Engine stumbled two back to back flights

[PT20J]

8 hours ago, Will.iam said:

I don’t think so. It’s the blade pitched more toward the wind like a fethered prop just not as extreme but the drag is way reduced at the low rpm in fact if i could pitch the blade into a full feather i. E. Blade is parallel to the air the propellor and thus the engine would stop and i would have zero rpm’s and the least amount of drag. If i push the prop control full forward and command a min pitch / blades are almost perpendicular to the air then the air is pushing the hardest and i get maximum rpm and i really feel the drag come on more so than speed brakes and about the same a applying full flaps. 

There is a difference between drag on the blades which affects windmilling rpm and the drag on the airplane which affects glide ratio. It takes power to rotate the propeller. When the engine is not running, that power comes from decreasing the potential energy of the airplane at some rate. The faster the rotation, the greater the rate of altitude loss.

[redbaron1982]

35 minutes ago, PT20J said:

There is a difference between drag on the blades which affects windmilling rpm and the drag on the airplane which affects glide ratio. It takes power to rotate the propeller. When the engine is not running, that power comes from decreasing the potential energy of the airplane at some rate. The faster the rotation, the greater the rate of altitude loss.

I guess a way of seeing this is that each rotation of the prop either produces energies (when there is combustion in the cylinders) or consumes it (when there is no combustion are you are using your engine as a sophisticated air compressor). Now to convert to power, you need to factor the speed at which this exchange occurs: here enters the RPM into the equation. As when the engine is producing power, higher RPM means higher power output (all other conditions the same), then when the engine is not producing power (i.e. is an air compressor), the higher the RPM, the higher the power consumption, hence the higher break effect and loss of potential energy.

[Hank]

40 minutes ago, PT20J said:

There is a difference between drag on the blades which affects windmilling rpm and the drag on the airplane which affects glide ratio. It takes power to rotate the propeller. When the engine is not running, that power comes from decreasing the potential energy of the airplane at some rate. The faster the rotation, the greater the rate of altitude loss.

Yes, pulling the prop control back under power reduces prop RPM because at higher blade angle it takes more power to rotate the prop, and you aren't changing power. When windmilling without power, pulling prop back and increasing blade angle reduces aerodynamic drag from the blades, reduces RPM, reduces descent and increases glide range.

1 minute ago, redbaron1982 said:

I guess a way of seeing this is that each rotation of the prop either produces energies (when there is combustion in the cylinders) or consumes it (when there is no combustion are you are using your engine as a sophisticated air compressor). Now to convert to power, you need to factor the speed at which this exchange occurs: here enters the RPM into the equation. As when the engine is producing power, higher RPM means higher power output (all other conditions the same), then when the engine is not producing power (i.e. is an air compressor), the higher the RPM, the higher the power consumption, hence the higher break effect and loss of potential energy.

Yes, airflow making the dead engine spin faster consumes more energy, increasing drag and descent rate, but I think the increase in aerodynamic drag from flattening the blades does more.

[PT20J]

5 minutes ago, Hank said:

Yes, pulling the prop control back under power reduces prop RPM because at higher blade angle it takes more power to rotate the prop, and you aren't changing power. When windmilling without power, pulling prop back and increasing blade angle reduces aerodynamic drag from the blades, reduces RPM, reduces descent and increases glide range.

Yes, airflow making the dead engine spin faster consumes more energy, increasing drag and descent rate, but I think the increase in aerodynamic drag from flattening the blades does more.

NACA did a test on that measuring drag on a windmilling vs stopped prop in a wind tunnel many years ago and found the windmilling prop had significantly more drag as I recall. 

The idea behind a constant speed prop is to set the advance ratio of of the prop to the region of maximum efficiency - essentially maximum L/D ratio. When the pitch is very high, blade lift (thrust) decreases and drag increases. 

