M20F Speed Checks... Comparison Please?

[carusoam]

This is one of the most spectacular threads...

International applied mathologists concur!

Speed has been proven a stronger driving force than efficiency.

I feel like I stayed at a Holiday inn last night.

Cheers,

-a-

[FloridaMan]

[aviatoreb]

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. 

 

But there is (essentially) never a zero wind condition at altitude.  Calm winds on the ground are often a boundary layer effect and even one thousand feet up the winds are always moving.  So it is not practical to say we will wait for a zero wind day - which yes if it existed would make the problem much easier.

 

OTOH I think some people were saying calm air - which makes for better TAS than turbulent air.

 

I misunderstood you - I thought you were talking about CAS errors, but there will be CAS errors when you crab nonetheless.

 

Quite right slipping to do a crab will cause slower TAS.  (I believe that is what you are saying now, right?)  So it is a real effect rather than a measurement error - you really are going slower rather than mismeasuring slower but you are really going faster.

 

Did you catch the thread entry where I suggested you can find direct head wind or tail wind even without an aspen, by solving argmin_theta ASI(theta).

[aviatoreb]

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?

 

The key to your confusion is this.  It is a signed and weighted sum. The vector sum (vectors have magnitude but also directions meaning bearings) between three vectors projects to scalars (numbers) involves weights (a fraction that multiplies how much of each number to add in).  The weights are sines and cosines (the trig functions) evaluated depending on the bearing of each vector.  Sines and cosines can be negative and they will be for some of the weights if you have 120 degrees apart.  So you are adding some positively weighted numbers and some negatively weighted numbers.  THis allows it to all work out.

 

At some fly in someday I will work a chalkboard full of vectors and trig.

[aviatoreb]

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.

 

That would work fine.  The three heading method requires no instruments other than the GPS (and other instruments in secondary role only to do things like maintain altitude pitch and so forth).

 

There is a principle in linear algebra - which includes the study of vectors: 2 points make a line. 3 points are required to describe 2 linearly independent vectors.  Meaning it takes 3 separate measurements to resolve something that is inherently two vectors ( the airplane movement is one vector, and the wind is the other vector).  So 3 separate speed measurements with bearings will do but one speed measurement could be replaced by the bearing read from your HSI.  I see no reason that wont work.

 

That said, all this stuff, if I were really doing this for work rather than for fun, if I worked at Boeing or some such, I would be expected to choose the best method amongst the 3 reasonable methods now proposed (4 runs at cardinal directions - use gps, 3 runs in arbitrary but measured directions and use only gps, 2 180 degree opposite directions and use gps for speed but hsi for bearing direction and to measure D=crab angle as difference between hsi bearing and gps ground track bearing).  Each measurement has an error.  And being different kinds of instruments, the errors may be different magnitudes.  So the next thing to do would be to do an error propogation analysis.

 

I.e., suppose you measure D+delta D instead of delta D=0 (zero) error. How does that effect the error the computed TAS?  I can tell you that the TAS error will vary quadratically, as delta D^2.  ANd likewise with the other errors.

 

If I am measuring roughly 200TAS.  Then +/1% is very important to me - since that is  important to me since that is the difference between 198 and 202 kts.  How much angular error measured in the HSI leads to 1% in TAS?  How accurate is the HSI?  A mechanical HSI has two kinds of error.  Intrinsic error (error in the device) and recording error (the human has a hard time reading the exact angle to within 1 degree when you are look at say 83 degrees vs 84 degrees and not near a tic mark - and not to mention in a bouncing cockpit).

 

So while these formulas each seem to have merit I bet one is best in terms of ease of use, or in terms of ease of flying, or in terms of smallest propagated errors given errors of various sorts of each measurement.

 

I like the 2 point method just put forward for ease of computing and elegance.  I like the 3 point method in that one can sorta keep heading generally toward their destination plus or minus 30 deg so I could imagine doing it for fun enroute somewhere on a long trip.

[aviatoreb]

 

Love it!

 

But its not exactly maths.

[Tony__L]

I did a few speed tests a couple of weeks ago. I will post the results in the morning.

1967 M20F Executive, 700 hr engine.

[aviatoreb]

I did a few speed tests a couple of weeks ago. I will post the results in the morning.

1967 M20F Executive, 700 hr engine.

 

I don't care how fast you went.  I want to know which method you used.

[ArtVandelay]

How about just flying in a square using 4 waypoints, at the end just divide the distance travelled by the time it took, removes variations of wind, etc by averaging speed over time, and you can create a flightplan in your GPS, let AP handle it while you monitor it. Try it ROP, then LOP, then using ram air, and then move weight around to see if you can go faster with CG more aft.

