M20J glide ratio

[Hank]

18 minutes ago, WAFI said:

When you say "Full Coarse" do you mean high rpm or low rpm?

Pull the knob towards you, low RPM, for max glide. But you do need oil pressure to hold the blades at any angle other than the flattest it will go.

Push the knob forwards, high RPM, for additional braking [since regular brakes don't work in the air].

[A64Pilot]

Slowing the prop works because the energy required to spin the motor over is coming from the airplanes potential energy from altitude, slow it down, less energy stolen, better glide.

In theory stopping the prop gives best glide, but in my airplane you have to slow to stall and hold it to stop the prop, and the altitude lost doing that and accelerating back to best glide I think is greater than any gained from stopping the thing, plus taking attention away from you finding a forced landing area, so I removed stopping the prop as a viable technique in my mind.

Funny thing is it’s easy to stop in my 140, maybe being a fixed prop is what’s different?

[PT20J]

At L/D max, half the drag is parasitic and half is induced. Assuming a good aerodynamic shape, the best way to minimize parasitic drag is to keep the wetted area as small as possible. Since induced drag comes from tip effects, an infinitely long wing has no induced drag. That's impossible to realize of course, so the next best thing is a high aspect ratio for the wing area required.

There's an old NACA report on propeller drag. Lowest was feathered, of course. Almost as low was windmilling but not attached to an engine. Stopped may or may not be lower drag than windmilling an engine -- it depends on the total blade area (lot of flat plate drag).

Skip

[DCarlton]

3 hours ago, Hank said:

2 miles per 1000' is a good general number for your Mooney.

Just be aware that your prop does NOT feather! But moving the prop control to Full Coarse is required to reach this glide number.

Makes sense but does it say full course in any of the Mooney docs? It would be nice to have a reference.  Thanks.

[RobertGary1]

2 hours ago, A64Pilot said:

Slowing the prop works because the energy required to spin the motor over is coming from the airplanes potential energy from altitude, slow it down, less energy stolen, better glide.

In theory stopping the prop gives best glide, but in my airplane you have to slow to stall and hold it to stop the prop, and the altitude lost doing that and accelerating back to best glide I think is greater than any gained from stopping the thing, plus taking attention away from you finding a forced landing area, so I removed stopping the prop as a viable technique in my mind.

Funny thing is it’s easy to stop in my 140, maybe being a fixed prop is what’s different?

Having done some experimenting with this in an Aeronca I can confirm that stopping the prop did improve glide ratio. We did a number of tests over a dessert airport some decades back.

[Sportster64]

someone asked about the reference to pulling the prop back, It actually states in this POH in regards to glide distances to pull prop back to increase the glide ratio..

http://www.avsoft.com.au/ext/m20tnpoh.pdf

 

[carusoam]

Engine dead?

Pull blue knob as far back as possible…

if at higher altitude, it can make sense to stop the engine…. But that is done at a very slow speed, and may not stay stopped…

 

If you have never felt the braking power of dropping your engine into low gear…

Push the prop all the way in, with throttle closed… even at traffic pattern speeds you can feel it…

Better to experiment in a cheap firebird, than to use an expensive aircraft engine…

 

The prop is a giant speed brake… employ it when you want it…

Take it out of the equation when you don’t want it…

 

prayers… if you need the decimal places in the glide ratio to get where you are going…  10:1 is easier for the mental math…

Selecting a good place to land is a better discussion…

Two Mooneys have been lost in recent history… selecting a roadway… with other fields available…

Best regards,

-a-

[A64Pilot]

On 11/22/2022 at 3:53 PM, PT20J said:

At L/D max, half the drag is parasitic and half is induced. Assuming a good aerodynamic shape, the best way to minimize parasitic drag is to keep the wetted area as small as possible. Since induced drag comes from tip effects, an infinitely long wing has no induced drag. That's impossible to realize of course, so the next best thing is a high aspect ratio for the wing area required.

There's an old NACA report on propeller drag. Lowest was feathered, of course. Almost as low was windmilling but not attached to an engine. Stopped may or may not be lower drag than windmilling an engine -- it depends on the total blade area (lot of flat plate drag).

Skip

A sailboat is just opposite of an airplane, a free wheeling prop is much less drag than a stopped one, your not spinning a dead engine on a boat.

The reason is a three bladed sailboat props blades encompass almost the entire disk area, but in comparison an airplane prop looks like long thin sticks. Maybe 10% of swept area?

[DCarlton]

33 minutes ago, carusoam said:

Engine dead?

