The effect on perf testing of non standard weather
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The effect on perf testing of non standard weather
To all you expert test pilots, with regard to climb performance testing.
If climb performance were tested in a standard atmosphere and the test repeated in a cold temperature and high atmospheric pressure scenario (like we had a week or so ago,) what would the effect on the figures be?
Is there a best time/place/atmosphere for the testing to make the figures look better?
Would smooth air allow better climb figures than turbulent air?
If climb performance were tested in a standard atmosphere and the test repeated in a cold temperature and high atmospheric pressure scenario (like we had a week or so ago,) what would the effect on the figures be?
Is there a best time/place/atmosphere for the testing to make the figures look better?
Would smooth air allow better climb figures than turbulent air?
A BN2T pilot from Hampshire, not police ASU at Lee are you by any chance? If so, I wrote the flight test plans and most of the clearance report for your Islander (although as it happens wasn't available for testing week so have never actually flown in the beast).
Anyhow for smaller aircraft like yours, it's acceptable and usual practice to reduce all climb perf data to density altitude, then for actual performance convert to local conditions. In other words, we assume that at a given density altitude the climb (and take-off) performance will be identical.
Given that, obviously an increased pressure setting on your altitimeter, will apparently give better climb performance at a given pressure altitude (since density altitude will be better assuming all else is equal). Similarly at ISA- temperatures you'll get better climb performance for a given pressure altitude, and at ISA+ temperatures you'll get poorer climb performance as the air density drops off.
You'll find this covered quite well by the BN2T operators manual, figures 5-12 to 5-16.
If you need performance data over a reasonable height bracket, it's normal to do matched pairs of performance climbs, into and downwind, which does a reasonable job of removing variation due to local conditions - although to be frank so long as you are 50 miles or so from the nearest front throughout the test, it's probably nugatory effort.
All aircraft performance will be better in still than in turbulent air (except possibly glide performance which may be better if the turbulence is primarily thermic), essentially because of the efficiency advantages of steady state conditions. Just compare GPS groundspeed on a long cruise leg, at constant IAS / Alt you'll lose anything up to 10% of groundspeed in moderate turbulence. I've never seen an actual analysis of loss of climb performance due to turbulence, so couldn't guess what magnitude of loss you'll see.
As to making the figures looking better, any grown-up pilot or engineer assessing an aircraft will be standardising to ISA / fixed weight conditions in any analysis they do (or comparing actual performance to published data at same conditions). But, if somebody is idiot enough not to do this, then clearly lightweight, cold, high pressure setting, nil turbulence will give better apparent results. And even more obviously, a good headwind component will give a better apparent climb gradient.
If you are looking at Islander performance data, virtually all of B-N's performance testing is done to the SW of the IOW which, with the usual prevailing wind, means no obstacles for about 7000 miles or so, and hence very little turbulence. So, real-world performance over land, particularly in summer flying conditions, will routinely be a little poorer.
Looking at your profile you surprise me with the question slightly, all this is covered pretty well by the ATPL(A) syllabus isn't it?
G
Anyhow for smaller aircraft like yours, it's acceptable and usual practice to reduce all climb perf data to density altitude, then for actual performance convert to local conditions. In other words, we assume that at a given density altitude the climb (and take-off) performance will be identical.
Given that, obviously an increased pressure setting on your altitimeter, will apparently give better climb performance at a given pressure altitude (since density altitude will be better assuming all else is equal). Similarly at ISA- temperatures you'll get better climb performance for a given pressure altitude, and at ISA+ temperatures you'll get poorer climb performance as the air density drops off.
You'll find this covered quite well by the BN2T operators manual, figures 5-12 to 5-16.
If you need performance data over a reasonable height bracket, it's normal to do matched pairs of performance climbs, into and downwind, which does a reasonable job of removing variation due to local conditions - although to be frank so long as you are 50 miles or so from the nearest front throughout the test, it's probably nugatory effort.
