M20J or GLIDER?

[kpaul]

9 hours ago, cnoe said:

That's pretty awesome! With the pointy nose and fully feathering prop I bet there's very little drag up front.

Yeah that thing will fly forever with a simulated feathered prop. And everything I have heard and read is that it actually glides better with a actual feathered prop.  We set about 2.5 psi on the torque to simulate.  We do a lot of practice emergency landing patterns (ELPs) from different altitudes and distances from airports, this in one place that AOA is awesome.  Pitch for AOA and fly on AOA until about 500' then pitch for slightly faster than AOA so there is speed to flare.

[cnoe]

I wanted to give a brief follow-up to my earlier post regarding the power-off glide in my J.

So... last week before the deluge came (Houston) I went out and performed a few odd maneuvers to get more data for my musings.

In my prior glide tests I pulled the power to idle and pulled the prop control out for the least drag. Airframe configuration was gear/flaps up. If you recall I managed a glide ratio of 13.85:1 over a 9,000' descent. That gave me a glide distance of 2.28 nm per 1,000' of descent AGAINST A HEADWIND.

A question raised was "how much is glide assisted by an "idling" engine as opposed to a true engine out?" So... 

During this recent test I did not complete the long descent again; I limited the descent to 2,200' total. But this time it was with a dead engine (windmilling prop) as I pulled the mixture to idle-cutoff for the test. There was NO POWER being made, and I have the JPI data to prove it. All other settings remained as before.

There was a slight quartering tailwind (~115 degrees to my side) this time as I descended from 3,020' MSL to 820' MSL covering a distance of 5.3 nm in a time of 3:35 (3.6 minutes). I was able to hold my airspeed much more precisely (@ 100 mias) this time.

Though slightly imprecise my calculations suggest a no-wind distance covered of 5.22 nm. This calculates out to a glide ratio of 14.4:1 or 2.37 nm per 1,000'.

Based upon this latest test along with my earlier findings I believe that the gliding distance in my J can be conservatively estimated as ~14:1 or 2.3 nm per 1,000'. Your mileage may vary, but that's a whole helluva lot better than the 1.6 nm/1,000' that I'd previously been counting on.

From now on when I'm at 9,000' AGL looking at the distance rings on Foreflight I'll no longer rule out airports 15-20 miles away in the event of an engine out.

[carusoam]

Thank you for sharing the data, CNOE.

Best regards,

-a-

[bonal]

You have some serious confidence in your airplane I cant imagine ever intentionally cutting my engine I wonder how the C would compare 14.1 seems pretty good I always like to get some decent AGL but not so easy with all these mountains I'm surrounded by when its a short flight. Nice report cnoa

Just for the record I have very high confidence in my plane but as long as the prop has power behind it I could never turn it off

 

[cnoe]

25 minutes ago, bonal said:

You have some serious confidence in your airplane I cant imagine ever intentionally cutting my engine I wonder how the C would compare 14.1 seems pretty good I always like to get some decent AGL but not so easy with all these mountains I'm surrounded by when its a short flight. Nice report cnoa

Consider that in comparison to my previous testing with the engine at idle, the distances weren't grossly different. Simply completing your own 2,000'-3,000' timed glide at idle will give you valuable information about your specific plane's glide performance.

While I use the Cloud Ahoy app to track all this, another good method is to set up a course 10 nm out (on your GPS) perpendicular to your airport's runway at 6,000' AGL and note your altitude as you cross it; you'll likely be at 1,500' AGL or above if similar to my results.

And regarding my confidence in the plane, I wouldn't do this over inhospitable terrain but I don't find it imprudent to run a tank dry or pull the mixture if it gives me some valuable data such as usable fuel or glide distance. I don't do this with my wife in the plane though.

[N201MKTurbo]

When I was young and foolish I used to fly long cross countries at 500 AGL or lower because I got bored flying the same routes day after day.
 

One day it occurred to me that my radius of action in case of an engine failure was very small and I wondered if it was better to decelerate from cruise speed to best glide speed straight ahead or zoom up to best glide speed. I did an experiment where I flew over the GLL VOR at 1000 ft., pulled the power to idle and see how far I got, on the DME, until I was at 500 ft. I don't remember the exact numbers, but I could go about 1/2 mile further by zooming up to best glide speed rather then decelerating at the same altitude.