[Bob_Belville]

How about just flying in a square using 4 waypoints, at the end just divide the distance travelled by the time it took, removes variations of wind, etc by averaging speed over time, and you can create a flightplan in your GPS, let AP handle it while you monitor it. Try it ROP, then LOP, then using ram air, and then move weight around to see if you can go faster with CG more aft.

Won't work for the reason explained earlier that flying a track rather than a heading will have you tacking into the crosswind component with consequent loss of ground speed. Think about it like this: suppose you fly a box with north, west, south, east legs returning to the starting point. Suppose the wind is exactly out of North. The average of gs of the north and south legs would give you your tas but not if you took the total distance divided by the total time since you would be flying longer on the northbound leg while your gs was slower. And the east and west legs with 90 deg cross winds would cover less ground than the same time spent on an east or west heading so the gs would be slower.

 

Got that?

 

[DonMuncy]

How do all the methods account for (or not) the potential change in wind speed and direction between the measurements, or is this a potential error factor we have to live with. 

[aviatoreb]

Won't work for the reason explained earlier that flying a track rather than a heading will have you tacking into the crosswind component with consequent loss of ground speed. Think about it like this: suppose you fly a box with north, west, south, east legs returning to the starting point. Suppose the wind is exactly out of North. The average of gs of the north and south legs would give you your tas but not if you took the total distance divided by the total time since you would be flying longer on the northbound leg while your gs was slower. And the east and west legs with 90 deg cross winds would cover less ground than the same time spent on an east or west heading so the gs would be slower.

 

Got that?

 

 

Tacking is for sailors isn't it?

 

Crabbing is for pilots.

[aviatoreb]

How do all the methods account for (or not) the potential change in wind speed and direction between the measurements, or is this a potential error factor we have to live with. 

 

Built in assumption - wind stays constant during the course of the legs of the test, and that all the other factors that contribute to TAS also stay constant, like density altitude type things like temp and pressure so don't cross any fronts during the test.  And the engine is assumed to be producing same power.  And you don't change the aerodynamics of the airplane any like driving through a rain storm for part of the test or picking up some ice.  If there is ice, stop the test and go home.

[Ragsf15e]

You guys have me laughing pretty hard... especially the part about brain surgery vs rocket science and the whole post about LOP, ROP, T&GOs, Flaps, etc.  Worth reading all 4 pages of this thing just for that.

 

Ok, I think Bob has nailed the easiest way to do this accurately that we can all agree on and do fairly simply - go fly, turn roughly into the forecast wind, fine tune your heading until GPS ground track and aircraft heading match (i.e. get pointed directly into the wind), note ground speed, turn exactly 180 degrees (perfect tailwind), stabilize and note ground speed.  The average of these two numbers is a pretty accurate take on your TAS. 

 

If you don't like this method, someone can read the attached document which is the most confusing and complicated method of determining TAS that I could find, but it does handle important factors like "curvature of the earth"!  

 

Rags

vol4no3_A01.pdf

[Lood]

Yeah, almost like the SA guys driving 4.5 EFI Land Cruuiser pick-ups - myself included. When asked what the fuel consumtion on the Cruisers are, we normally answer in decimals, like 4.83km/L or 5.61km/L. So one day, friend of mine actually interupted me and asked why we are so adament on giving it in .this or .that, rather than simply 5km/L or 6km/L. Cruiser are fuel guzzlers and uneconomical, full stop.

 

I'm rather curious to know how much different the result will be, using the the different methods of determining TAS? Is it really going to be substantial or are we only going to see a knot or so?

[Tony__L]

I don't care how fast you went.  I want to know which method you used.

   

[podair]

Hi, I have a pretty stock 1967 M20F with 3 blade hartzell.

 

I consistently get 143/144ktas at 9500-11000ft density altitudes, about 15deg LOP , working out 32 litres an hour (8.5 us gal).

I use the e6b function on my gps on every flight once in cruise, and been flying the same routes/settings/altitudes over the last six years. 

That's 2500rpm and about 21.5in map with ram air, wot

A knot or two a bit lower with a litre or two more, a couple knots and a litre or so less higher up.

 

It is so consistent that a few weeks I was getting sub 140 at those settings, turned out the retractable step no longer retracted, the vacuum tube had disconnected!

 

Setting 47 (map+rpm) and 32 litres (so 65%)  seems to be the best compromise at all altitudes. So at 3k/4k feet I tend to set 2350/23.5 , and 2500/22 higher up etc

At all altitudes, I find if I try to go faster than 145/150mph IAS I really need to pour fuel and rpm, and it is not worth it. Even lower down, I need 25/25 to get to 160-165mph and 10gph.