[Hank]

19 minutes ago, PT20J said:

NACA did a test on that measuring drag on a windmilling vs stopped prop in a wind tunnel many years ago and found the windmilling prop had significantly more drag as I recall. 

The idea behind a constant speed prop is to set the advance ratio of of the prop to the region of maximum efficiency - essentially maximum L/D ratio. When the pitch is very high, blade lift (thrust) decreases and drag increases. 

Yep, a stopped prop is a lot less drag than one turning at any blade angle. But the only Mooneys I'm ware of with fully-feathering props are Rockets, unless the Acclaims and Ultras do also (I can't even dream about flying one of those).

So for the overwhelming majority of us who can't feather our props, pulling it back will reduce drag and increase glide range when it is no longer producing sufficient (or any) power. If you're lucky enough, pull it to Feather position, also "all the way back" I believe.

Yes, it is possible to pull back the yoke and trade airspeed for altitude and stop the prop on your dead engine. But I've read studies across multiple aircraft types and the general consensus is so much energy is lost doi g this that pitching back down for best glide speed gives a shorter total glide distance than simply pulling prop all the way back and pitching for best glide from the beginning, or sometimes the best glide speed will start the prop turning again with reduced range from energy lost while stopping the prop.

So my plan, and periodic practice, is to pitch for best glide, then feel the acceleration as I pull the prop all the way back. Wheeee!

We are now officially way out in the weeds, having started with engine stumbling and now into dead engine gliding.

[redbaron1982]

So, I think, although there is not consensus on the why, that having the prop full back gives better glide distance.

Now, what about the throttle? Technically full forward would reduce the energy required for the cylinders to pull air in, and should extend the glide slope.

[Will.iam]

4 hours ago, redbaron1982 said:

So, I think, although there is not consensus on the why, that having the prop full back gives better glide distance.

Now, what about the throttle? Technically full forward would reduce the energy required for the cylinders to pull air in, and should extend the glide slope.

Humm that’s an interesting hypothesis, i will test that out next time I’m in the air and find out. It might be so little change to not feel a difference between full throttle and closed. 

[hazek]

15 hours ago, Will.iam said:

Blade is parallel to the air the propellor and thus the engine would stop and i would have zero rpm’s and the least amount of drag.

For propeller blades relative wind is not from head on though? It’s at an angle due to the spinning of the propeller. It’s the sum vector of the forward speed of the plane and the rotational speed of the propeller. It’s head on only if the plane is moving forward and the propeller is stopped.

[Will.iam]

3 hours ago, hazek said:

For propeller blades relative wind is not from head on though? It’s at an angle due to the spinning of the propeller. It’s the sum vector of the forward speed of the plane and the rotational speed of the propeller. It’s head on only if the plane is moving forward and the propeller is stopped.

That’s cute. When the blades go to feather all that relative wind doesn’t keep it spinning and in fact the propeller slows to a stop. 

[Pinecone]

10 hours ago, redbaron1982 said:

Now, what about the throttle? Technically full forward would reduce the energy required for the cylinders to pull air in, and should extend the glide slope.

There are two losses.

Sucking the air in

Compressing it.

Full throttle lowers the first but increases the second.

[Shadrach]

10 hours ago, redbaron1982 said:

So, I think, although there is not consensus on the why, that having the prop full back gives better glide distance.

Now, what about the throttle? Technically full forward would reduce the energy required for the cylinders to pull air in, and should extend the glide slope.

Nope…close the throttle.

[EricJ]

13 minutes ago, Pinecone said:

There are two losses.

Sucking the air in

Compressing it.

Full throttle lowers the first but increases the second.

You get much of the compression effort back in the expansion during the power stroke, though, so it's not a total loss.

[PBones]

I am just curious to learn, why in normal circumstances would anyone want to run an airplane tank dry on purpose? In my automotive world, it is the worst thing you can do to older classic and muscle cars. AV gas stays well and is not like auto gas with deteriorating additives, alcohol and low grade processes. Going low on gas to that point will drag every particle into the fuel system. I would drain the tank on the ground if I wanted to remove fuel or debris, not run it dry.