When I did these tests, I was over a series of SOD farms, the turf was so smooth it made a golf course look rough. 

[jlunseth]

My 231 is 12.7:1 according to the POH in cruise configuration with the prop windmilling and at best glide.  I use 12 to 1 to be conservative and because it is easy math.  12,000 feet per thousand feet AGL, or 2 nm per thou.  I would think your lighter J would get that certainly.  I have never tested it though, and don't have the cojones to test it engine out.  There are some complications if you let a turbo engine cool too much and then power up, you can't go full power until you warm the engine and it is just too complicated to deal with, so I don't go there. 

 

[ArtVandelay]

Are hot starts easier with windmilling prop as oppose to on the ground?

[N201MKTurbo]

Hot starts are caused by a heat soaked engine vaporizing the fuel in the fuel injection lines and doing two things, forcing the fuel out of the injectors which floods the engine and also leaving the lines empty so they have to be refilled.

None of that happens when you are wind milling. My experience is that the engine will come back to life as soon as it has fuel. The engine will be running before you get the knob all the way to the panel. It is better on the engine if you bring the throttle back to idle before advancing the mixture, then advance the throttle.

[N201MKTurbo]

35 minutes ago, jlunseth said:

My 231 is 12.7:1 according to the POH in cruise configuration with the prop windmilling and at best glide.  I use 12 to 1 to be conservative and because it is easy math.  12,000 feet per thousand feet AGL, or 2 nm per thou.  I would think your lighter J would get that certainly.  I have never tested it though, and don't have the cojones to test it engine out.  There are some complications if you let a turbo engine cool too much and then power up, you can't go full power until you warm the engine and it is just too complicated to deal with, so I don't go there. 

 

I wouldn't be too worried, The engine doesn't know that it is turbocharged. The major thing people worry about is coking the turbo bearings by shutting it down when it is blazing hot or spooling it up before it has adequate oil flow when it is cold. None of that will happen with a wind milling engine. The oil won't cool down that much before you hit the ground unless you start at FL240 and the oil will continue to flow because the engine is still turning. In reality, starting a wind milling engine is probably easier on it then starting it before a flight. As long as you let it cool a bit before you shut it down you shouldn't have any cylinder shock cooling issues and if you are worried about shock heating just bring it up to enough power to maintain altitude for a minute until you go back to climb power.

Unless you are about to hit the ground, then all knobs to the fire wall and think happy thoughts about your engine. 

[cnoe]

Hot starts are caused by a heat soaked engine vaporizing the fuel in the fuel injection lines and doing two things, forcing the fuel out of the injectors which floods the engine and also leaving the lines empty so they have to be refilled.

None of that happens when you are wind milling. My experience is that the engine will come back to life as soon as it has fuel. The engine will be running before you get the knob all the way to the panel. It is better on the engine if you bring the throttle back to idle before advancing the mixture, then advance the throttle.

Exactly, to restart I'd crack the throttle slightly and with a couple twists of the mixture she promptly comes to life. Add throttle and away you go.

At 100 mias the prop got pretty slow and had me wondering if an electric start might be necessary. It also set up some undesirable vibration when windmilling slowly so I wouldn't do this for a long period of time. I also set it up from low power with well-controlled CHT prior to shutdown to minimize shock cooling issues (OAT was ~75 F. too which should help).

Lastly, my POH shows a 1.89 nm/1,000' best glide (graphically) which equates to an 11.5:1 ratio which is significantly less than what I'm seeing in actual tests (20% less in fact)!

[ArtVandelay]

So, on a related topic, I'm going to use some approximate numbers for easy math:

You take off, 90k And climbing 1000/min. At a 1000', you lose your engine, standard rate turn 180 takes 1 min, you've lost 600', pointed back towards airport you have 400' left, or 40 seconds to cover 1.5 nm, at 90 knots you end up 1/2nm short, or 20 seconds of glide time, or an extra 200'.

With the absence of wind looks like you need 1200' of altitude to make the impossible turn.

Do I got that right?

[N201MKTurbo]

1 hour ago, cnoe said:

 It also set up some undesirable vibration when windmilling slowly so I wouldn't do this for a long period of time.

