M20J Owners - Glide Slope

[WAFI]

I’ve scoured the internet researching different opinions on Glide Slope, referring to the J models. It seems the big discussion always goes back to feathering props and or stopping your constant speed prop. Full feathering, etc… 

Plugging everything into Forefilght my GS is 11:1 not sure if this is correct. Referencing different sources is seems the base GS varies from 10:1 - 13:1 depending on what year the literature was printed. 

I have a standard two blade McCauley prop. 

Fellow M20J owners, what’s your GS?

[1964-M20E]

If you are asking about glide distance on an engine out scenario then pulling the prop  (blue) knob all the way out does increase your glide ratio.  Go out an practice engine out and pull the knob you can feel the difference.

 

 

[A64Pilot]

Our props are single acting which means oil pressure is used to increase pitch and a spring decreases it. Double acting takes oil pressure to move in any direction.

What this means is that anytime oil pressure is lost, the prop will go to fine pitch. In order to have oil pressure of course the engine has to be spinning, if the engine isn’t running on its own when it’s spinning, then of course the energy to spin the engine is coming from the aircraft, and that will decrease glide distance. Pulling the  blue knob won’t do anything if the prop is stopped, and pulling full pitch on an idling engine may not give a realistic glide as well, the engine is still running.

Best glide distance is achieved with a stopped prop, feathered if possible of course, but I assume most of us don’t have feathering props.

Many aerobatic props work backwards, that is with oil pressure loss, they go to full pitch, reason is if the engine momentarily loses oil pressure it won’t go to very high RPM  like it would with a normal prop, but again I doubt many have an aerobatic prop.

Stopping a prop is really no big deal, just slow down with fuel or ignition turned off and it will stop, and if it’s a healthy engine it will take significant speed to get it going again so it will stay stopped at best glide speed. You can restart merely by lowering the nose and turning fuel or ignition back on. Of course unless your at real significant altitude, your not going to have time to do much except run the checklist and land if the engine quits for real.

I wouldn’t shut down a “hot” engine myself, low power cruise with maybe cowl flaps open to drop the cyl head temp first.

You have to look at the very end of the video, but on the 140 it took 120 mph, right at the yellow and way above best glide to restart.

For a 1946 airplane a 140 is surprisingly efficient, 110 MPH on 85 HP, and with the prop stopped it glides very well, it’s actually way more aerodynamic than later Cessna’s.

Edited October 25, 2021 by A64Pilot

[Shadrach]

On 10/25/2021 at 9:59 AM, A64Pilot said:

Our props are single acting which means oil pressure is used to increase pitch and a spring decreases it. Double acting takes oil pressure to move in any direction.

What this means is that anytime oil pressure is lost, the prop will go to fine pitch. In order to have oil pressure of course the engine has to be spinning, if the engine isn’t running on its own when it’s spinning, then of course the energy to spin the engine is coming from the aircraft, and that will decrease glide distance. Pulling the  blue knob won’t do anything if the prop is stopped, and pulling full pitch on an idling engine may not give a realistic glide as well, the engine is still running.

Best glide distance is achieved with a stopped prop, feathered if possible of course, but I assume most of us don’t have feathering props.

Many aerobatic props work backwards, that is with oil pressure loss, they go to full pitch, reason is if the engine momentarily loses oil pressure it won’t go to very high RPM  like it would with a normal prop, but again I doubt many have an aerobatic prop.

Stopping a prop is really no big deal, just slow down with fuel or ignition turned off and it will stop, and if it’s a healthy engine it will take significant speed to get it going again so it will stay stopped at best glide speed. You can restart merely by lowering the nose and turning fuel or ignition back on. Of course unless your at real significant altitude, your not going to have time to do much except run the checklist and land if the engine quits for real.

I wouldn’t shut down a “hot” engine myself, low power cruise with maybe cowl flaps open to drop the cyl head temp first.

You have to look at the very end of the video, but on the 140 it took 120 mph, right at the yellow and way above best glide to restart.

For a 1946 airplane a 140 is surprisingly efficient, 110 MPH on 85 HP, and with the prop stopped it glides very well, it’s actually way more aerodynamic than later Cessna’s.

