M20F Speed Checks... Comparison Please?

[FloridaMan]

I'm not quite sure as I don't have a FF gauge. I'd guess it's probably between 10 and 11 gph. I tend to fly long x/c flights and I switch tanks after three hours. The empty tank usually will take 30-34 gallons when I fill up and that includes fuel burn on climb out.

I honestly expected to see 153-155 TAS as that's what I seemed to "average" over the ground. I guess I just assumed the wind would be with me 1/2 the time instead of almost never. Also, that trip my IAS seemed a few mph higher than what I'm used to. It was cooler than usual here in FL and I was lower than I usually travel. The plane was full of fuel and I was flying by myself. IAS was 160-163mph. I usually see IAS at 155mph in cruise at higher altitudes.

Sent from my iPhone using Tapatalk

[TTaylor]

I figure on 139 knots at 8.4 gph for our 1975 that is stock other than a three blade prop.

I hope to do a windshield and cowl mod someday so that I can see closer to 150 knots.

[bonal]

Finally got around to trying a speed check. Conditions were density altitude 8,100 @ 2500 WOT wind was 21.6 from 183 degrees.

OAT 50F about 600lbs load

4 Heading IAS 132.75 average calculated TAS 153 knots

4 Heading GPS 148 knots

3 Heading GPS 147.5 knots

[Marauder]

For you guys reporting speeds, what mods do you have?

[bonal]

Nothing more than a cowl closure flap gap seals and the all important landing light cover

[Bob_Belville]

I did a speed check last week using the 2 headings technique discussed above.

 

GS directly into the wind 138k, down wind 169k, average 153.5. (5500' 75% power, '66E with 201 w'shield, 201 style cowl, gap seals, FG belly, reversed brakes, PFS.)

 

The Aspen was displaying TAS as 155/156k. It appears my ASI might read about 2 knots high.

 

But... I live on the lee side a pretty sizable mountain ridge so it is likely there is a wave effect most of the time the wind is blowing. (For example, yesterday the Aspen was showing 170k TAS holding 7000' with a 35 knot wind coming over the ridge. I was probably 20 miles from Mt. Mitchell [6683' -- highest peak east of the Mississippi]  I was really doing 150-155 but the A/P ALTHLD was pushing the nose down to offset the lifting wave.) 

[Ragsf15e]

I only have the 201 windscreen and the LASAR cowl closure.  My speeds were both about 140 KTS true at 100 ROP, 2500 RPM, RAM on, at around 10k D.A. (exact details in the first post).

 

Bob, while the wave might be affecting your reading like you mentioned, the ASPEN is using the IAS from the pitot system to computer it's TAS, so it's not unexpected to see it wrong.  The operators manual has a 3 knot correction (slower) at 140 kts, so that kind of matches up with what you saw.

[Marauder]

I only have the 201 windscreen and the LASAR cowl closure. My speeds were both about 140 KTS true at 100 ROP, 2500 RPM, RAM on, at around 10k D.A. (exact details in the first post).

Bob, while the wave might be affecting your reading like you mentioned, the ASPEN is using the IAS from the pitot system to computer it's TAS, so it's not unexpected to see it wrong. The operators manual has a 3 knot correction (slower) at 140 kts, so that kind of matches up with what you saw.

Aside of the speed mods, engine hours, rigging and stuff dragging (like antennae) will all have an impact. Be interesting if someone could compile all of these data points and figure out what is affecting it both positively and negatively.

[Bob_Belville]

I only have the 201 windscreen and the LASAR cowl closure.  My speeds were both about 140 KTS true at 100 ROP, 2500 RPM, RAM on, at around 10k D.A. (exact details in the first post).

 

Bob, while the wave might be affecting your reading like you mentioned, the ASPEN is using the IAS from the pitot system to computer it's TAS, so it's not unexpected to see it wrong.  The operators manual has a 3 knot correction (slower) at 140 kts, so that kind of matches up with what you saw.

Yeah, I included the Aspen TAS (calculated using the pitot info) to see how it compared to the GPS GS "reality". That's the 2 knots low info. 

 

The mountain wave comments are meant to be a caveat on the data being compared to someone else's plane. My airspace introduces another variable (soaring effect) that I don't think has been mentioned.

 

My data was not balls to the wall max speed. It was 75%. Power boost was closed (worth 1.1MAP) 

[Ragsf15e]

Yeah thats about the airspeed difference for the "computed tas" i got too. Pretty fast plane for ram closed!

[Bob_Belville]

Yeah thats about the airspeed difference for the "computed tas" i got too. Pretty fast plane for ram closed!