Interesting, two things:

1.  The oil holes in the crank are drilled to supply oil to the highest pressure point which is at ~15 deg. ATDC. When the engine is wind milling the pressure is being applied to the least lubricated side of the journal.

2. Torsional vibration, which is responsible for the yellow arcs on our tachs, has probably not been evaluated for a wind milling engine and could possibly be severe enough to damage an engine.

It would be good to get an expert opinion both of these.

[Hank]

1 hour ago, teejayevans said:

So, on a related topic, I'm going to use some approximate numbers for easy math:

You take off, 90k And climbing 1000/min. At a 1000', you lose your engine, standard rate turn 180 takes 1 min, you've lost 600', pointed back towards airport you have 400' left, or 40 seconds to cover 1.5 nm, at 90 knots you end up 1/2nm short, or 20 seconds of glide time, or an extra 200'.

With the absence of wind looks like you need 1200' of altitude to make the impossible turn.

Do I got that right?

That's why the suggestion is to bank ~45° when turning around like this. 

[cnoe]

1 hour ago, cnoe said:  It also set up some undesirable vibration when windmilling slowly so I wouldn't do this for a long period of time.

Interesting, two things:

1.  The oil holes in the crank are drilled to supply oil to the highest pressure point which is at ~15 deg. ATDC. When the engine is wind milling the pressure is being applied to the least lubricated side of the journal.

2. Torsional vibration, which is responsible for the yellow arcs on our tachs, has probably not been evaluated for a wind milling engine and could possibly be severe enough to damage an engine.

It would be good to get an expert opinion both of these.

Interesting thoughts. It wasn't too bad but I would characterize it as moderate. I've seen more vibration in a couple of planes under power but it still concerned me and was unusual. I rarely push the prop like that as I hear it can cause ring flutter. And though I don't fully understand it I've also heard that counterweights on the crank can be damage by abnormal operations. The good news is that with the application of power everything smoothed out and seemed perfectly normal.

I'd love to know more about this subject if anybody can offer more info or expertise. "Anyone, anyone?" "Bueller?"

[clh]

Can and will are two different concepts.   The engine is designed and operates pushing against the bearings.  If you let the prop push the engine, you can effect the counterweights, even spin the bearings. It does not mean that it will happen. (If it did, we would be doing a teardown every time we get starter kick-back)  

It is generally, not good for the engine to be pushed with the prop, but it is not always going to lead to a catastrophic failure.  you will likely get bad vibrations from the engine and the vibrations may add up over time.  but there are many out there that run their engines to exhaustion frequently before the last tank change.  We also stop the engines by pulling the mixture. This does not appear to be detrimental to the engine.  (it is also usually for short duration and at low power)

At work, our 20,000 hp motors and compressors definitely do not like to be spun backwards. They shake the buildings when they vibrate.

 

[gsxrpilot]

Ok, Chuck...  I'll be your Safety Pilot this weekend at the Formation Clinic if you'll come be my Safety Pilot for engine out glide practice in my C.  I'd love to practice this with your experience riding along in the right seat.

[cnoe]

Paul, all this testing came about after attending a safety seminar by Bruce Bohannon where he described engine out procedures and the need to practice such things BEFORE they happen. As I recall his team had an engine quit following their record setting climb to 41,600 msl in a modified (piston engine) Vans RV4. In the seminar I queried him about the risk of shock cooling during a glide and he convinced me that the risk of not practicing surpassed the chance of engine damage. Like Craig said above I don't think it's necessarily a huge risk if managed with some reasonability.

The other point I want to make is that you get very similar glide results with the throttle pulled to "idle". Still I have no qualms about running a tank dry if there's a reason to do so. Looks like it'll be a great weekend for flying formation! CU there.

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[jlunseth]

7 hours ago, N201MKTurbo said:

I wouldn't be too worried, The engine doesn't know that it is turbocharged. The major thing people worry about is coking the turbo bearings by shutting it down when it is blazing hot or spooling it up before it has adequate oil flow when it is cold. None of that will happen with a wind milling engine. The oil won't cool down that much before you hit the ground unless you start at FL240 and the oil will continue to flow because the engine is still turning. In reality, starting a wind milling engine is probably easier on it then starting it before a flight. As long as you let it cool a bit before you shut it down you shouldn't have any cylinder shock cooling issues and if you are worried about shock heating just bring it up to enough power to maintain altitude for a minute until you go back to climb power.