 

My experience is that temp and altitude have an impact on whether the prop will remained stopped. During my first flight in a conventional gear aircraft (11AC), the owner/instructor stopped the prop at ~2500’. It was a cold winter day in Indiana. The prop rotated slowly a few degrees but would not move through the compression stroke. Diving at 120MIAS made no difference. We were close to the airport and deadsticked no problem. 
 

On the other hand, the only time I stopped the prop on my F model it was challenging to stop. I actually dropped the flaps a bit because the stall horn was blaring the whole time. Once stopped, the prop really did not want to stay that way.  I was using 105MIAS as a target speed. Much more than that and it would work start turning albeit slowly. This was on a hot august day at 7500’.

Edited October 29, 2021 by Shadrach

[Hank]

@BumpyAcorn, your Owners Manual / POH should have a page a lot like this one for my C. It's the graph at the bottom, note the two lines for "prop windmilling" and "prop stopped":

Of course, pulling the prop knob out should reduce blade pitch and drag, and can certainly be felt when practicing. 

[A64Pilot]

I think it’s more of an engine compression issue, local flying club in Ga had an old tired 172. We couldn’t get it to stop even in a full stall.

My C-85 has relatively low time cylinders and even though a C-85 is not a high compression engine as you can see it stops and stays stopped.

I’ve not stopped my J model but would hope with its high compression engine it would stop really well.

I’ve not done it as it’s really sort of a stupid pet trick, meaning there isn’t really a lot of value in it in my opinion. more to satisfy curiosity.

In the Video I was in downwind at Sylvester Ga airport and at an altitude that I’m sure would take a 360 to land deadstick, so yes if your going to stop a prop be at altitude and have a nice landing area picked out, even though I’ve never heard of one not restarting 

Edited October 25, 2021 by A64Pilot

[Immelman]

Years ago I was curious and decided to try it out as Shadrach did. And felt completely ready to dead stick it in safely if needed. The data I gathered in my E confirmed the book #s for glide ratio prop windmilling and prop stopped. Prop windmilling in low RPM was slightly worse, but not much, than prop stopped.

But if the engine fails in a spectacular fashion I would not count on the ability to control it with the governor.

[ArtVandelay]

The J is 13.2 if you do the math, the speed is dependent on weight, no mention of prop position (just that it’s windmilling).

[A64Pilot]

1 hour ago, ArtVandelay said:

The J is 13.2 if you do the math, the speed is dependent on weight, no mention of prop position (just that it’s windmilling).

That’s another thing, surprisingly weight doesn’t hurt glide distance, but best speed of course increases with weight, and I believe rate of descent does too

[201Mooniac]

3 hours ago, ArtVandelay said:

The J is 13.2 if you do the math, the speed is dependent on weight, no mention of prop position (just that it’s windmilling).

For my 1980 J, doing the math from the chart in the POH yields about 11.4 as best as I can figure.  I looked at 10K ft and found about 21.6 statue miles so did 21.6 * 5280 / 10000 and got 11.4.  Am I doing something wrong or did the chart change over the years?

BTW, I checked the POH issued on 9/28/1982 and found the same chart

[Shadrach]

3 hours ago, A64Pilot said:

I think it’s more of an engine compression issue, local flying club in Ga had an old tired 172. We couldn’t get it to stop even in a full stall.

My C-85 has relatively low time cylinders and even though a C-85 is not a high compression engine as you can see it stops and stays stopped.

I’ve not stopped my J model but would hope with its high compression engine it would stop really well.

I’ve not done it as it’s really sort of a stupid pet trick, meaning there isn’t really a lot of value in it in my opinion. more to satisfy curiosity.

In the Video I was in downwind at Sylvester Ga airport and at an altitude that I’m sure would take a 360 to land deadstick, so yes if your going to stop a prop be at altitude and have a nice landing area picked out, even though I’ve never heard of one not restarting 

My comp reading are all >77/80.  It’s not a tired engine.

my theory is that the C85 in a the 11AC I was flying had straight weight oil and perhaps the cold (~10f) caused the oil to thicken enough during shutdown to inhibit a restart. I also think the type of prop installed makes a difference in term of both pitch and weight. The Hartzell on the Mooney weighs nearly twice that of the Sensenich on the Chief.
 

