backing a 172 into a parking spot, probably silly question

[Matthew]

I know guys who like to turn the prop verical so birds have less area to perch and poop on.

 

[MAKG1]

Not disagreeing with you, but can you give a documented example from a typical piston single? Key word here is documented.* Call it a "prove it to me" challenge.

For most piston singles I have zero qualms about gently turning a prop backward if it's needed for some reason.

Next challenge, describe the only time that's really necessary.** It does relate to the tow-bar on singles.

* caveat: Many people will immediately go to saying that a dry vacuum pump can break a vane if turned backward. If you've ever seen a cutaway cross section of one, and the manufacturers of them used to make such things to hand out, you'll see that the "catch" angle of the vane isn't steep enough to do that UNLESS the vane is so worn or the inside surface of the pump is, that it wasn't long for this world anyway -- and you might as well find out during the next run-up before the IFR flight than afterward. You also won't find any warnings documented much about it from the vacuum pump people. And if you have access to one of those demo devices you can see, it's just not going to happen unless there's already something wrong or the vane is worn down to the limits anyway, in a typical pump.

**No, props don't need to be horizontal on the ramp, but many of us were taught that out of a perhaps misguided sense that a wing may pass over it and not strike it if someone taxis too close. While true, the real problem is, of course, that they taxied too close.

Disclaimer: I bet you can find an aircraft that states you shouldn't. I've just never flown one.

Not an airplane, but you won't like the results if you turn a worn Chevy small block backwards. Not a big deal when freshly overhauled, but a worn timing tensioner and sprocket can lead to valves that don't move when they should.

 

[denverpilot]

Not an airplane, but you won't like the results if you turn a worn Chevy small block backwards. Not a big deal when freshly overhauled, but a worn timing tensioner and sprocket can lead to valves that don't move when they should.

Haha. No plans to go out of my way to do that!

 

[DaleB]

Agreed and understood. So there's one.

Not only that, but it's normal procedure to pull the prop through at least a dozen or so blades as part of preflight.

Yeah, it does take a little getting used to, as does cruising at 5,000-plus RPM. Darned fine little engine though.

 

[Tom-D]

But turning backwards can damage some engines.

Tell me how?

 

[Tom-D]

Anything with a Rotax 912/914 series engine.

You'd have to show me that in a Rotax manual.

 

[denverpilot]

Not only that, but it's normal procedure to pull the prop through at least a dozen or so blades as part of preflight.

Yeah, it does take a little getting used to, as does cruising at 5,000-plus RPM. Darned fine little engine though.

Having not flown one, I didn't know that either, but the airplane equipped with one from the local flight school is literally in the hangar next to mine and I've *never* seen anyone do it. Ever.

It's been there for well over a year and no grass grows under that thing. Their door is open and they're headed somewhere nearly every time I go to my hangar. Or coming back.

Not sure what that says about it, just an observation. All the students and CFIs just pull it out, climb in, and start it.

It's a Gobosh. Pretty little thing, too. I haven't read the manual on it.

But I've watched it leave a lot of times and nobody's been turning the Rotax over by hand.

It kinda sounds like my summer bedroom fan when it's taxiing.

 

[eetrojan]

You'd have to show me that in a Rotax manual.

The Rotax has a "dry sump" lubrication system that has an external oil tank mounted next to the crank case and connected by hoses. I presume it's called a dry sump because it literally drains dry when the engine isn't running.

The dip stick is beneath the cap of the dry sump.

In order to check the oil level during the pre-flight, you have to rotate the propeller to pump oil into the dry sump. You need to rotate the prop until you hear a burp or a "gurgle" coming out of the sump (see step 4).

Here is a blurb from the Rotax 912 "Operators Manual." The highlighted note in step 2 is the apparent source of the admonition that the propeller should not be rotated backwards:

Also, from a little Googling, here's an excerpt from a Rotax Service Instruction that suggests that rotating the prop in reverse is bad because it "allows "air to be injested (sic) into the valve train," whatever that means:

 

[denverpilot]

The Rotax has a "dry sump" lubrication system that has an external oil tank mounted next to the crank case and connected by hoses. I presume it's called a dry sump because it literally drains dry when the engine isn't running.

The dip stick is beneath the cap of the dry sump.

In order to check the oil level during the pre-flight, you have to rotate the propeller to pump oil into the dry sump. You need to rotate the prop until you hear a burp or a "gurgle" coming out of the sump (see step 4).

Here is a blurb from the Rotax 912 "Operators Manual." The highlighted note in step 2 is the apparent source of the admonition that the propeller should not be rotated backwards:

Well that makes me even more curious about the folks next door. How would they check the oil without turning the prop?