An engine running at 2000 plus rpms will not like gasping for fuel, cylinder imbalance and losing the lubricating effects to the cylinder walls (extreme lean condition) this is probably the last thing I would ever want to happen to one of my automotive high performance engines running at 5500 rpms. It will probably survive, but longevity will decrease.

[201er]

2 hours ago, PBones said:

I am just curious to learn, why in normal circumstances would anyone want to run an airplane tank dry on purpose?

To avoid running it dry inadvertently during a more critical phase of flight 

[PT20J]

I wonder how hard it is in the dynamic counterweight bushings to abruptly go from high power to power off at cruise speed on an IO-360-A3B6(D)?

[Pinecone]

3 hours ago, PBones said:

I am just curious to learn, why in normal circumstances would anyone want to run an airplane tank dry on purpose?

In my autos will fuel pumps in the fuel tank, I try to fill up before 1/4 tank to keep the pump in fuel.  A straw poll on one of the BMW forums convinced me of this.

The only time I would intentionally run a tank dry is if I was very tight on fuel and wanted to have all the fuel on board in one tank.   I have not ended up in that situation.   I did have one were I ran one tank to the low fuel light (2.5 - 3 gallons) to have about 12 - 15 in the other tank

[PBones]

 201er, I am trying to figure this out you said "To avoid running it dry inadvertently during a more critical phase of flight".

How does purposely running it dry anytime, in flight or otherwise, avoid doing it in flight? And what does it prove except the accuracy of the gauges, which can be done by draining the fuel on the ground or using a stick.

Sorry I am just trying to learn why anyone would run their fuel tanks down to engine starvation, to prove what?

 

Edited Thursday at 04:22 PM by PBones

[wombat]

@PBonesA more critical phase of flight is short final.

Let's say I want to fly VFR from KPHP to KTTD... It's 890 miles, which is near the edge of the range of my plane. According to the current weather reports, that leaves me with 11.5 gallons.   When you factor in reserve fuel (18 GPH, * 30 minutes = 9 gallons).

If I run one tank dry, I will have 11.5 gallons in one tank and 0 in the other.  Minus 1 gallon unusable fuel.   No problem, even if winds are a little worse than forecast.

If I don't run one tank dry, I'll have about 6 gallons per side and risk running out on final is a real risk.  Particularly if you had slightly higher headwinds than planned.

[201Mooniac]

2 hours ago, PT20J said:

I wonder how hard it is in the dynamic counterweight bushings to abruptly go from high power to power off at cruise speed on an IO-360-A3B6(D)?

I've been told (many years ago by a Lycoming factory rep) that it is bad enough to not let the RPMs drop more than 400 on a prop check during the runup.  I would imagine high power to power off would be worse. 

I wouldn't expect that to be the case though when running the tank dry if you are ready for it and switch quickly.  I never flight plan close enough on fuel that I've had a need to run a tank dry so I'm not sure but I forgot to check the fuel selector position once after annual and started up and the engine began to die and I realized the selector was in off and switched it to the left tank without much more than the stumble which wasn't a huge drop in RPMs.

[201Mooniac]

1 hour ago, Pinecone said:

In my autos will fuel pumps in the fuel tank, I try to fill up before 1/4 tank to keep the pump in fuel.  A straw poll on one of the BMW forums convinced me of this.

Seems to be the consensus on many of the auto type forums.  I'm told it is due to the fuel helping to cool the pump but haven't verified if this is really true or an owt.

[PBones]

I completely understand the calculations and planning and see that you are highly qualified, Thanks. I am new to this and have been studying intensely and the reason I am reading as many posts as I can. I still don't understand purposely running out a tank while the engine is running.