[EricJ]

You might want to change the thread title to 'glide ratio' rather than 'glide slope', which is a specific part of an instrument approach.

I don't know whether there's enough oil pressure to keep the prop at high pitch if it's windmilling after an engine failure.    I also don't know whether the combination of pulling the prop and reducing airspeed to near stall is sufficient to stop the engine rotation.    I do know that a C150 with an O-200 and nearly any kind of associated prop (climb, cruise, whatever), can stop the prop if you drop the airspeed sufficiently, and it won't start spinning again unless you gain a lot more airspeed.    In that application it does extend glide significantly.

 

[ArtVandelay]

For my 1980 J, doing the math from the chart in the POH yields about 11.4 as best as I can figure.  I looked at 10K ft and found about 21.6 statue miles so did 21.6 * 5280 / 10000 and got 11.4.  Am I doing something wrong or did the chart change over the years?

BTW, I checked the POH issued on 9/28/1982 and found the same chart

Opps, yes I was thinking nautical miles when I did my math.

[PT20J]

Best glide is achieved at the angle of attack for L/D max which doesn't depend on weight. But, if you increase the weight, the lift has to increase to compensate and since the angle of attack is fixed for best glide, the airspeed has to increase to make up the difference. If the airspeed increases, the sink rate has to increase to keep the angle the same. So, at higher weights, you just slide down the 11:1 slope at a faster speed and get to the ground sooner. But you still go the same distance.

Skip

[WAFI]

Well said PT20J that makes better sense. Airspeed is definitely key depending on the the planes gross weight. 

[A64Pilot]

22 hours ago, Shadrach said:

My comp reading are all >77/80.  It’s not a tired engine.

my theory is that the C85 in a the 11AC I was flying had straight weight oil and perhaps the cold (~10f) caused the oil to thicken enough during shutdown to inhibit a restart. I also think the type of prop installed makes a difference in term of both pitch and weight. The Hartzell on the Mooney weighs nearly twice that of the Sensenich on the Chief.
 

Could be, you never know until you try it.

Next time I’m out without the Wife along I’ll try it in the Mooney, J model with factory prop.

I believe our flat pitch setting is courser than average, I say that as it takes way less RPM for me to taxi up our slight uphill than my neighbors Bonanza.

Of course it could be their flat pitch is finer than average too, or maybe it’s just the airplane weight?

I won’t pull the pitch knob, I’ll leave it be. Once that engine stops it’s not going to matter, the prop is going flat in a short time, anyone who flies turbines shut down without feathering, by the time you get out most likely the prop will be in feather

Edited October 26, 2021 by A64Pilot

[kortopates]

Definetly need to go by your POH - as long as you have the same Prop listed in your POH. But here are a number of Mooney models among some other I have done the calc's for a class I teach. My J model comes from a more recent year though that uses knots but the units (SM or NM) shouldn't change the Glide Ratio - and their nearly identical. None of the Vintage POH's I have publish a Glide ratio.

Note the K mid-body has the best Glide Ratios, followed by the J and all the longbody's.

[PT20J]

Low pitch is going to be set to absorb rated horsepower at rated rpm on a standard day. 

There was an old NACA study where they tried various configurations in a wind tunnel. Lowest drag, of course, was feathered. The next lowest was windmilling but not connected to the engine -- just freely rotating. Whether stopped or windmilling in high pitch is lowest drag depends on the aircraft. When windmilling, it is driving the engine against friction and compression. When stopped, you have the blades flat against the free stream airflow which is a pretty high drag condition. For many airplanes you have to get pretty close to stall to stop the prop.

Personally, I would just pull the prop back and keep the throttle wide open to reduce the pumping loss.

Skip

[A64Pilot]

6 hours ago, PT20J said:

Low pitch is going to be set to absorb rated horsepower at rated rpm on a standard day. 

The governor plays into that too of course, What your saying would be correct if and only if your prop hit it’s RPM set point when the engine was at full throttle. but I bet it will hit that point just prior to full throttle, once governed RPM is hit the governor will of course control RPM with pitch. 