Pull blue knob as far back as possible…

if at higher altitude, it can make sense to stop the engine…. But that is done at a very slow speed, and may not stay stopped…

 

If you have never felt the braking power of dropping your engine into low gear…

Push the prop all the way in, with throttle closed… even at traffic pattern speeds you can feel it…

Better to experiment in a cheap firebird, than to use an expensive aircraft engine…

 

The prop is a giant speed brake… employ it when you want it…

Take it out of the equation when you don’t want it…

 

prayers… if you need the decimal places in the glide ratio to get where you are going…  10:1 is easier for the mental math…

Selecting a good place to land is a better discussion…

Two Mooneys have been lost in recent history… selecting a roadway… with other fields available…

Best regards,

-a-

This leads to yet another another question....  at what point in a normal approach do most folks push the prop all the way forward?  Same time you drop to full flaps?  :>  

[A64Pilot]

2 hours ago, DCarlton said:

This leads to yet another another question....  at what point in a normal approach do most folks push the prop all the way forward?  Same time you drop to full flaps?  :>  

I push prop forward on final, when I’m sure it won’t cause the engine to accelerate. Not that it hurts anything but I was taught to be a Commercial pilot, and we were taught no alarms or revving engines should be done because it could make Pax nervous, bedsides if the engine accelerates when you push it forward it changes your glide slope.

Just the technique I was taught.

I don’t think there is any “right” answer

 

[Scottknoll]

I push prop forward on final, when I’m sure it won’t cause the engine to accelerate. Not that it hurts anything but I was taught to be a Commercial pilot, and we were taught no alarms or revving engines should be done because it could make Pax nervous, bedsides if the engine accelerates when you push it forward it changes your glide slope.
Just the technique I was taught.
I don’t think there is any “right” answer
 

Agreed, the moment your governor can no longer maintain selected RPM, moving the Blue knob forward has no effect on current propeller blade angle. You’re already at the low pitch stop, it can’t reduce prop blade angle any further.

[Hank]

25 minutes ago, Scottknoll said:

Agreed, the moment your governor can no longer maintain selected RPM, moving the Blue knob forward has no effect on current propeller blade angle. You’re already at the low pitch stop, it can’t reduce prop blade angle any further.

For me, that's typically right after Takeoff Flaps go down and I slow to 100 or so. NLT pattern entry on downwind. On approach, sometime after Gear Down to Go Down--there is so much variation in distance from FAF to DA / DH / MAP that I can't offer a generalization that fits very many.

[PT20J]

The only reason for pushing the prop forward is to have full power available in the event of a go around. You should do it after power and airspeed are low enough to be outside the prop governing range so that you don't announce your presence in the pattern with a mighty roar, but anytime after that up until the point where you initiate a go around is a matter of choice and habit. In the Mooney, I do it after reducing power and putting flaps down abeam the numbers when I do my first GUMP check. I think the airlines used to do it on short final which is what we did in the museum DC-3 and B-25. That's also what I did in the float Beaver.

Skip

[M20F]

On 11/22/2022 at 7:55 AM, N201MKTurbo said:

 cranked it into a 60 degree turn. 

59.5 degrees :-)

[N201MKTurbo]

7 minutes ago, M20F said:

59.5 degrees :-)

Oh, did I go too far?

[DCarlton]

16 hours ago, PT20J said:

The only reason for pushing the prop forward is to have full power available in the event of a go around. You should do it after power and airspeed are low enough to be outside the prop governing range so that you don't announce your presence in the pattern with a mighty roar, but anytime after that up until the point where you initiate a go around is a matter of choice and habit. In the Mooney, I do it after reducing power and putting flaps down abeam the numbers when I do my first GUMP check. I think the airlines used to do it on short final which is what we did in the museum DC-3 and B-25. That's also what I did in the float Beaver.

Skip

I'm learning something here or I haven't thought about it in a long time.   If I'm early in the pattern I will slowly roll in the prop to avoid the surge.  At other times, I've never noticed exactly when the governor can no longer maintain the setting.  If I have the prop set to 2400 for example, and then reduce the throttle to where the RPMs drop below 2400, is that the point where it can no longer maintain the range?  I'm gonna look at the manifold settings next time I see that behavior.  Curious now.  

[Hank]

After lots of "how to land" discussions here, I finally paid attention in the pattern the other day.

Downwind at 90mph, Takeoff flaps, is 13-14"; dropping gear abeam intended point of landing to start descent, reduce power to 12-13" and I'll be on glideslope when I turn final--any lower power and I have to add power to reach the runway. 

[A64Pilot]

I get the gear on entry to downwind at idle, then add throttle, above 15” puts me in the yellow prop avoid range so from 15”MP to 20” isn’t usable, but even 20” won’t keep me level so it’s a non issue, just have to watch it is all. I stay at cruise RPM until final.

