M20F Speed Checks... Comparison Please?

[N601RX]

The early F model POH is pretty basic and doesn't include a CAS chart, but here is one for a J. It appears to match what I see on my F.

[jnisley]

Here's a flight I did earlier this summer in my 1969 M20F TN, it has most if not all of the Laser mods but has old style windshield.

Estimated weight 2350

17,800'

MP 24.5

RPM 2500

FF 11gph

IAS 143MPH

TAS 195MPH 169.6 knots

GS 171MPH

Oil temp 194 degrees

OAT 22 degrees F

Here's a flight I did Saturday on a cross country from Iowa-Pa

Estimated weight 2600

15,500'

MP 24.5

RPM 2450

FF 10.5-11gph

IAS didn't record

TAS 185MPH 160.8 knots

GS 200+MPH

Oil temp 195-200 degrees

OAT 28 degrees F

[carusoam]

Nice data collection, John!

Not bad for 1960's technology.

Best regards,

-a-

[jnisley]

Most times I fly in 6,000 to 11,000 range and don't use the turbo, I have an Insight TAS 1000 fuel/air computer (10 year old technology) that's coupled with my 430W which calculates my TAS (plus a host of other things), I would say that my plane averages between 143 and 153 knots TAS when flying ROP and 8 knots or so slower when flying LOP

[Jerry 5TJ]

I use the 4 quadrant method for speed checks, but I fly four orthogonal ground tracks, not headings.  That takes care of the wind issue with no trigonometric calculations.

 

As for IAS errors in half-century old Pitot Static systems: I find my "C" model's computed TAS is generally many knots faster than the figures I get from the ground track averages.   Based on IAS and temperature I have calculated 157 "KTAS" at 8000 feet with 2400 RPM and wide-open throttle ~23"  but that's a flat out instrumentation lie:  GIGO as we used to say.  Since the speed run data for my plane shows it is really doing about 144 KTAS under those conditions the IAS is about 157/144 or about 9% high. 

 

The P46 I fly has two identical pitot probes, one at each wing tip.  They drive two identical 3" airspeed indicators.  They were calibrated in 2006 and the left one shows 2-3 knots faster than the right at cruise of 160 indicated.    The left system is connected to the G500 which, to no surprise, shows the same reading on its tape as the round ASI on the left indicates.  

 

I suspect if two nearly new systems installed on the same platform differ by several percent then the reported values in our 30 to 50 year old Mooney fleet may well spread 10 percent or more.   That alone would account for all the "Fast" and "Slow" planes debated on this forum. 

 

 

[Ragsf15e]

Bob, copy, you're right, I didn't see the couple of important distinctions in your data.  I'd still like to see a 3 way test to take out the pitot error.

 

Jerry - I'm no "mathologist", but flying 4 ground tracks instead of headings is not negating the wind.  In fact, you're probably going faster than indicated by the test, so that should make you feel good.  If you fly a ground track you must be crabbed into the wind to hold the track - probably on all four legs unless two of them happen to be directly into/away from the wind.  The greater the crab, the less your TAS down the intended track.  Imagine flying with a 100kt direct crosswind.  You'd be pointed something like 40 degrees into it to hold a track and you're now using 60 of your 140 knots TAS just to hold you against the wind.  Get a strong enough crosswind and you have to point directly into it just to avoid being blown across your track and you won't be moving along that track at all.

 

Unless we're talking about strong winds, I doubt there's more than a knot or two difference in the 2 or 3 methods talked about here, but I still think the 3 way test is the most accurate since the Navy Test Pilot School has some papers posted online about it with a lot of squiggly math symbols that I forgot 20 years ago.

 

We need a mathematician on Mooneyspace!

 

Rags

[carusoam]

We have a really good one of those...!

Best regards,

-a-

[Hank]

Erik Bollt! Calling Dr. Erik Bollt . . .

[aviatoreb]

 I'm no "mathologist", 

 

I'm a mathologist.

 

We have a really good one of those...!

Best regards,

-a-

 

You aren't talking about me.

 

Erik Bollt! Calling Dr. Erik Bollt . . .

 

Norm!  It's like Cheers around here.  I never felt so welcome!

 

 

Ok guys - Dr Erik Bollt mathologist at your service.  Professor of Mathology.  Math'o'-logician.

[aviatoreb]

Bob, copy, you're right, I didn't see the couple of important distinctions in your data.  I'd still like to see a 3 way test to take out the pitot error.

