too close to the stall, if you're trying to achieve a particular cruise speed, it doesn't really matter) - get the aeroplane roughly trimmed there. Then very slowly, with no large pitch or throttle inputs (different aeroplanes and pilots favour the two controls for this, but the odds are your light aircraft instructor will favour the yoke), creep the speed down to what you're looking for making very small and relaxed inputs. That way, you shouldn't have a problem with the stick.
If you've got into a PIO, you need to do what a Test Pilot would call "opening the loop", which means stop making active inputs. So, clamp the stick or yoke in the middle and let the aeroplane settle down, or make a single (gentle!!!) input to pitch up or down below or above the required airspeed then start sneaking up on it again.
The other thing is, as JT says, the ASI is only really an approximation to AoA - which is what is really important. So, just as a matter of flying technique in a small aeroplane - learn the attitude for various standard manoeuvres (that is the relationship between the horizon and the aeroplane references - such as the top of the instrument panel), fly to that, then make small changes with the occasional glance at the ASI. Chasing airspeed does you no favours in this context anyway.
G
GD&L, all speed indicators, as you indicate may be compensated. Older ASIs and sensing devices had inherent damping from the sensors and the mechanical display mechanism; generally these gave an adequate compromise between accuracy and usability in dynamic situations.
The newer sensors, display media, and computation allows the designer to fine tune the display or seek other compromises; in the early days of Air Data Computers and EFIS the overuse of the newfound technology led to interesting mistakes.
Modern sensor systems provide accurate source information; some smoothing can be applied prior to the ADC. On larger aircraft the pitot static systems may be cross coupled so as to average out transient errors due to local airflow / yaw during dynamic maneuvers.
Some speed tapes have relatively high damping, but any adverse effects of this can be offset by using a speed trend vector. This vector primarily replaces the loss of speed trend and acceleration cues that are inherent in dial displays via angular awareness and a much longer display scale. Trend vectors may be include a function of inertial acceleration for damping / quickening, but this requires a good balance of terms and often complicated inertial axis resolution. A poor system for example could indicate speed loss at the stall with an accelerating speed trend; this is unacceptable for a civil aircraft certification.
Some speed tapes now have stall margin indicators which are computed via an AOA input. A really clever system could use actual aircraft wt, ‘g’, configuration, and lift coefficients to correlate vane AOA with indicated speed. Less complex systems have simpler routines and make assumptions about lift characteristics based on trim position, the end results are surprisingly accurate.
Another aspect of your observation of speed fluctuation may be that larger aircraft (high mass) are less prone to speed fluctuation due to inertia; this may also be a function of the aircraft design – speed / AOA stability. In really turbulent conditions where there are large fluctuations the crew may add a small value to the required speed to counter any hazardous error; generally the crews do not worry about small fluctuations. However on smaller aircraft due to less inertia, etc, speed fluctuations can be disconcerting. The significance of the fluctuation is proportional to time, the shorter the period the less significant, the longer then the more attention that has to be paid to the parameter. Judgment of this significance has to be learnt and may be encompassed under general airmanship.
The newer sensors, display media, and computation allows the designer to fine tune the display or seek other compromises; in the early days of Air Data Computers and EFIS the overuse of the newfound technology led to interesting mistakes.
Modern sensor systems provide accurate source information; some smoothing can be applied prior to the ADC. On larger aircraft the pitot static systems may be cross coupled so as to average out transient errors due to local airflow / yaw during dynamic maneuvers.
Some speed tapes have relatively high damping, but any adverse effects of this can be offset by using a speed trend vector. This vector primarily replaces the loss of speed trend and acceleration cues that are inherent in dial displays via angular awareness and a much longer display scale. Trend vectors may be include a function of inertial acceleration for damping / quickening, but this requires a good balance of terms and often complicated inertial axis resolution. A poor system for example could indicate speed loss at the stall with an accelerating speed trend; this is unacceptable for a civil aircraft certification.
Some speed tapes now have stall margin indicators which are computed via an AOA input. A really clever system could use actual aircraft wt, ‘g’, configuration, and lift coefficients to correlate vane AOA with indicated speed. Less complex systems have simpler routines and make assumptions about lift characteristics based on trim position, the end results are surprisingly accurate.
Another aspect of your observation of speed fluctuation may be that larger aircraft (high mass) are less prone to speed fluctuation due to inertia; this may also be a function of the aircraft design – speed / AOA stability. In really turbulent conditions where there are large fluctuations the crew may add a small value to the required speed to counter any hazardous error; generally the crews do not worry about small fluctuations. However on smaller aircraft due to less inertia, etc, speed fluctuations can be disconcerting. The significance of the fluctuation is proportional to time, the shorter the period the less significant, the longer then the more attention that has to be paid to the parameter. Judgment of this significance has to be learnt and may be encompassed under general airmanship.
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If you want accurate performance data from flight tests the first thing is to find some non turbulent air.
The same goes for a lot of handling tests, especially stalling or stability and control. Some handling assessments do need strong gusty winds to provide a qualitative answer as to whether the handling is up to flying in really bad conditions.
Bottom line - gusty conditions are no use for other than finding out how the aeroplane handles in such conditions.
The same goes for a lot of handling tests, especially stalling or stability and control. Some handling assessments do need strong gusty winds to provide a qualitative answer as to whether the handling is up to flying in really bad conditions.
Bottom line - gusty conditions are no use for other than finding out how the aeroplane handles in such conditions.
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Some speed tapes now have stall margin indicators which are computed via an AOA input. A really clever system could use actual aircraft wt, ‘g’, configuration, and lift coefficients to correlate vane AOA with indicated speed. Less complex systems have simpler routines and make assumptions about lift characteristics based on trim position, the end results are surprisingly accurate.
Having determined I'm using up "50% of my alpha" I can reasonably translate that into "50% of my lift" and use the inverse square relationship to determine what my stall speed factor is. Dividing my current speed by the ratio thus obtained, I get the stall speed for my current configuration etc. By multiplying by known ratios I could present stall warning speed, too.
The advantage of this method is that it removes any need to determine aircraft weight, etc., and it implicitly accounts for load factor, and will increase the 'stall speed' presented to account for load factor. It therefore gives a better indication to the pilot of the manoeuvre margin he has above stall in turning flight - margin to the 1'g' stall speed isn't much direct use in that case.
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Thanks to all, this is valuable info to me. 
Genghis the Engineer not a student yet. I am close to start my PPL studies, just beginning to really really learn the art
Thats precisely my question, safetypee , because you have to deal with so many parameters and if one of those things isn't stable enough for you to do a relaxed instrument check, then you can put yourself at unnecessary risk, by start to give to much attention to one detail. Ok, for you guys its like driking water, but for me it makes me a little dizzy 
John Farley I was on a C172 flight on those gusty wind conditions and was mentaly "driving" for fun, checking the instruments and looking at the reactions of the pilot and thought "how can he assess/feels the lift if the airspeed needle sometimes jumps like crazy."
Again thank you all for your replies.
Genghis the Engineer not a student yet. I am close to start my PPL studies, just beginning to really really learn the art
The significance of the fluctuation is proportional to time, the shorter the period the less significant, the longer then the more attention that has to be paid to the parameter.
John Farley I was on a C172 flight on those gusty wind conditions and was mentaly "driving" for fun, checking the instruments and looking at the reactions of the pilot and thought "how can he assess/feels the lift if the airspeed needle sometimes jumps like crazy."
Again thank you all for your replies.