Unless you are about to hit the ground, then all knobs to the fire wall and think happy thoughts about your engine. 

Well, the oil does cool down and so do the cylinders.  When I was practicing steep spirals and power off 180's for commercial I found that a turn and a half or two turns in a steep spiral would bring both the OT and the CHT below the operating limits in the POH.  The biggest issue is having the oil cool below 100, and not getting good oil flow to the turbo when you power up.  This shows up in several places in my POH.  If doing an air restart, for example, the POH prohibits operating above 16-18" MP until the oil is back above 100.  It warns that catastrophic engine failure is possible.  There are also a couple of spots where it warns not to power up until the CHT's are 250.

A few years ago I had to do a prolonged emergency descent from the flight levels with the engine throttled back.  We made a safe landing, oil all over the belly but it turned out we still had the bare minimum oil in the pan to keep the engine safe.  Nonetheless, not long after a few of the cylinders were losing compression.  The diagnosis was "ring slap" during that descent. The compression rings are normally sealed against the cylinder walls by compression, but with the engine throttled back there is no compression and the rings are free to bounce around in their slots, which hurts both the rings and the cylinder walls.  So we IRAN'd the engine and it has been very happy for several hundred hours as long as I keep the OT and CHT warm.

So I am not a big fan of prolonged, power off descents in my 231.

[cnoe]

11 hours ago, jlunseth said:

Well, the oil does cool down and so do the cylinders.  When I was practicing steep spirals and power off 180's for commercial I found that a turn and a half or two turns in a steep spiral would bring both the OT and the CHT below the operating limits in the POH.  The biggest issue is having the oil cool below 100, and not getting good oil flow to the turbo when you power up.  This shows up in several places in my POH.  If doing an air restart, for example, the POH prohibits operating above 16-18" MP until the oil is back above 100.  It warns that catastrophic engine failure is possible.  There are also a couple of spots where it warns not to power up until the CHT's are 250.

A few years ago I had to do a prolonged emergency descent from the flight levels with the engine throttled back.  We made a safe landing, oil all over the belly but it turned out we still had the bare minimum oil in the pan to keep the engine safe.  Nonetheless, not long after a few of the cylinders were losing compression.  The diagnosis was "ring slap" during that descent. The compression rings are normally sealed against the cylinder walls by compression, but with the engine throttled back there is no compression and the rings are free to bounce around in their slots, which hurts both the rings and the cylinder walls.  So we IRAN'd the engine and it has been very happy for several hundred hours as long as I keep the OT and CHT warm.

So I am not a big fan of prolonged, power off descents in my 231.

Your excellent comments got me to thinking so I analyzed the data from my JPI-830 and found some interesting tidbits. Here's the summary:

Note: The JPI assumed my flight had ended 26 seconds after I pulled the mixture (though the instrument was still powered up) but it seamlessly started producing data again shortly after I re-started the engine.

1) Within the first 22 seconds following shutdown the average CHT fell 24 F. from 322 F. to 298 F.  This is an average cooling rate of 65 F./min. I can live with that.

2) During the entire 3.6 minute descent the average CHT fell from 322 F. to a low of 141 F. (a 181 F. drop). This is an average cooling rate of 50 F./min. which also seems acceptable.

3) The prop speed fell to ~663 rpm following shutdown (and feathering) which is 110 rpm less than what I see on an idle-power glide.

4) Once the prop speed fell my oil pressure dropped from its normal of 73 psi to a steady 53 psi. This increased to 59 psi 3.6 minutes later as the oil cooled some.

5) In the first 22 seconds following shutdown the oil temperature actually INCREASED from 192 F. to 198 F. but then fell to a low of 178 F. during the 3.6 minute descent. For comparison during my previous idle-power glide test lasting nearly 16 minutes the oil temperature fell from 199 F. to 139 F.  In both cases the oil temperature remained well above 100 F. (good to know).

6) This test was conducted in fairly warm temperatures with OAT starting at 61 F. and ending at 70 F. which likely aids in keeping all temps in a safe range.