 

Jerry - I'm no "mathologist", but flying 4 ground tracks instead of headings is not negating the wind.  In fact, you're probably going faster than indicated by the test, so that should make you feel good.  If you fly a ground track you must be crabbed into the wind to hold the track - probably on all four legs unless two of them happen to be directly into/away from the wind.  The greater the crab, the less your TAS down the intended track.  Imagine flying with a 100kt direct crosswind.  You'd be pointed something like 40 degrees into it to hold a track and you're now using 60 of your 140 knots TAS just to hold you against the wind.  Get a strong enough crosswind and you have to point directly into it just to avoid being blown across your track and you won't be moving along that track at all.

 

Unless we're talking about strong winds, I doubt there's more than a knot or two difference in the 2 or 3 methods talked about here, but I still think the 3 way test is the most accurate since the Navy Test Pilot School has some papers posted online about it with a lot of squiggly math symbols that I forgot 20 years ago.

 

We need a mathematician on Mooneyspace!

 

Rags

 

Rags, I am not certain specifically what you are saying but I will backup what I think you are saying - and in another of your posts too?

 

It seems you are saying two things.  

a) there is CAS errors due to crabbing to maintain ground track in a direction other than parallel the wind direction.  This seems entirely correct.  Remember I am not an aero prof, but a math prof with more aero training than usual maybe, but I have done very little professional work with Pitot tubes.  Still I am confident to say that yes if you are crabbing that introduces a CAS error that would be expected to be increasingly worse the greater the crab angle.  And so likewise commuted TAS from uncorrected IAS will also have corresponding errors.

using trigonometry alone, you can compute TAS from ground tracks, at least 3 ground tracks "in general configuration" (meaning linearly independent meaning not all 3 in the same line - 2 always make a line) so at least 3 means 4 is fine and more is fine too.  Its standard trig even if one needs to be skillful with it.  The pitot tube is not involved in GPS ground speed computations so any CAS errors cannot be part of any error in using any GPS based method.  

 

So why use a formula with 3 directions rather than the easier to understand/compute with 4 cardinal direction method?  Each measurement (of anything in physics) has errors so the fewer the better.  Crabbing causes drag so this is a real cause of reduced speed, so this is a systematic error to true airspeed max potential so the fewer measurements the better there.  But more measurements is generally considered good if errors are unbiased since you expect errors to average out in such case, but not in this case if the errors are systematically biased.  Also it is a nuisance and takes time and lots of space to do 4 cardinal directions and so a 3 arbitrary directions is operationally easier - I bet this is a major reason NTPS likes 3 directions - easier to data collect vs easier to compute with.  

 

In fact, I see no reason (other than it amplifies measurement errors - which is important for real test pilots I am sure - but not amateur hacks like me) you could not use the 3 directions method in roughly aligned directions without turing around.  I.e., try out 180, 190, and 200 degrees while in cruise flight to bearing 190 with brief deviations from course.

 

You mentioned earlier that 3 directions is better because it uses math, but 4 directions uses math too - averaging 4 cardinal directions is a special case of a trig method where you have sines and cosines of perpendicular angles so they work out to 0's and 1's instead of irrational numbers.  But it is every much a method of mathology.  (Funny every time I try to spell the fun made up mathology word here, apple-autocorrect "fixes" it to mythology - maybe that should tell me something about what I do).

[Bob_Belville]

Alternate test procedure, simpler and quicker. (Less dependent on steady state winds.)

 

Since the Aspen displays the wind direction (E6B calculation comparing heading to track) why not turn to that heading and note the GPS GS, then turn 180 deg note GPS GS again. The TAS should be the average of the 2, independent of any pitot/static error. And if you compare that average to the displayed TAS you'll have the ASI error, at least for that IAS. 

 

If not, why not?

[aviatoreb]

Alternate test procedure, simpler and quicker. (Less dependent on steady state winds.)

 

Since the Aspen displays the wind direction (E6B calculation comparing heading to track) why not turn to that heading and note the GPS GS, then turn 180 deg note GPS GS again. The TAS should be the average of the 2, independent of any pitot/static error. And if you compare that average to the displayed TAS you'll have the ASI error, at least for that IAS. 

 

If not, why not?

 

That should work.

 

In fact you can solve an optimization problem on the fly and get the same result of flying directly into the wind.

 

Solve argmin_(theta) GS(theta) 

 

In other words in your steam gauge airplane, find the bearing that minimizes your GS.  Then write down that airspeed.  Then turn 180 degrees which should be your maximum GS and write that number down and take the average.  No induced drag due to crabbing - quite right Bob.