Now I want to watch even closer. This could be excellent popcorn eating material if they're not doing it.

 

[Tom-D]

The Rotax has a "dry sump" lubrication system that has an external oil tank mounted next to the crank case and connected by hoses. I presume it's called a dry sump because it literally drains dry when the engine isn't running.

The dip stick is beneath the cap of the dry sump.

In order to check the oil level during the pre-flight, you have to rotate the propeller to pump oil into the dry sump. You need to rotate the prop until you hear a burp or a "gurgle" coming out of the sump (see step 4).

Here is a blurb from the Rotax 912 "Operators Manual." The highlighted note in step 2 is the apparent source of the admonition that the propeller should not be rotated backwards:

Also, from a little Googling, here's an excerpt from a Rotax Service Instruction that suggests that rotating the prop in reverse is bad because it "allows "air to be injested (sic) into the valve train," whatever that means:

EXCESSIVELY ? explain that.

 

[Tom-D]

What does the checklist say?

check list is in the Owner's operating manual. written long before they had the AD written

 

[mtuomi]

Well that makes me even more curious about the folks next door. How would they check the oil without turning the prop?

Now I want to watch even closer. This could be excellent popcorn eating material if they're not doing it.

Checking the oil in a Rotax is a technique you just have to learn. It's like leaning. You can ask 20 people and you get 20 different opinions You can quickly turn the prop forever and you'll never hear the noise you want to hear (it's like the oil sump burping). The way I do it is hold the prop against compression for a moment for every turn, and it seems to do it faster.
You can't check the oil without turning the prop.

 

[eetrojan]

EXCESSIVELY ? explain that.

Rotax is Austrian, so the English phraseology is probably a little bit off.

Some of the Rotax 912s have a gear box with an overload clutch. A couple of pages earlier, it describes a procedure for checking the gear box which says "Turn the propeller by hand to and fro, feeling the free rotation of 30* before the crankshaft starts to rotate."

So, just a guess, but I suspect that a full revolution is "excessively reverse," but a little "to and fro" is OK.

 

[DaleB]

Well that makes me even more curious about the folks next door. How would they check the oil without turning the prop?

Now I want to watch even closer. This could be excellent popcorn eating material if they're not doing it.

If they are not turning the prop over, they're probably not checking the oil. Not AS big a deal since the Rotax normally doesn't burn oil, but I wouldn't skip the check anyway. I've added about a cup in the last 30 or so hours. Maybe they check it on a schedule rather than daily.

 

[Tom-D]

Rotax is Austrian, so the English phraseology is probably a little bit off.

Some of the Rotax 912s have a gear box with an overload clutch. A couple of pages earlier, it describes a procedure for checking the gear box which says "Turn the propeller by hand to and fro, feeling the free rotation of 30* before the crankshaft starts to rotate."

So, just a guess, but I suspect that a full revolution is "excessively reverse," but a little "to and fro" is OK.

So are you saying the backwards in English, doesn't mean backwards in German? Austria speaks German ya know.
Next
Are you implying the overload clutch will slip if you turn the prop backwards?
The procedure you quoted is to check backlash in the PRU, that's a bit different than rotating the engine backwards.
I know of no engine we use that will be damaged by rotating it backwards.

 

[Tom-D]

The Rotax has a "dry sump" lubrication system that has an external oil tank mounted next to the crank case and connected by hoses. I presume it's called a dry sump because it literally drains dry when the engine isn't running.

Dry sump means the crankcase has no oil stored in it. the oil is scavenged out and is stored in a tank or other location, Radial engines are a dry sump type of engine.
Why Rotax chose that engineering I don't know. but that alone would not preclude turning it backwards.
the oil pump on the other hand will turn backwards too, which would remove oil from the pressure side and pump and send it back to the tank. this may be your reason, put in other words it may cause the oil pump to lose its prime, thus the warning.

 

[eetrojan]

So are you saying the backwards in English, doesn't mean backwards in German? Austria speaks German ya know.
Next
Are you implying the overload clutch will slip if you turn the prop backwards?
The procedure you quoted is to check backlash in the PRU, that's a bit different than rotating the engine backwards.
I know of no engine we use that will be damaged by rotating it backwards.

Rotax is from Austria it seems. I wasn't focusing on some strange meaning of "backwards" in another language, nor indicating what languages may be spoken in Austria, but rather on the overall sentence structure sounding a little weird because of translation issues. You should read an Evektor POH if you want to really appreciate this issue.

And, I was just trying to read the Rotax manual to glean a reasonable answer to your question about what Rotax might mean by telling users not to turn it "excessively reverse" as opposed, I presumed you meant, to an unqualified admonition to not turn it in a reverse direction at all.