All aircraft performance will be better in still than in turbulent air (except possibly glide performance which may be better if the turbulence is primarily thermic), essentially because of the efficiency advantages of steady state conditions. Just compare GPS groundspeed on a long cruise leg, at constant IAS / Alt you'll lose anything up to 10% of groundspeed in moderate turbulence. I've never seen an actual analysis of loss of climb performance due to turbulence, so couldn't guess what magnitude of loss you'll see.
As to making the figures looking better, any grown-up pilot or engineer assessing an aircraft will be standardising to ISA / fixed weight conditions in any analysis they do (or comparing actual performance to published data at same conditions). But, if somebody is idiot enough not to do this, then clearly lightweight, cold, high pressure setting, nil turbulence will give better apparent results. And even more obviously, a good headwind component will give a better apparent climb gradient.
If you are looking at Islander performance data, virtually all of B-N's performance testing is done to the SW of the IOW which, with the usual prevailing wind, means no obstacles for about 7000 miles or so, and hence very little turbulence. So, real-world performance over land, particularly in summer flying conditions, will routinely be a little poorer.
Looking at your profile you surprise me with the question slightly, all this is covered pretty well by the ATPL(A) syllabus isn't it?
G
Slim , further to G`s input a few suggestions; if you can fly early morning, or late evening and preferably over flat terrain/sea, you will minimise any turbulence effects.If there is any wind, preferably constant with increasing height, then I always try to set it on a beam and do a climb, descend and do another on the reciprocal hdg.
It is most important to set the a/c up before your start height at the correct power and balance/trim, otherwise you may find yourself still fiddling about and not stable in the climb. Depending on the a/c`s ROC, that may be 500-1000 ft below your start alt.A good flt.test observer should assist with adjusting power as you climb to ensure you keep max. Most important if you are doing S/E climbs.
Some a/c altimeter systems are a little "sticky" and lag and if you are looking for s/e performance every foot or so of ROC is a bonus, so if you can ,get your FTE to borrow a "vibrator",or even a battery toothbrush and hold it against the panel near the altimeter you are using.
He could of course just keep tapping the panel, not the gauge,
but I think it has a slightly more professional nuance and he can use it as a good excuse, ie "flight-test equipment"!!
I say "he", but of course it may be otherwise !!
Further, if you can do two sets of climbs , one at high weight, and one at low weight, and take stabilised temperature readings at the mid-climb point you will minimise errors.
Smooth flying and minimal control inputs are the secret, ie if you are a couple of knots too fast or slow, don`t chase it, just maintain it as a steady speed..
Happy climbing !!
It is most important to set the a/c up before your start height at the correct power and balance/trim, otherwise you may find yourself still fiddling about and not stable in the climb. Depending on the a/c`s ROC, that may be 500-1000 ft below your start alt.A good flt.test observer should assist with adjusting power as you climb to ensure you keep max. Most important if you are doing S/E climbs.
Some a/c altimeter systems are a little "sticky" and lag and if you are looking for s/e performance every foot or so of ROC is a bonus, so if you can ,get your FTE to borrow a "vibrator",or even a battery toothbrush and hold it against the panel near the altimeter you are using.
He could of course just keep tapping the panel, not the gauge,
but I think it has a slightly more professional nuance and he can use it as a good excuse, ie "flight-test equipment"!!
I say "he", but of course it may be otherwise !!
Further, if you can do two sets of climbs , one at high weight, and one at low weight, and take stabilised temperature readings at the mid-climb point you will minimise errors.
Smooth flying and minimal control inputs are the secret, ie if you are a couple of knots too fast or slow, don`t chase it, just maintain it as a steady speed..
Happy climbing !!
Shawn If doing reciprocals, yes, of-course. The principal being that you can most easily then spiral down (allowing the aircraft to drift downwind in the process) before repeating the test in the same slice of air. However, because the time-to-height may be affected by crossing the Isobars (i.e. the relationship between geometric and pressure altitude will be inconstant) I'd only do the test crosswind if I were flying reciprocals in that way (because the mean of the two runs will give a pretty good equivalence to true still air). If not doing reciprocals I'd prefer to plan into or downwind, so as to remove that effect. If doing a typical light aircraft 1000'-5000' or thereabouts climb perf with a limited flying hours budget and a limited test area, I'd probably fly climbing downwind, and performance glides on the reciprocal into wind (in, of-course, good conditions with not too many Isobars and the nearest front on the next continent.)
