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Carson Speed/Altitude/Engine & Prop Settings M20K (231)


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16 hours ago, jaylw314 said:

Huh, that's interesting...I figured best glide would be similar at similar weights??  That's like a  7-8 knot difference?

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Max glide range occurs at max L/D. L=weight (unaccelerated flight). L is a function of airspeed squared. So best glide speed varies as the square root of weight.

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49 minutes ago, PT20J said:

Max glide range occurs at max L/D. L=weight (unaccelerated flight). L is a function of airspeed squared. So best glide speed varies as the square root of weight.

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I was referring to equivalent weights:

M20J @ 2700 lbs - 91 KIAS

M20K @ 2700 lbs - 83 KIAS

That's quite different??

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1 minute ago, jaylw314 said:

I was referring to equivalent weights:

M20J @ 2700 lbs - 91 KIAS

M20K @ 2700 lbs - 83 KIAS

That's quite different??

Ah, I misunderstood. Since the lift would be the same, it must be that the M20K has more parasitic drag. A windmilling K engine/prop. must have more drag than a J engine/prop. 

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13 minutes ago, PT20J said:

Ah, I misunderstood. Since the lift would be the same, it must be that the M20K has more parasitic drag. A windmilling K engine/prop. must have more drag than a J engine/prop. 

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I know, but 8 knots worth of extra drag??  That seems like a huge difference?

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2 hours ago, jaylw314 said:

I know, but 8 knots worth of extra drag??  That seems like a huge difference?

It is a huge difference, one must be incorrect or calibrated A/S vs true or something.

I can’t see same airframes at same weight having a significant difference in L/D.

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FYI glide ratio does not change with weight, but best glide speed does.

When lift is good, competition sailplanes add ballast to fly efficiently at higher speeds.  But they can dump the ballast if things change.

It makes no sense with the same fuselage and same wing and tail, that a J and K would have such different best glide speeds at the same weight.

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FYI glide ratio does not change with weight, but best glide speed does.
When lift is good, competition sailplanes add ballast to fly efficiently at higher speeds.  But they can dump the ballast if things change.
It makes no sense with the same fuselage and same wing and tail, that a J and K would have such different best glide speeds at the same weight.

It may have to do with the fact the J is better balanced, the K is more nose heavy with a heavier engine and extended nose.
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7 minutes ago, ArtVandelay said:

It may have to do with the fact the J is better balanced, the K is more nose heavy with a heavier engine and extended nose.

It may have to do with the fact that the AFM/POH for both airplanes is probably 50% testing and 50% guesswork.  I would also bet that two consecutive M20Js or M20Ks coming off the line would have different characteristics.

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27 minutes ago, PT20J said:

The drag curve is more U shaped than V shaped at it’s minimum. It doesn’t take a lot of drag change to change the airspeed for the minimum. 

That curve is “bucket” shaped, for an AH-64 the two corners of the bucket were 64 and 98 Kias, so if we were in trouble, lost an engine or both etc, we immediately trimmed for bucket airspeed, went for 80 KIAS as it’s pretty much in between. 64 kts worked out as min rate of descent power off, and 98 as max glide distance.

Knowing the bucket airspeeds is good information, if for instance your engine isn’t making good power.

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30 minutes ago, ArtVandelay said:


It may have to do with the fact the J is better balanced, the K is more nose heavy with a heavier engine and extended nose.

I bet the K has real close to the same CG as a J, may have ballast or not I don’t know, but I would expect the same CG range.

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1 hour ago, A64Pilot said:

It is a huge difference, one must be incorrect or calibrated A/S vs true or something.

I can’t see same airframes at same weight having a significant difference in L/D.

I mean, both POH's refer to KIAS specifically, and KCAS tends to only be couple knots off, even with flaps out and power on (which they wouldn't be if the engine is out)...

I could understand some flight testing error, but 8 knots?

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The difference in L/D speeds is really interesting. On both airframes, the speeds are calculated with cowl flaps closed, prop windmilling, and flaps up - so clean. I have heard told that the M20J is faster than the K up to about 8,000 feet after which the K is faster because of the turbo. The better speed, lower-down for the J must be from lighter weights and less drag. The only significant difference drag-wise between the two is the shorter cowl on the J and cooling drag. The displacement of the engines is the same, horsepower about the same.  The only significant differences with the engine is the K has a 6 cylinder 360 rather than the J's 4 cylinder 360, as well as the K's relatively lower compression engine.  

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2 hours ago, A64Pilot said:

That curve is “bucket” shaped, for an AH-64 the two corners of the bucket were 64 and 98 Kias, so if we were in trouble, lost an engine or both etc, we immediately trimmed for bucket airspeed, went for 80 KIAS as it’s pretty much in between. 64 kts worked out as min rate of descent power off, and 98 as max glide distance.