Rotax recommends moving the prop "to and fro" in one place. Moving it "to and fro" is turning it in both directions, forward and reverse, albeit through a small angle. Thus, they couldn't say you should never turn it "reverse" direction.

But, Rotax says, please don't turn it "excessively" reverse.

On what is "excessively" reverse, your guess is as good as mine - probably better.

 

[eetrojan]

... the oil pump on the other hand will turn backwards too, which would remove oil from the pressure side and pump and send it back to the tank. this may be your reason, put in other words it may cause the oil pump to lose its prime, thus the warning.

Perhaps. I added a Rotax "Service Instruction" to my post #128 above. It says it's bad if the engine "had the prop spun in reverse direction allowing air to be injected into the valve train."

I'm not sure what a "valve train" is, but maybe it's the pump mechanism?

 

[RJM62]

Dry sump means the crankcase has no oil stored in it. the oil is scavenged out and is stored in a tank or other location, Radial engines are a dry sump type of engine.
Why Rotax chose that engineering I don't know. but that alone would not preclude turning it backwards.
the oil pump on the other hand will turn backwards too, which would remove oil from the pressure side and pump and send it back to the tank. this may be your reason, put in other words it may cause the oil pump to lose its prime, thus the warning.

It pulls air into the hydraulic lifters. See Page 3:

http://docusearch.flyrotax.com/files/pdf/d05187.pdf

Rich

 

[Tom-D]

When you rotate the engine/pump backwards the oil must come from somewhere. So it comes from the engine and goes to the tank. backwards to normal.

but here is the question, why does it not immediately come back when you crank the engine on the starter? pump cavatitated ? all the engines we build it does.

 

[Tom-D]

Perhaps. I added a Rotax "Service Instruction" to my post #128 above. It says it's bad if the engine "had the prop spun in reverse direction allowing air to be injected into the valve train."

I'm not sure what a "valve train" is, but maybe it's the pump mechanism?

the valve lifters are a part of the engine valve train they open and close the valves, the lifters are normally oiled directly from the oil galleys cast into the engine case. when you rotate the pump backwards the oil would be taken from there first. "Injected" probably a wrong word. "drawn" would be a better description of what would happen.

 

[Clark1961]

The Rotax has a "dry sump" lubrication system that has an external oil tank mounted next to the crank case and connected by hoses. I presume it's called a dry sump because it literally drains dry when the engine isn't running.

The dip stick is beneath the cap of the dry sump.

In order to check the oil level during the pre-flight, you have to rotate the propeller to pump oil into the dry sump. You need to rotate the prop until you hear a burp or a "gurgle" coming out of the sump (see step 4).

Here is a blurb from the Rotax 912 "Operators Manual." The highlighted note in step 2 is the apparent source of the admonition that the propeller should not be rotated backwards:

Also, from a little Googling, here's an excerpt from a Rotax Service Instruction that suggests that rotating the prop in reverse is bad because it "allows "air to be injested (sic) into the valve train," whatever that means:

You've got it backwards in your description. The dipstick is in the oil tank and the prop is turned to pump the oil from the sump into the tank.

 

[eetrojan]

You've got it backwards in your description. The dipstick is in the oil tank and the prop is turned to pump the oil from the sump into the tank.

Thanks. I was mistakenly under the impression that the "oil tank" and the "dry sump" were the same thing.

I now understand that "sump" refers to the thing I normally call an oil "pan" bolted to the bottom of an automotive engine. A wet sump being the kind of oil pan that holds a bunch of oil, and a dry sump being one that is kept generally dry by a "scavenger pump" that pumps oil from the pan (dry sump) to an external oil tank.

Sound right?

 

[Clark1961]

Thanks. I was mistakenly under the impression that the "oil tank" and the "dry sump" were the same thing.

I now understand that "sump" refers to the thing I normally call an oil "pan" bolted to the bottom of an automotive engine. A wet sump being the kind of oil pan that holds a bunch of oil, and a dry sump being one that is kept generally dry by a "scavenger pump" that pumps oil from the pan (dry sump) to an external oil tank.

Sound right?

Correct.

 

[RJM62]

Rich

 

[Capt. Geoffrey Thorpe]

Well that makes me even more curious about the folks next door. How would they check the oil without turning the prop?

Now I want to watch even closer. This could be excellent popcorn eating material if they're not doing it.

Depends on the details of the installation.
Some tend to let oil accumulate in the crankcase and you need to burp it to get an accurate reading - others not so much. I only burp mine if the oil looks low and I want to make sure before I add any. Normally when I pull the dipstick the oil level is in the "OK" range and that's good enough for me.