 

Still the arbitrary 3 bearings method is most convenient since you could in principle due one speed on course, then 2 more say 20 degrees left and right of course.

[N601RX]

The 3 angles need to be spread out to be accurate. When the 3 tracks are close together there is very little difference in the gps speed speed between the 3 angles. Keep in mind that most gps will round the speed off to a whole number.

[aviatoreb]

The 3 angles need to be spread out to be accurate. When the 3 tracks are close together there is very little difference in the gps speed speed between the 3 angles. Keep in mind that most gps will round the speed off to a whole number.

 

I agree.  I already said, "I see no reason (other than it amplifies measurement errors - which is important for real test pilots I am sure - but not amateur hacks like me)"

 

But how much do they need to be spread out to get a decent result?  I don't know.  I would not be surprised if 20 or 30 degrees is good enough.  I could be wrong.  None of us here knows specifically how much error is involved in each measurement (errors in measured GS and errors in measured bearing)  but if someone would tell me then it is straight forward to compute the error propagation.  I can do that as I am sure most of the engineers here can too.  Without knowledge of these errors, then we are just guessing how much angular separation is required.

 

​Now thinking about it, I am tempted to try 20, 30 and 120 degree angular separation 3 speed measurements and see how they differ.  I will guess 1 or 2 knots.

[Hank]

So, Doc Erik, Prof of Mathology, please confirm one little detail for this here Engineer with atrophied mathological skills.

All of this airspeed calculating should be done using data generated by flying headings, not ground tracks, and we should record both IAS and GS numbers?

I flew somewhat over 1300 nm westbound with average block GS of 127-128 knots, and a barely different return track several days later, averaging 151 knots. So I'm happy saying my block speed is 140 knots, but I failed to record IAS, altitude and temp. Mostly 8500 westbound and mostly 9500 eastbound.

Guess I should start over, call it 2-3 minutes in each direction, on the same day at the same altitude. Allow 1-2 minutes to stabilize on each heading, then run an additional minute or two and record both speeds. Repeat for as many altitudes as I feel like doing (say, my favorite cruising altitudes). While I'm at it, I should record power settings, altimeter setting and OAT at each; no wind direction, alas, due to inability to display same on G430W, Aspen funding not currently available.

Then run everything through the handy TAS Calculator referenced above? How, then, do I figure ASI error?

Sure, I could write out and solve the vector sum, but if I'm flying anything close to 120° apart, the sum will be near zero; if I fly three headings 90° apart, two will come close to canceling out, leaving me with the other number and a small correction. If I fly three headings only 10-20° apart, the sum will be close to 3X IAS. Whassup wi' dat??

Or am I more mathologically atrophied than I think I am?

[DonMuncy]

I'm not a math genius, but it seems to me that if you are not flying directly into or away from the wind, you are going to have some cross wind component. And whether you are flying a compass direction or a heading would not make any difference in the calculation. If you fly 4 headings that are 90 degrees apart and average the figures it should be exactly the actual no wind speed.

Am I wrong? Erik?

[bonal]

Why not just wait till its calm with little wind affect and get your numbers to be as close to equal as possible for the least amount of cross wind component then make your test legs. This post has gotten me all jacked up to get out and do some tests of my own.

[adrian]

Old E model (probably 350# below gross)

 

9500' OAT 5C

21.6/2540 70% ROP  10.2 GPH

136 KIAS

158 KTAS 

 

I get almost exactly the same results as Bob for a similarly old E, with cowl closure and 201-style windshield.

 

As for determining TAS, it is really quite easy.  I once had the pleasure of working with a test pilot named Ralph Kimberlin, who had worked for Piper, Rockwell and Beechcraft then freelanced while working for 26 years as a professor at the University of Tennessee Space Institute.  He wrote a definitive text book - Flight Testing of Fixed Wing Aircraft.  He's pretty good at the math, as well as being a superb pilot and thoroughly nice gentleman.

 

Here's his method:

 

1.  In calm air fly a constant, known heading at a constant altitude and a constant indicated airspeed for long enough for your GPS to show groundspeed and track.  It should only need a few seconds of stable flight with a modern GPS.

2.  Write down GS, track and heading.

3.  Turn to a reciprocal heading (not a reciprocal track!), repeat the test at the same altitude and indicated airspeed and write down the results.  

4.  Land!

5.  Use the heading and track figures to determine drift angle.  Let us call it "D".

6.  The very simple formula:  cosine(D)  *  GS   gives the component of ground speed along the aircraft heading.  Determine this value for both headings.  The reciprocal headings cancel any wind effects.