I have way more experience with turbines and on those we would adjust the flat pitch to obtain desired braking force when the engine was reduced to idle, the flatter pitch of course gave the most drag. You have “beta” nuts on a turbines prop so min pitch is easily adjusted simply by turning one nit per blade.

On a piston engine the pitch can’t be too flat as the aircraft has to be able to maintain flight with a failed governor.

[PT20J]

2 hours ago, A64Pilot said:

The governor plays into that too of course, What your saying would be correct if and only if your prop hit it’s RPM set point when the engine was at full throttle. but I bet it will hit that point just prior to full throttle, once governed RPM is hit the governor will of course control RPM with pitch. 

I was just describing how the low pitch stops are set. During runup, the rpm is a good check that the engine is putting out rated power because the governor is out of the picture and the prop is sitting on the low pitch stop. As the airspeed increases during the takeoff, the prop will want to increase speed and then the governor will control it. The low pitch stops on the Hartzells can be adjusted but the McCauley's require disassembling the hub and changing a spacer.

Skip

[ZuluZulu]

On 10/25/2021 at 5:54 AM, BumpyAcorn said:

I’ve scoured the internet researching different opinions on Glide Slope, referring to the J models. It seems the big discussion always goes back to feathering props and or stopping your constant speed prop. Full feathering, etc… 

Plugging everything into Forefilght my GS is 11:1 not sure if this is correct. Referencing different sources is seems the base GS varies from 10:1 - 13:1 depending on what year the literature was printed. 

I have a standard two blade McCauley prop. 

Fellow M20J owners, what’s your GS?

You know the J's OEM two-blade McCauley does not feather, right?

Edited October 27, 2021 by ZuluZulu

[WAFI]

8 hours ago, ZuluZulu said:

You know the J's OEM two-blade McCauley does not feather, right?

Yes I am aware but thanks for pointing that out anyway. 

[A64Pilot]

I went out this morning and flew, first on run up, my prop hit full governed RPM at about 25 to 26 inches, so at full manifold pressure which was for me today about 28 inches or so I was definitely on the governor before the aircraft moved, I didn’t video that as in my neighborhood I was right beside someone’s house and didn’t want to spend much time at full RPM due to noise.

Anyway went out and flew and shut down the motor and very, very slowly decreased airspeed until the prop stopped, it wasn’t a stall, but it wasn’t far off either.

I’m in the country and it took forever for my phone to upload, I’ll try to upload here, but no promises, it’s a cell phone video so it’s not very good etc.

A few opinions, 

First I don’t really see it being of much good to extend the glide, reason is your rate of descent and glide distance are pretty bad when approaching a stall, and recovering  airspeed back to best glide, it’s unlikely in my opinion that the increased glide distance from a stopped prop will overcome the altitude loss from stopping the prop.

Maybe if your at the flight levels flying over one of those big lakes you guys have up North and need every foot of glide distance, then maybe?

Secondly with the prop stopped your giving up on the engine coming back on its own, while unlikely it’s possible the engine quit due to a slug of water etc and it could come back once it pumps the water through or whatever, plus when stopped you lose your vacuum, sure it’s low at low RPM windmilling, but at least there is some, stopped there is none of course. Of course IFR I wouldn’t pull it close to stall myself.

It’s only a 2min 43 sec video, but apparently an enormous file size, even seems to be slow motion at times? Can one of you smart guys fix it?

Its not much to look at really, 

Once prop was stopped it took 120 kts to get it going again, and even with the cowl flaps closed, the cyl head temp drops off of the scale

Edited October 27, 2021 by A64Pilot

[ZuluZulu]

8 hours ago, BumpyAcorn said:

Yes I am aware but thanks for pointing that out anyway. 

I guess I don't understand why you mentioned it twice, then, but I appreciate the response.

[WAFI]

On 10/27/2021 at 4:24 PM, ZuluZulu said:

I guess I don't understand why you mentioned it twice, then, but I appreciate the response.

I wasn’t referencing my prop but just noting where most conversations go when talking about Glide Ratio / Glide Slope, etc..