 

Hoping to install a sloped windshield and powerflow exhaust in the future, maybe I ll eke out another 5kt. Seems like I hit a drag barrier at 145mph ias.

 

Although it seems my 1967 M20F is not that bad for a stock airframe, I am nowhere near the figures in the 1967 poh. However, I also looked at the figures quoted in an early 70s M20F and they are lower and more accurate, pretty close to what I get! That's Sixties marketing for you!

[Tony__L]

Okay, here goes.  I went and did a few runs the other day to see what LOP ops would do to my speeds and fuel burn.  I was also a little concerned that I had lost some speed since doing a flight test a year ago in the same plane as part of my loonnngggg purchase process and so wanted to see what the performance was at those same settings.

 

The attached PDF shows my average speeds using the 4 cardinal points method all at 4000'.  2 runs were at 30 LOP, one at 24/24 and the other at 25/25.  The 3rd was 50 ROP at 24/24.  I applied this data to the 3 turn method and did 3 iterations of each (starting at a different point to see if made any difference). So Erik, I hope you are satisfied  .  Net result, there is no difference in the result.

 

SpeedTests.pdf

 

When I did a 4 point cardinal test last year the plane was flying at 144kts TAS at 4000' at 24/24 and 50 ROP.  From this test it seems like I have lost around 4kts and possibly another knot or two as the plane was lighter this time around.  Perhaps I also rushed the test a bit because I did not give the plane as much time to settle after each turn, so there is perhaps a knot or two to be gained (but who can fly that straight and level continuously anyway?).

 

As far as LOP ops go.  I think I still have to practice this as I think that I am losing too much speed when going LOP.  Especially since I have GAMI injectors and an EDM 700 to fine tune the mixture (so no guess work).  I found that during LOP ops the temps would take a very long time to settle and that perhaps with more experience I might be able to shorten and fine tune the process and get more out of the plane wrt speed.  The speed loss just seems to be too much!!!!!!

 

I have never tried to see what the plane is capable of at say 25/25 100 ROP (max power) or WOT/25 at lower altitudes to see what she is capable of.  I have done a 25/25 50 ROP at 4000' and did a TAS calc using IAS OAT QNH and got a TAS of 146kts @ 42 Lph.

 

 

[cliffy]

Used to pull out the ol' CR-3 wiz wheel in the 757 and the F/O would say "What RU doing? Did it just to screw with him :-)

After 1400 hrs in my M20D/C I'm confident in 135 kts TAS, variation due to day, weather, phase of the moon, gravitational pull of Venus, my attention span, plus or minus 4 kts at @ 8gph,  usually 9500' to 12,500 MSL, FT, 2500 RPM. Now I know this is not an F BUT you guys don't have me by much.

Also only 727 era steam gauges and a non-precision GPS tied to a rudimentary wing leveler.  

I love the two fancy panels but they're worth more than my entire airplane.  But OH are they nice!

Aspens remind me of the 757 panel

[carusoam]

Tony,

Nice data collection!

Delivered in a timely fashion!

Using the four leg method, it is easy to understand and the same data can be used to prove the value of the three leg method.

How many continents do we have represented in this conversation here today? NA, Africa, Europe,

Go MooneySpace,

-a-

[Ragsf15e]

Tony - 

Just curious, why so low?  You could be a lot more efficient up around 9-11,000' at full throttle, RAM on, and LOP.  

 

Your speed would be around 140 kts as the 4 pages of this forum can attest to.

 

Rags

[Tony__L]

Tony - 

Just curious, why so low?  You could be a lot more efficient up around 9-11,000' at full throttle, RAM on, and LOP.  

 

Your speed would be around 140 kts as the 4 pages of this forum can attest to.

 

Rags

Rags, I agree it would be more efficient up there. I was down at 4k because that was where we did previous speed tests. I had planned to go up to 8k (as I had also previously done) too but ran out of time.

PS: no RAM air on my F :-(

[FloridaMan]

I did the 3 direction check yesterday. 1967 m20f at 6500 ft. OAT 10C at altitude. 24" ram air open 2500 RPM ROP was 158kts. Mods include LoPresti cowl, powerflow exhaust, GAMIs, misc gap seals

Sent from my iPhone using Tapatalk

[Ragsf15e]

Antares, that's one fast F model!  That's got to be 10+ knots faster than the average bird.  You're getting your money's worth out of those mods at that speed!

[Tony__L]

I did the 3 direction check yesterday. 1967 m20f at 6500 ft. OAT 10C at altitude. 24" ram air open 2500 RPM ROP was 158kts. Mods include LoPresti cowl, powerflow exhaust, GAMIs, misc gap seals

Sent from my iPhone using Tapatalk

YOWZA!!!!!!!!

Fuel flow?