7.  Take the average of the two results found in step 6.  That gives your TAS.

[Ragsf15e]

Someone nailed it about 5 posts ago... the easiest way is to fly in calm air!  Or maybe to fly directly into the wind... however, that pretty Aspen displayed wind value is using your pitot system because it uses IAS, temp, and pressure to figure TAS, then uses TAS, GS, HDG, and Track to figure the winds.  

 

Sorry to our mathologist, but I wasn't talking about CAS being affected by a crosswind.  Here's why holding a ground track (say North) will always be slower than holding the same HEADING (North) with any crosswind... You will have to crab into the crosswind to hold your ground track.  That will always increase the headwind component (if it was a headwind or direct cross) or it will decrease the tailwind component if it was a quartering tailwind.  No matter what, holding a ground track crabs you into the wind and slows your forward progression. 

[Bob_Belville]

That should work.

 

In fact you can solve an optimization problem on the fly and get the same result of flying directly into the wind.

 

Solve argmin_(theta) GS(theta) 

 

In other words in your steam gauge airplane, find the bearing that minimizes your GS.  Then write down that airspeed.  Then turn 180 degrees which should be your maximum GS and write that number down and take the average.  No induced drag due to crabbing - quite right Bob.

 

Still the arbitrary 3 bearings method is most convenient since you could in principle due one speed on course, then 2 more say 20 degrees left and right of course.

ISTM that absent the Aspen type display of wind, if you steer to have the GPS

track match the GPS heading you're in line with the wind and you can note GS on that heading and it's reciprocal.

[Bob_Belville]

Someone nailed it about 5 posts ago... the easiest way is to fly in calm air!  Or maybe to fly directly into the wind... however, that pretty Aspen displayed wind value is using your pitot system because it uses IAS, temp, and pressure to figure TAS, then uses TAS, GS, HDG, and Track to figure the winds.  

 

Sorry to our mathologist, but I wasn't talking about CAS being affected by a crosswind.  Here's why holding a ground track (say North) will always be slower than holding the same HEADING (North) with any crosswind... You will have to crab into the crosswind to hold your ground track.  That will always increase the headwind component (if it was a headwind or direct cross) or it will decrease the tailwind component if it was a quartering tailwind.  No matter what, holding a ground track crabs you into the wind and slows your forward progression. 

I think Adrian's reference to calm air was smooth, non-turbulent conditions to get a stable airspeed reading. Not that you should or could find a day without wind. Since we'd like to get a reading accurate to a knot or 2, we will have to use a technique that accounts for wind. But we can eliminate the high school trig calculations by flying in line with the wind.  

[Marauder]

When you rocket scientists figure out the correct way to launch this rocket, will one of you summarize it in a format that us astronauts know what buttons to push?

I have a nephew who just got hired as an aerodynamic engineer -- should I ask him?

[N601RX]

I agree.  I already said, "I see no reason (other than it amplifies measurement errors - which is important for real test pilots I am sure - but not amateur hacks like me)"

 

But how much do they need to be spread out to get a decent result?  I don't know.  I would not be surprised if 20 or 30 degrees is good enough.  I could be wrong.  None of us here knows specifically how much error is involved in each measurement (errors in measured GS and errors in measured bearing)  but if someone would tell me then it is straight forward to compute the error propagation.  I can do that as I am sure most of the engineers here can too.  Without knowledge of these errors, then we are just guessing how much angular separation is required.

 

​Now thinking about it, I am tempted to try 20, 30 and 120 degree angular separation 3 speed measurements and see how they differ.  I will guess 1 or 2 knots.

Assume flying directly into an 8 knot headwind and turning 20 deg in each direction.

8knots= direct headwind.

8cos20=7.51 =+-20 deg headwind component (which most GPS will round back up 8)

 

Unless you have a GPS that will display decimal places, at best your measurement will have a resolution of 8 knots.

 

Selecting the 3 headings 90 deg apart also makes the math easier.

[ArtVandelay]

When you rocket scientists figure out the correct way to launch this rocket, will one of you summarize it in a format that us astronauts know what buttons to push?

I have a nephew who just got hired as an aerodynamic engineer -- should I ask him?

All this to figure out who is fastest?

[Marauder]

All this to figure out who is fastest?

Yep... At least Timmy is not here to talk about doing this or not doing this using LOP while doing or not doing a touch and go with or without flaps using or not using CamGuard while looking or not looking at his Aspen or Garmin glass panel instead of steam gauges while listening to music on his Bose or Halo headset.

Did I miss any?