 

[Capt. Geoffrey Thorpe]

Thanks. I was mistakenly under the impression that the "oil tank" and the "dry sump" were the same thing.

I now understand that "sump" refers to the thing I normally call an oil "pan" bolted to the bottom of an automotive engine. A wet sump being the kind of oil pan that holds a bunch of oil, and a dry sump being one that is kept generally dry by a "scavenger pump" that pumps oil from the pan (dry sump) to an external oil tank.

Sound right?

Correct. Except, on the Rotax, there is no scavenger pump - it relies on blowby to push the oil back to the tank.

 

[Capt. Geoffrey Thorpe]

When you rotate the engine/pump backwards the oil must come from somewhere. So it comes from the engine and goes to the tank. backwards to normal.

but here is the question, why does it not immediately come back when you crank the engine on the starter? pump cavatitated ? all the engines we build it does.

The oil usually comes back, but Rotax has blamed valve train damage on air in the lifters.

Overall, it's a quirky lubrication system with unique issues.

 

[Capt. Geoffrey Thorpe]

Checking the oil in a Rotax is a technique you just have to learn. It's like leaning. You can ask 20 people and you get 20 different opinions You can quickly turn the prop forever and you'll never hear the noise you want to hear (it's like the oil sump burping). The way I do it is hold the prop against compression for a moment for every turn, and it seems to do it faster.
You can't check the oil without turning the prop.

If you check and it's not low, there is no point in burping it.

 

[eetrojan]

And, OP, sorry for completely DERAILING your thread.

 

[Capt. Geoffrey Thorpe]

And, OP, sorry for completely DERAILING your thread.

I guess I'm not helping either.

 

[Clark1961]

Correct. Except, on the Rotax, there is no scavenger pump - it relies on blowby to push the oil back to the tank.

Perhaps it depends on the Rotax model. Pretty sure there is a pump on the Rotax used in the Gobosh.

 

[Capt. Geoffrey Thorpe]

Perhaps it depends on the Rotax model. Pretty sure there is a pump on the Rotax used in the Gobosh.

There is an oil pump, but no scavenging pump on the 912/914 - they all rely on blowby.

 

[Clark1961]

There is an oil pump, but no scavenging pump on the 912/914 - they all rely on blowby.

And after a bit of digging, you are correct. I guess it is easier to say it pumps the oil rather than using a more accurate description.

 

[mtuomi]

If you check and it's not low, there is no point in burping it.

Ok, makes sense. On the Zephyr I flew, it was always low before burping, but it flew a lot, maybe that's why it was always low? Quirky system anyway.

 

[exncsurfer]

And, OP, sorry for completely DERAILING your thread.

Haha, hey, I got my answers, you guys carry on.

 

[SoCal RV Flyer]

Thanks. I was mistakenly under the impression that the "oil tank" and the "dry sump" were the same thing.

I now understand that "sump" refers to the thing I normally call an oil "pan" bolted to the bottom of an automotive engine. A wet sump being the kind of oil pan that holds a bunch of oil, and a dry sump being one that is kept generally dry by a "scavenger pump" that pumps oil from the pan (dry sump) to an external oil tank.

Sound right?

Yes! For automotive applications, a dry sump is helpful in a performance application for a couple of reasons. First, it's lower-profile than a wet-sump oil pan, so the engine can be mounted lower in the chassis, for a lower center of gravity, and still maintain sufficient ground clearance. Second, with an external oil tank, there's always adequate oil supply to the engine...with a wet sump, lateral/longitudinal g-loads loads can slosh oil away from the pickup and cause oil starvation, though a well-designed wet sump with proper pickup location, baffling, etc. can be made to work well. Depositing the oil back in a tank can also reduce oil aeration issues.

 

[Dan Thomas]

Dry sumps are used on inverted aircraft engines, too. Those engines have the cylinders hanging off the bottom of the case, and the crankshaft at the top. The cylinders are made so that they protrude an inch or so into the crankcase, forming a dam to discourage oil flowing into them, and there are a couple of oil scavenge pumps to draw the oil out of the front and rear of the case and send it to the tank. deHavilland's Gipsy engines, the Czech Walters, the old Rangers and Menascos, and the Renault were all inverted engines. Might be more that I don't know of or recall.


Gipsy Major

Ranger

Ranger made an inverted V-8, and the Germans had the Argus V-8.

Argus

This thread is thoroughly derailed....

 

[denverpilot]

We were just killing time until he realized he can just put the 172 in reverse and back in. You just need one of those OSHA backup beepers installed via STC.

 

[Tom-D]

4360 dry sump as are all radial engines R-500 thru 4360