Speed Comparisons

[201er]

I've seen a bunch of topics comparing speed. A few questions about that.

What is the standard speed test at which they are comparing? When someone says plane X is a 150kt airplane, do they mean at 100%? 75%? 65% power? At what altitude? What kind of mixture? It would be silly to compare one plane flying at low power in LOP to another at high power unless gas saving was the standard to measure by.

Also are people measuring this in knots or mph? True airspeed or indicated?

Lastly, I've seen people post about cruising faster in the winter than in the summer. This doesn't make sense to me. I thought if anything, at the exact same power setting, altitude, etc, you would cruise faster because effectively you are at a higher density altitude with hotter air (unless you're too high).

When people are posting the speeds they get, are they being objective or is it all about bragging? Cause I'm still trying to figure out why I'm not getting the kind of speed I've been hearing about for the 201.

[Shadrach]

How on earth does would one cruise at 100%??? You'd have to stop for traffic lights, which is tough in a Mooney at full power!

We're talking true airspeed. There is little point in discussing indicated or corrected when talking cruise numbers.

My take is that these speeds are at optimum cruise altitudes but there is not a huge difference in speed at 4000 and 8000 probably a knot or 3... Best speeds are achieved at the lower altitudes, best fuel burn for speed is higher. the intersection ~5000ft DA... 

%HP in a NA AC for cruise is a pretty old school approach. I only know of a few that still throttle back for a specific MP, and they're super old school and have little interest in hearing anything other than what they know they know.  I think most pilots just climb to their cruise alt bring the RPMs back to a comfortable level (~2500 is peak efficiency for most of our props), and lean according to their goals/comfort level... Whether it's 4K or 8K the delta in fuel burn will be greater than the delta in speed.  The number you're reading are probably the upper end of the averages they see. I know of know one that flies normally aspirated machines by power % except for during instrument approaches. Turbo ACs are another story.

What kind of speeds are you seeing? The upper range for a 201 is ~160kts from 4 to 8K, some do even better...

 

 

[Jeff_S]

201er, as Ross says, the only way to do a true comparison is by measuring TAS. Newer planes (not sure of the date) will have their ASIs in knots, while older models will show MPH. The general convention is to use knots because then we sound cooler and more sophisticated, although if you want to show raw numbers, you should use MPH. After all, the "201" is based on MPH, not knots. The "Mooney 174ish" just doesn't sound as fast!

Two more issues to consider. In colder weather, the air is denser as you note in your post, so there are more air molecules for the prop to chew through. This is effectively what Ross was joking about when he said you'd have to stop for traffic lights...the only way to get 100% HP out of a normally aspirated engine is at sea level. So yes, in colder air you will get a greater TAS for a given altitude than you would in warmer air, all else being equal.

Second, how you actually measure your TAS will be subject to some errors in calculation. If you have the adjustable doohickey on your ASI you can set it for altitude and corrected OAT and that will give you an approximate TAS. If you have an air-data computer, that will show it as well. I have both, and I find the ADC to usually be within a few knots of the ASI doohickey. The alternative is to use your E6B which I'm sure you carry and use routinely while in flight! (ha)

Finally, why are you not seeing the speeds you think you should? That could be any number of reasons, including an older engine, excess drag somewhere on your airframe, a tired prop, or (quite possibly) somewhat inflated expectations. Remember that "201" was based on an entirely new airplane in perfect spec and with a new engine and prop. In real world work, I'll agree with Ross that my J can do ~160 KTAS if I really want to push with hotter engine temps and higher fuel burn, but I can also get about 153 KTAS flying LOP and save at least 2.5 gph and have a much cooler and happier engine. For me the choice is a no-brainer.

[Magnum]

IMHO 201er is right. To get apples to apples comparison you need %HP, DA and mixture setting. If it makes sense to throttle back for a normal cruise is different question. My Mooney is faster in the summer with the same %HP due to a higher DA (and needs more MP to achieve the same power)

I get 160kts at 65%HP, 50dF ROP, DA 7500ft, 150lbs below MGW, 10 GPH (the FF in the picture reads 3-4% high)

 

Edit: Added readable cutouts of the picture...

[Magnum]

Quote: Jeff_S

Two more issues to consider. In colder weather, the air is denser as you note in your post, so there are more air molecules for the prop to chew through. This is effectively what Ross was joking about when he said you'd have to stop for traffic lights...the only way to get 100% HP out of a normally aspirated engine is at sea level. So yes, in colder air you will get a greater TAS for a given altitude than you would in warmer air, all else being equal.

[Jeff_S]

Magnus, you are right. I realized after I posted that this was an oversimplification. The real issue is that the engine can generate more power with more air. And if we want to get all scientific, I will also agree that if you truly keep power output constant for any given density altitude you should see the same TAS. But most folks do as Ross described, which is to settle into a standard RPM/MP combo that they like and where the plane feels right, so in that case, cooler air should generally yield higher TAS than warmer air. That's the part I meant about "all else being equal."

[Magnum]

Let's keep it simple: With the same FF we are faster than brand B, P or C.

[Jeff_S]

Quote: Magnum

Let's keep it simple: With the same FF we are faster than brand B, P or C.

[trjymr]

I fly a 65E model and am considering moving up to a 201.  In generally how much faster and more fuel burn should I expect? 

[MooneyMitch]

Jolie's E is consistenly 144 kts.  I believe the J is close to 155 kts.  J owners will chime in on that figure.

Both great airplanes, in my opinion.