Lots of caveats... I was alone, with near full fuel so about 400# below gross. 

[FloridaMan]

I think that the main speed advantages with the LoPresti cowling are from cooling efficiency and prop pressure. The three round holes of the cowling are pushed almost right up to the prop, which I assume allows for prop pressure to really force air through the cowling and the ram air. I gain 1-2 inches of MP with the ram air open on the LoPresti cowling. 

[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.

Anyone got any data on speed loss when comparing ROP versus LOP ops at the same power settings? I am finding around 10-12kts slower at LOP versus ROP and wondered if that was normal.

[Bob_Belville]

I lose at least 10-12. And more if you're at an altitude where you can, and want to, run 75% power ROP since LOP is not recommended above 65%, is it? 

[Lood]

About the same here Tony. I lost around 10 kts and used about 6 liters less Avgas. Fortunately, I can claim VAT back on Avgas, so I fly my F at 75 - 80 deg ROP.

[Ragsf15e]

Interesting, I've done all the speed tests and have been pretty thorough in my data collection... I'm only losing about 5-6 knots going LOP.  There's probably some differences in our altitude, temperature, or technique to explain it though.

 

I'm losing those 5-6 knots between ROP / LOP at approximately 10-12,000' Density Alt, WOT, 2500 rpm.  As Bob mentioned, I try not to run LOP unless I'm below around 65%, so I'm up pretty high when I do this.  I have GAMI injectors and a JPI 930.  I use the lean function on the JPI and I lean it out until the richest cylinder just goes maybe 5-10 degrees LOP.  The leanest one is probably 40-50 degrees LOP.  

 

That setting is about 5-6 KTAS slower than 100 ROP, but it's burning a bit over 2 GPH less fuel!  It's a good trade when you consider it takes about 5 minutes longer to get where you're going on a 300 NM cross country but you saved 4+ gallons of gas.

 

Are you guys that are losing 10-12 knots doing it any differently or at a lower altitude?

[Tony__L]

Rags,

I also have GAMI injectors and use a JPI EDM 700 to lean accurately. I however was leaning the richest cylinder to 30 degrees LOP and not 10 like you (is there a reason why you choose 10?). I was also,doing this at lower levels, 4000 ft indicated and probably around 5000' DA. According to the POH at 5000' and 25/25 or 24/24 at best power mixture you are running well over 65% power, more like 82% and 77% respectively. So,I probably wasn't being very wise trying LOP at that altitude and power setting configuration (although I suspect that at LOP the power produced drops radically).

I will be doing a trip to Cape Town this week and will be at the higher levels and will try a bunch of different power configurations and will see what the data shows.

Lood and Bob,

I'm glad to see that you experience similar loss of speed to me.

Tony

[fantom]

Interesting, I've done all the speed tests and have been pretty thorough in my data collection... I'm only losing about 5-6 knots going LOP.  

 

 

Very much like my J....~ 8,000 to 9,000 DA, WOT, 2500 RPM,

 

About 162 kts @  80 degress ROP, and 154 kts 10 or 15 degrees LOP on the richest cylinder. Use JPI and the 'big pull.'

 

So, I save 1.5 to 2.0 GPH, and about 8 kts an hour, LOP.

[Ragsf15e]

Tony - 

 

To be honest, I'm only pulling it that far back because the speed drops off pretty rapidly with only a little change in fuel flow.  That probably explains our difference in 5-6 knots vs 10-12 knots.  If I pull the richest back to 40 LOP, I think I'd be about the same as you.

 

Since I'm doing it up pretty high and the engine is at 65% or so, I'm not really worried about being close to peak.

 

I'll be looking forward to hearing what you come up with on your trip at higher altitudes.

 

Rags

[Tony__L]

Did a trip to Cape Town last week and had plenty of time (on the way there at least, on the way back had to 'fly' the plane the whole way back because of turbulence) to test a whole lot of mixture settings and get some data together.  In summary I found that at anything more than 10 LOP you loose a lot of speed.  Your MPG improves a little but I would guess that the extra fuel saved is spent again in maintenance due to the extra time spent in the air.  I also found that at anything below 2500 RPM the loss in speed is drastic when getting to Peak and even more so when moving to LOP.  So much so that I did not bother trying out 30 LOP at 2400 RPM.  I have showed some 2400 RPM data in the data table but not on the chart.