For me, and this is just me,  I will never "plan" to be that low on fuel at any point when flying because calculating fuel use is one thing, what can actually happen is another. I would rather make a stop than to risk my engine stalling and not restarting for some reason or doing damage to the engine by running out of fuel in one tank.

I have taken online classes that discuss fuel management where they calculate their fuel based on a percentage being a state of empty ie: 25 gallon tank is empty after use of 20 gallons.

I flew with a friend and during the preflight I stuck the tanks and found each had between 8 to 10 gallons. We were only going to stay in the pattern and he was OK with these levels, but I said, no and paid to put 10 gallons in each tank. No harm in having more. As everyone here keeps telling me "airplanes are expensive". I think fuel is the cheapest part of that!

Too many stories of pilots running out of fuel cutting it too close. One small plane crashed near my home because the pilot kept stretching his fuel even though he passed multiple airports he planned on landing at. Unfortunately, he missed the final runway by 3 miles.

[0TreeLemur]

On 8/6/2025 at 9:43 PM, PT20J said:

pax HATE it when the engine quits.

So do spouses!  When we flew a C model I used to like to run a tank dry when doing long cross-country flights so I was left with all useable in one tank.  Normally I noticed the fp drop and switched without anybody else knowing.   That one time I didn't, I got her attention.   From that point forward, when I would work to dry a tank I'd tell her.  Man, she was all over that fp gage.   Never happened again.

In our J with LOP ops burning 8-8.5 gph and 64 useable, I practically never need to run one dry.   Six hours is about the longest leg I want to fly.

[0TreeLemur]

1 hour ago, wombat said:

@PBonesA more critical phase of flight is short final.

Let's say I want to fly VFR from KPHP to KTTD... It's 890 miles, which is near the edge of the range of my plane. According to the current weather reports, that leaves me with 11.5 gallons.   When you factor in reserve fuel (18 GPH, * 30 minutes = 9 gallons).

If I run one tank dry, I will have 11.5 gallons in one tank and 0 in the other.  Minus 1 gallon unusable fuel.   No problem, even if winds are a little worse than forecast.

If I don't run one tank dry, I'll have about 6 gallons per side and risk running out on final is a real risk.  Particularly if you had slightly higher headwinds than planned.

Exactly.    WWII pilots routinely ran tanks dry.  Nothing quite as worthless as an unknown quantity of fuel sitting in the bottom of a tank when you are trying to get somewhere.   The POH for the older Mooneys more or less says: Take off and fly 1 hour on one tank, then empty the other tank, and land back on the tank you took off with.

[EricJ]

25 minutes ago, 201Mooniac said:

I've been told (many years ago by a Lycoming factory rep) that it is bad enough to not let the RPMs drop more than 400 on a prop check during the runup.  I would imagine high power to power off would be worse. 

The 100-hour/Annual inspection checklist from Mooney (Form 100B, the Aug 2018 edition) covers all models and in the Post Inspection Operational Check section it says:

1. Check propeller governor operation with engine running at 2000 RPM & pitch control at low pitch
(High RPM): When propeller control is pulled out to high pitch (low RPM), engine speed should
decrease at least 500 RPM.

I've seen other general inspection guidance that 500 rpm is a minimum drop to look for, not a limit.   I'd think if there was an important limit it would placarded.

[201Mooniac]

32 minutes ago, EricJ said:

The 100-hour/Annual inspection checklist from Mooney (Form 100B, the Aug 2018 edition) covers all models and in the Post Inspection Operational Check section it says:

1. Check propeller governor operation with engine running at 2000 RPM & pitch control at low pitch
(High RPM): When propeller control is pulled out to high pitch (low RPM), engine speed should
decrease at least 500 RPM.

I've seen other general inspection guidance that 500 rpm is a minimum drop to look for, not a limit.   I'd think if there was an important limit it would placarded.

Good to know it won't cause any issues.  I wonder why the Lycoming guy mentioned that to me then, I guess another OWT.