The reason I get gear so early is to get them down, the later in the approach things get busier often and I want to make sure I have the gear down, so it’s the first action I take, it may be more logical to wait until you want the descent, but I figure about the only thing I can screw up that’s not recoverable is forgetting the gear, I don’t think I’ll stall, but those that do didn’t think they would either.

My airspeed is a lot lower on short final than I read about here, I don’t really watch it, but I’ve looked and it’s in the mid 60’s kts on short final, but I don’t float either, only time I may hear the stall horn is just prior to or just after touchdown and my warning sounds about 4 or 5 kts prior to stall break, power off flaps and gear down

[Pinecone]

On 11/23/2022 at 5:12 PM, A64Pilot said:

A sailboat is just opposite of an airplane, a free wheeling prop is much less drag than a stopped one, your not spinning a dead engine on a boat.

The reason is a three bladed sailboat props blades encompass almost the entire disk area, but in comparison an airplane prop looks like long thin sticks. Maybe 10% of swept area?

It depends on the sailboat prop.

One the sailboat I owned, stopping the prop was lowest drag.  

But that was because when it stopped, it folded the blades.

Jets have high L/D because they have low drag.  The T-38 (25 foot wingspan) has a glide ration of 9.6 to 1.  Versus the C-172 at 8.8 to 1 (according to my 2972 POH).

[carusoam]

On 11/23/2022 at 4:12 PM, A64Pilot said:

A sailboat is just opposite of an airplane, a free wheeling prop is much less drag than a stopped one, your not spinning a dead engine on a boat.

The reason is a three bladed sailboat props blades encompass almost the entire disk area, but in comparison an airplane prop looks like long thin sticks. Maybe 10% of swept area?

The reason is…

The sail boat has a transmission, with neutral as an option….  
 

-a-

[PT20J]

A good approximation for most single engine airplanes is 5 nm for every 3000'.

[jaylw314]

On 11/24/2022 at 10:53 AM, DCarlton said:

I'm learning something here or I haven't thought about it in a long time.   If I'm early in the pattern I will slowly roll in the prop to avoid the surge.  At other times, I've never noticed exactly when the governor can no longer maintain the setting.  If I have the prop set to 2400 for example, and then reduce the throttle to where the RPMs drop below 2400, is that the point where it can no longer maintain the range?  I'm gonna look at the manifold settings next time I see that behavior.  Curious now.  

If the RPM drops when you reduce the throttle any, you've hit the fine pitch stops.  at that point, you can crank the blue knob forwards.  In the pattern, you can just listen for the RPM drop on downwind or base, you can keep your eyes outside

Edited November 28, 2022 by jaylw314

[carusoam]

On 11/24/2022 at 1:53 PM, DCarlton said:

I'm learning something here or I haven't thought about it in a long time.   If I'm early in the pattern I will slowly roll in the prop to avoid the surge.  At other times, I've never noticed exactly when the governor can no longer maintain the setting.  If I have the prop set to 2400 for example, and then reduce the throttle to where the RPMs drop below 2400, is that the point where it can no longer maintain the range?  I'm gonna look at the manifold settings next time I see that behavior.  Curious now.  

you might be seeing…

1) governor wear issues…

2) fly weight system may be at fault…. Wear and stickiness…

3) the valve the flyweights are attached too may be a bit worn….

4) The gear pump inside the gov may be running out of pumping volume at lower rpm…

 

Lots of moving parts inside the gov…

Outside the gov…

5) prop oil pressure gets drained to the sump

6) there are a few places MSers have found internal leaks that drained too much pressure…

Swapping out the gov is often a quick fix…

hunting down internal leaks… one is easy… the other one, took disassembly to find… 

PP thoughts only,

-a-

[DCarlton]

56 minutes ago, carusoam said:

you might be seeing…

1) governor wear issues…

2) fly weight system may be at fault…. Wear and stickiness…

3) the valve the flyweights are attached too may be a bit worn….

4) The gear pump inside the gov may be running out of pumping volume at lower rpm…

 

Lots of moving parts inside the gov…

Outside the gov…

5) prop oil pressure gets drained to the sump

6) there are a few places MSers have found internal leaks that drained too much pressure…

Swapping out the gov is often a quick fix…

hunting down internal leaks… one is easy… the other one, took disassembly to find… 

PP thoughts only,

-a-

I don't think I have a problem.  Was asking about the point where the governor can no longer maintain RPM due to throttle setting when you reduce power in the pattern.  A feature; not a problem. 

[Pinecone]

Yes, when you reduce throttle to the point the RPM drops, you are at the fine pitch stop.  At that point, there is no change in RPM by pushing in the blue knob