Sycamore I challenge you to propose that particular technique and use of equipment to any lady member of aircrew, of any level of experience and qualifications, and (should you survive) report back on her response
More seriously, I believe that glider pilots routinely tap the instrument glass with a soft pencil and find that does the job well. But, it's really only turbo-jets and gliders that do suffer routinely from this, and I doubt that our man's Islander might not have enough vibration at the cockpit to keep the pressure instruments from sticking.
Mentioning data recording, there are two bodies of opinion - some TPs / FTEs record time at fixed heights, some record height at fixed times. It doesn't really matter, other than the data analysis is done slightly differently - but do make sure that whichever approach is taken both the pilot and FTE are expecting to do the same thing. I also often alternate recording IAlt and IRoC, say the first at odd-numbered hundreds of feet, and the second at evens. If there's an inconsistency I usually find it's the VSI that has something wrong with it.
G
Sycamore I challenge you to propose that particular technique and use of equipment to any lady member of aircrew, of any level of experience and qualifications, and (should you survive) report back on her response
More seriously, I believe that glider pilots routinely tap the instrument glass with a soft pencil and find that does the job well. But, it's really only turbo-jets and gliders that do suffer routinely from this, and I doubt that our man's Islander might not have enough vibration at the cockpit to keep the pressure instruments from sticking.Mentioning data recording, there are two bodies of opinion - some TPs / FTEs record time at fixed heights, some record height at fixed times. It doesn't really matter, other than the data analysis is done slightly differently - but do make sure that whichever approach is taken both the pilot and FTE are expecting to do the same thing. I also often alternate recording IAlt and IRoC, say the first at odd-numbered hundreds of feet, and the second at evens. If there's an inconsistency I usually find it's the VSI that has something wrong with it.
G
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More about perf
Those that know me would also question my grip of the subject, but the main reason for my question is this.
Aircraft has new "thing" bolted to same place as last "thing", empty weight is, surprisingly, only 20 lb greater. Aircraft has to be test flown to ensure new "thing" doesn't affect handling and performance.
The "things" are very similar in size and shape.
Why then does the new thing cause a significant reduction in declared single engine climb performance, when there ain't much to spare.
The reason I cling to my anonymity is that the quote "it must be because we did the testing when it was cold and high pressure so the air must have been descending," comes from very clever people who I respect, yet sounds odd to me.
Aircraft has new "thing" bolted to same place as last "thing", empty weight is, surprisingly, only 20 lb greater. Aircraft has to be test flown to ensure new "thing" doesn't affect handling and performance.
The "things" are very similar in size and shape.
Why then does the new thing cause a significant reduction in declared single engine climb performance, when there ain't much to spare.
The reason I cling to my anonymity is that the quote "it must be because we did the testing when it was cold and high pressure so the air must have been descending," comes from very clever people who I respect, yet sounds odd to me.
Do a Hover - it avoids G
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Do you have any evidence of the ACTUAL performance before the mod?
Unless the tests that are to be carried out post mod are carried out immediately pre mod there will always be doubt about the actual effect of the mod
Unless the tests that are to be carried out post mod are carried out immediately pre mod there will always be doubt about the actual effect of the mod
There are a lot of factors to be taken into consideration when fitting mods to a/c and then perf. checking;ie, how long since the previous perf check,; deterioration/improvement maybe because an engine/prop. may have been changed; same pilot doing the tests, using his technique; instrument calibration recently( including flight instruments). As JF says, before any mod , which may look similar to previous equipment, it is always adviseable to do a datum check so you know what the change has been.
If these back-to-back tests can be carried in the same time-frame,and weather conditions, then you will have a good comparison.