 

After this exercise I think that 2500 RPM and 10 LOP is a sweet spot.  The CHT's are lower than ROP and fuel burn improvement is substantial.  The improvement in fuel burn and the negligible change in TAS as you fly higher is quite impressive and confirms the talk about flying higher to get better figures all around.

 

Below is the RAW data and a chart that I put together.  The chart is quite busy but very legible when you print it.  I had to change some of the typical reporting formats to make it readable (e.g. NM/10gal instead of NM/gal).  Have a look and let me know what you think.

 

PerformanceChart.pdf

PerformanceData.pdf

 

 

[Ragsf15e]

Tony - 

 

Nice job with the data collection and the graph is awesome!  With that much info it's usually pretty difficult to understand someone else's work, but I got it without too much study.  

 

I get a similar drop off ROP to LOP and I agree - 2500rpm and about 10 LOP at around 8-12000' is pretty good.

 

You seem to be about 4 or 5 knots faster than me.  What speed mods do you have?  I only have the 201 windshield, but I do use my RAM air and I know you said you don't have that on yours.  I'm wondering why else you're faster?  Is your engine new?  Aftermarket exhaust?  Cowing?

 

Thanks,

Rags

[Tony__L]

Rags

Thanks for the comment on the graph, it took a long time to get it readable for those that didn't put it together. Was still worried that it was too much.

Just another comment on the data, there is a gap between FL065 and FL095. This was due to unfavorable winds at higher or lower levels for the route either on the way to CT or on the way back. Being a 490NM trip meant that I did not want to spend time in a head wind for the sake of collecting data. I will add to the data as and when the opportunity arises to fly at the 'missing' levels.

With respect to mods on the plane; I still have the original split windshield. It has the cowl front end enclosure mod, gap seals at the wing roots, wing tips (M20J style), the step has been removed and minimal antennas. The paint is newish and I gave it a full compound polish earlier this year (I don't believe it myself but I believe that alone added 2 to 3 kts). I don't have ram air anymore, not sure of why it was removed, I think it might have something to do with the cowl closure and the air filter position. I also have a 3 blade prop which doesn't help with speed but boy oh boy can I beat the POH figures on take off. I also have a draggy wheel at the moment, see my thread on this, but will hopefully get this sorted out the next time the plane goes in for maintenance.

I know we calculate the weight of the plane relative to MAUW but nonetheless I think that my planes empty weight is quite light for such an old plane. The last time it was weighed it came in at 1817 lbs. I did see a figure somewhere, but I can not find it at the moment, that showed that the plane was heavier when it was new.

Also note that I was calculating TAS using pressure altitude, OAT and indicated airspeed. I have found that this seems to produce 2 to 3 kts higher results than the 4 or 3 point GPS method.

Tony

Tony

[Lood]

Hi Tony,

Wow, you did indeed go through a lot of trouble getting all that data together! It actually motivates me to go through the same exercise next time I do a long flight - which is going to be tomorrow .

My engine runs very cold as is, so I don't intend to go LOP to improve on engine temps and I am able to claim VAT back on Avgas, so the price also doesn't bother me much.

 

What I do want check with the new engine and overhauled prop though is the TAS. Then I also want to settle in the cruise at my normal 75 deg ROP setting and note down the FF and IAS. Then do the same at 10 deg LOP. After that, I want to go back to my normal 75 deg ROP setting, let everything settle and then pull the power back to the same FF that I got at the LOP setting and compare the speeds.

Have you maybe done anything similar.?

[Tony__L]

Rags,

 

Something I forgot to mention in my response to you questions.  My engine is at +/-800 SMOH (factory overhaul) but is a 4000+ hour total time engine.

 

Tony

[Tony__L]

Hi Lood,

 

I can also claim VAT on my Avgas and my engine runs cool too, but a saving of 32% in fuel burn for a sacrifice of 4% loss in speed is hard to turn down.  Let's put some real world figures to this.  Flying to CT at FL065 (as I did last week) at 100 ROP would have taken 3.31hrs and 146 L of avgas.  At 10 LOP it would have taken 5.6 minutes longer and I would have saved 33 L of avgas.  At 33 L per hour at 10 LOP that means that I get a whole free hour of avgas for my next trip     

 

To answer your question, no I haven't performed your proposed exercise yet (will try it next time out).  However, I fly with a bunch LSA aircraft on trips that we do and I will occasionally loiter with them at around 100kts.  I always end up with much the same fuel burn as they have, around 20 Lph at a ROP setting.  This is very economical but you lose out on the maintenance costs on the other end of the equation (my tach turns slightly faster than the clock all the time, no matter what the power setting is ).

 

Tony