[201er]

What I forgot to ask is... the numbers people end up giving. Are these the numbers they typically see (more of an average) or the best case scenario? Little things here and there can cause slight fluctuations in the speed by a few knots (as much as +/-5kts easily). So are people normally talking about the average they see per given configuration or the optimistic number based on the best they've had?

Mine doesn't have much in terms of antennas or noticeable modifications. I could snap 160ktas as a best case scenario sort of thing. But typically I'm seeing 150-155ktas. For example recently I took a flight at 7,500ft MSL (but 9,000' DA). With WOT and 2400 RPM, I was getting about 154ktas. My guess is this is about 67-70% power.

[David Mazer]

I think the numbers quoted depend on the perspective of the person making the statement.  Someone selling an airplane may be very optimistic while someone trying to buy the same airplane may be a little more pessimistic.  When I quote speeds (since I'm not buying or selling), I generally quote what I would expect as a reasonable cruise at a typical altitude for a given plane.  Someone above said 145 for an E and 155 for a J.  I think they are using the same criteria I would use. You know - about this or that speed.  I wouldn't use it for flight planning just comparison.

[MooneyMitch]

For the E model, we typically do our planning at 140kts.  With that said, we get 144kts with wide open throttle, 2450 RPM's at 6500-9500 foot alititude range.  100 ROP.  Of course, lots of varibles but that is pretty consistent overall.  Hope that helps.

By the way.  This IO-360A1A runs well at 20 LOP, losing about 4kts in the process.

[bd32322]

These were taken on a colder than standard day - OAT at 8500 was -15C in Massachusetts. The day had super smooth air - which is why I took the readings then. THe aircraft is a 1987 M201-LM - it might be called M205 I think.

Measured speeds in the 4 directions using GPS and took the average to get a TAS. At 8500 feet throttle fully open, 2500 RPM, full fuel, 2 passengers (260 lbs total in front seats), 30 lb baggage in baggage compartment, 100 deg ROP I was getting 158 KTAS at 10.7 gph. This was approx 70% power from the charts that I created based on my data collection.

 

At 75% power at 8500 feet, 23.6 MP, 2700 RPM, 100 deg ROP (book specified values), I got 161 KTAS at 11.4 gph.

[flight2000]

Quote: Mitch

For the E model, we typically do our planning at 140kts.  With that said, we get 144kts with wide open throttle, 2450 RPM's at 6500-9500 foot alititude range.  100 ROP.  Of course, lots of varibles but that is pretty consistent overall.  Hope that helps.

By the way.  This IO-360A1A runs well at 20 LOP, losing about 4kts in the process.

[Shadrach]

Easy buddy...  Mitch's numbers seemed slow to me as well.  I think they're in the "something needs tweaked range".  My F bests those numbers even when LOP. A good E model should have about 2-3kts on an F.  Yours seems to be flying to its potential, Jolie's not so much... 

[MooneyMitch]

"Mitch,

I'm consistently in the 154-156 range running the same numbers except I'm at 2500 RPM.  I flight plan my IFR trips at 155 and have always reached my destinations at +/- 5 minutes of my ETA.  Those numbers seem to be in the F range.  Just curious.

Brian"

Hi Brian.

Your E speed numbers seem close to the J model numbers.  We can get more speed by turning up the RPM's but 2450 seems to be where Maggie likes to run.  Also, my F model was very close to these E speeds as well.  Possibly you have more speed due to what appears to be the 201 windshield mod. 

Happy and fast E model Mooney flying to you! 

[fantom]

At 100 ROP and 2500 PRM, maybe 22 MP, and 8000 ft, I easily see at least 162 KTAS.

At 20 LOP, with the same set up, I'm at about 154 KTAS.

[Immelman]

Another E model data point: If I want to run it fast, full throttle, 100 ROP, 2500RPM (yes, that is turned down a bit) at 7000-10,000 density altitude I can squeak out 150kts. No doubt the hangar rash my ailerons have, the little extra bit the LMG hangs down (versus where it should be), antennas, etc rob precious speed.

I have found that 145kt is a more honest number for my airplane and use that for planning. If I am flying higher up, in the low to mid teens, that will drop off to 140kt or so (but the gas mileage is better)

Flat-out speed is something else entirely. Just for kicks I've leveled off on a cold day near sea level at about 1,000' and kept the throttle wide open... the airspeed slowly built to close to redline (185mph indicated). Does this mean the E is a 160-knot cruising machine? Not mine... that's just something to make me feel good; pushing the engine that hard and burning 17-18gph is only fun for a few minutes

[flight2000]

Quote: Mitch

Hi Brian.

Your E speed numbers seem close to the J model numbers.  We can get more speed by turning up the RPM's but 2450 seems to be where Maggie likes to run.  Also, my F model was very close to these E speeds as well.  Possibly you have more speed due to what appears to be the 201 windshield mod. 

Happy and fast E model Mooney flying to you! 

[carusoam]

 

Brian,

Welcome home...

-a-

[carusoam]

 

Brian,

Welcome home...

-a-

[scottfromiowa]

I SECOND THAT BRIAN....WELCOME HOME!!!!!!  IS YOUR BIRD IN MN?

[MooneyMitch]

Thank you for your service to our Country Brian.  Take care and enjoy the heck out of that Mooney!

[flight2000]

Quote: scottfromiowa

I SECOND THAT BRIAN....WELCOME HOME!!!!!!  IS YOUR BIRD IN MN?