I have spoken to a forecaster who says it is very difficult to be precise about the effect of a low/high pressure system with respect to airmass rising/falling rates as it is dependent on too many factors,ie smoke and mirrors magic,isobars,temperature at upper and lower levels etc,etc., although actual rates are fairly low.That said, if previous climbs have been reduced correctly, then you should have a good basis for comparison.
What you can do, however is to do a series of "partial climbs" at slightly differing airspeeds. ie if normal climb speed is 90 kts, then try climbs at 95, and 85 kts as well. It should give you a spread of performance. BEWARE,if you are looking at s/e perf, as you may be getting close to Vmca! The figures are only illustrative, so you use the POH.
I presume its a turbine you fly , so you may find that a slightly faster speed is beneficial.
If these back-to-back tests can be carried in the same time-frame,and weather conditions, then you will have a good comparison.
I have spoken to a forecaster who says it is very difficult to be precise about the effect of a low/high pressure system with respect to airmass rising/falling rates as it is dependent on too many factors,ie smoke and mirrors magic,isobars,temperature at upper and lower levels etc,etc., although actual rates are fairly low.That said, if previous climbs have been reduced correctly, then you should have a good basis for comparison.
What you can do, however is to do a series of "partial climbs" at slightly differing airspeeds. ie if normal climb speed is 90 kts, then try climbs at 95, and 85 kts as well. It should give you a spread of performance. BEWARE,if you are looking at s/e perf, as you may be getting close to Vmca! The figures are only illustrative, so you use the POH.
I presume its a turbine you fly , so you may find that a slightly faster speed is beneficial.
"it must be because we did the testing when it was cold and high pressure so the air must have been descending," comes from very clever people who I respect, yet sounds odd to me.
How hot, cold, high-pressure, low-pressure it was then is only relevant insofar as you should have done enough measurement to establish actual test conditions throughout your own testing - it's the analysis that then becomes important. (And obviously that you tested the right things at the right conditions - for example are you absolutely sure there was no external change that might have affected the static, and thus the indicated best climb speed?)
G
Genghis raises an important point about the static ports, and it also applies to the pitot head(s) as well. In the original configuration,was there any flight testing carried out to determine PEC`s, was it checked with the manufacturer, ie approved?If not then you may have to seek their advice. There also may be a CofG effect depending on the position of the equipment as well. To exaggerate somewhat,you will have minimum trim drag with the a/c at an aft CofG,and more at a fwd one.This may mean in doing s/e climbs especially in "light" twins that you may well be using more control and hence have more drag.
There are references on this forum as to carrying out PEC`s in a/c not equipped with full test insrumentation..Genghis will offer advice I`m sure, as he`s doing a thesis. BS
There are references on this forum as to carrying out PEC`s in a/c not equipped with full test insrumentation..Genghis will offer advice I`m sure, as he`s doing a thesis. BS
And it can be quite fiddly telling the difference!
As Bristol Sycamore says, if I can help out a bit with analysis or test methods, please just ask. If you'd prefer not to do so in open forum by all means Email me on [email protected] ; testing relatively light and simple aircraft with minimum instrumentation is a bit of a long term interest of mine. (And yes, doing a PhD thesis but that's largely for the benefit of my ego rather than anything else.)
G
As Bristol Sycamore says, if I can help out a bit with analysis or test methods, please just ask. If you'd prefer not to do so in open forum by all means Email me on [email protected] ; testing relatively light and simple aircraft with minimum instrumentation is a bit of a long term interest of mine. (And yes, doing a PhD thesis but that's largely for the benefit of my ego rather than anything else.)
G
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Me again
I'm not involved with flight testing myself, hence the urge to consult with those that are.
The aircraft returned from modification with the new manual supplements describing a greatly reduced single engine climb performance.
My job is to live with it, and the reduced fuel load at night in crap weather etc.
The aircraft returned from modification with the new manual supplements describing a greatly reduced single engine climb performance.
My job is to live with it, and the reduced fuel load at night in crap weather etc.