Knowing the bucket airspeeds is good information, if for instance your engine isn’t making good power.

Think you are referencing the power required curve. 

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45 minutes ago, PT20J said:

Think you are referencing the power required curve. 

May be but as there as there is no power required for autorotation, it apparently corresponds. Point is knowing your L/D airspeed could be useful

But it’s been over 20 years ago, so who knows

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2 hours ago, jaylw314 said:

I mean, both POH's refer to KIAS specifically, and KCAS tends to only be couple knots off, even with flaps out and power on (which they wouldn't be if the engine is out)...

I could understand some flight testing error, but 8 knots?

No idea, but unless someone can come up with a rational explanation, I’m not buying 8 kts.

However you wouldn’t believe how many mistakes there are in TCDS’s etc. I’d be willing to believe it’s just a mistake, before I’d believe there is an 8 kt difference.

Ref the J being faster below 8,000, if it’s real I’d bet the J’s engine is making more power, the rated power of an engine is a min spec number, and some engines beat the rated power by more than others. I doubt the J has enough difference in drag to give it a greater speed.

Of course it may not be real too

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54 minutes ago, A64Pilot said:

May be but as there as there is no power required for autorotation, it apparently corresponds. Point is knowing your L/D airspeed could be useful

But it’s been over 20 years ago, so who knows

Sure there is :) The power comes from the wind in your sails. The power required is simply the drag multiplied by forward velocity. 

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There is probably one or two people that know the answer…

He was the engineer / test pilot for both the J and K at the time…

And/or designed the fiberglass cowling and wing tips at the same time….

:)
 

Best regards,

-a-

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  • 2 weeks later...

Well I flew my airplane at Carson Speed today because I wasn't in a big hurry and heck, gas is costly:

  • 2300 RPM/24.5" MAP -  112 KIAS/137 KTAS.
  • 10,500 feet (29.91) from Medford, Oregon (KMFR) to Hillsboro, Oregon (KHIO)
  • 13 degrees Centigrade outside (+19 ISA).
  • Winds were about nil at that altitude today.  17.5 NM/Gallon.

I ran ROP, leaned to 1550 TIT. It was nice and quiet. Of course the temps were all good and since I was set at such a low power setting I could close the cowl flaps despite the high OAT.

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No intercooler, FF about 7.6 - 8 gph. 

I went flying today for some IFR practice to Newport, OR (KONP). I took some photos of the panel to show the data. 25", 2200 RPM. I was flying at 5,500 feet (too low for a turbo really). 126 TAS, 7.5 GPH, 1550 TIT. At that power setting, I could shut the cowl flaps and easily fly LOP with only the smallest change in mixture. The RPM shows too high in the picture. What looks like 2200 is really 2300 (old, inaccurate Tach) I have a portable (visual) tach on the top of the panel.

IMG_1798.jpeg

IMG_1799.jpeg

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Do not underestimate the drag required to trim a nose heavy aircraft. I truly believe the 8 knot difference is real because of the additional trim drag the M20K has to have to keep the nose up. In tankers we would routinely burn the fuel out of the center tank first and bring our CG back to 35 to 37% as our cruise range went up significantly. Once an hour out from top of descent we would burn aft tank to bring our CG back to 22 to 24% for better stability especially crosswind landings to put more pressure on the nose on ground rollout. In fact i’ve proved to myself in my M20K, put 2 40lbs water jugs in the cargo compartment and i pickup 5 knots with the same power settings as i have without the water jugs despite the additional weight which would normally slow me down. 
this is the biggest reason i want to put an MT prop on my plane to get 12 lbs. off the nose for better CG trim in flight and the extra 12 lbs of useful load is also nice. 

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35 minutes ago, Will.iam said:

Do not underestimate the drag required to trim a nose heavy aircraft. I truly believe the 8 knot difference is real because of the additional trim drag the M20K has to have to keep the nose up. In tankers we would routinely burn the fuel out of the center tank first and bring our CG back to 35 to 37% as our cruise range went up significantly. Once an hour out from top of descent we would burn aft tank to bring our CG back to 22 to 24% for better stability especially crosswind landings to put more pressure on the nose on ground rollout. In fact i’ve proved to myself in my M20K, put 2 40lbs water jugs in the cargo compartment and i pickup 5 knots with the same power settings as i have without the water jugs despite the additional weight which would normally slow me down. 
this is the biggest reason i want to put an MT prop on my plane to get 12 lbs. off the nose for better CG trim in flight and the extra 12 lbs of useful load is also nice. 

Good point. Most of the trim drag comes from the increment of induced drag caused by the extra lift required to offset the tail down force. The tail down force is, of course, greater at forward CG. And the induced drag is a greater proportion of total drag at lower airspeeds. So, it makes sense that power required at Carson speed would be more sensitive to CG than in high power cruise.

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