So, my hopeful conclusion is that the amount of stress imparted on the engine due to cooling was somewhat minimal. I do, though, still have concerns about frequent and/or prolonged operation with the prop being "pushed", particularly when the engine is completely shut down. The vibration and possible piston-ring effects associated with this practice will deter me from doing such tests unnecessarily. Still, I believe that the knowledge gained from measuring the gliding characteristics of my plane is invaluable. I would encourage others to practice power-off glides though my recommendation would be to typically do this at idle-power and then reduce their best-glide distance by perhaps 5%.

I hope this information is of some value and I welcome any comments or questions.

Cnoe

 

 

[Steve Dietrich]

18 hours ago, N201MKTurbo said:

Interesting, two things:

1.  The oil holes in the crank are drilled to supply oil to the highest pressure point which is at ~15 deg. ATDC. When the engine is wind milling the pressure is being applied to the least lubricated side of the journal.

2. Torsional vibration, which is responsible for the yellow arcs on our tachs, has probably not been evaluated for a wind milling engine and could possibly be severe enough to damage an engine.

It would be good to get an expert opinion both of these.

 

21 hours ago, jlunseth said:

My 231 is 12.7:1 according to the POH in cruise configuration with the prop windmilling and at best glide.  I use 12 to 1 to be conservative and because it is easy math.  12,000 feet per thousand feet AGL, or 2 nm per thou.  I would think your lighter J would get that certainly.  I have never tested it though, and don't have the cojones to test it engine out.  There are some complications if you let a turbo engine cool too much and then power up, you can't go full power until you warm the engine and it is just too complicated to deal with, so I don't go there. 

 

It's important  to note the different speeds based on weight (think of the rule as   XX Knots - Y Knots per 200# under gross) to achieve max L/D.  

The glide ratio does not change substantially but the speed to achieve that glide distance changes.

  Also to check if it assumes reduction of rpm to minimum. 

As others indicated above it's useful to think in terms of NM/ 1,000 feet

A few flights in a sailplane is a great investment

 

Edited April 20, 2016 by Steve Dietrich

[jlunseth]

cnoe the concerns are probably a lot different in your NA aircraft than in my turbo.  I think the CHT cooling rates I saw are probably not a lot different than what you saw.  A couple of turns in a steep spiral probably takes 3-4 minutes, so at 50dF that would be 150-200dF, and if I started even at 400 (which is high) I would be at the CHT mimimum operating range of 250 in 3 minutes.  That (minimum CHT) does not bother me so much though, the bigger concern is oil cooling.  I have an oil cooler in my turbo as do most turbos, and it is quite efficient.  It was possible to get OT down in the uncomfortable range of around 100 pretty quickly.  I was doing this late summer, early fall (I took the checkride in early Nov.) so the OAT's were about the same as what you saw. 

I think the situation is different for NA aircraft, among other things you don't have to push cold oil through a skinny turbo bearing, the turbo turning at 100,000.  I am just trying to get the turbo guys not to get too happy about trying long, power off descents just to see what their glide ratio is.  My POH is very explicit about what you need to do to power up again, after a prolonged engine out descent.   

When I took my checkride I showed the POH and the results of my attempts at spirals and power off 180's to my Examiner, and told her I really did not want to do anything involving a prolonged descent with the potential for some kind of power up or go around at the end of it.  We would probably survive it just fine but I have to pay for the new parts.  She agreed.  So we didn't.

"CAUTION

Operating the engine at too high an RPM before reaching minimum oil temperatures may cause loss of oil pressure."

"CAUTION

Should the engine excessively cool during engine out care should be exercised during restart to avoid excessive oil pressure.  Allow the engine to warm up at minimum governing RPM and 16-18 inches MP."  [In my 2312, 16-18" is a final approach descent setting.)

"NOTE

Following descent, do not exceed 20" Hg. Manifold Pressure if cylinder head temperature is below 250 dF."

And there is more in the POH.

OK, OK I get the point.  Don't slam the power in after a long cooling descent. 

[cnoe]

Thanks Jlunseth; that all makes sense. I should qualify my suggestion to "practice power-off glides" to those flying normally aspirated planes. I'm not qualified to offer advice to turbo-normalized or boosted owners.

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