pushing on a propellor

J

JOhnH

Touchdown! Greaser!
Joined
May 20, 2009
Messages
14,506
Location
Florida
Display Name
Display name:
Right Seater
When I bought my cessna 172 (my first and only plane so far) the previous owner would push it with one hand on the propellor near the spinner. The other hand was used to steer and push with the tow bar. This is how I push my plane up the slight incline into the hangar. It is a fixed-pitch two blade sensenich prop.

Now I read that you should not push on the propellor. What is the suggested method of moving this plane.
 
JOhnH said:
When I bought my cessna 172 (my first and only plane so far) the previous owner would push it with one hand on the propellor near the spinner. The other hand was used to steer and push with the tow bar. This is how I push my plane up the slight incline into the hangar. It is a fixed-pitch two blade sensenich prop.

Now I read that you should not push on the propellor. What is the suggested method of moving this plane.
Click to expand...
What you said above will work. The propeller is under enormous forces when turning. Far more than what your hand is putting on it near the spinner when you push. You can do more damage to the plastic around the prop when you push on that. I would not push or pull a plane by grabbing on the middle or ends of the prop because you can bend the prop.
 
JOhnH said:
Now I read that you should not push on the propellor. What is the suggested method of moving this plane.
Click to expand...

Where did you read that you shouldn't push on the prop?

That's the way I was always trained with singles. Where else are you going to push? It's a lot more stress to push on the spinner.
 
The blades dont remain in a single plane during operation to start with. Depending on angle of attack, speed, number of cylinders and amount of power deposited they are subject to considerable forces perpendicular to the axis of rotation.

On CS props you have to be careful not to impart a twisting motion on the blades against the direction that it is exposed to under power as there is some linkage thingy in there that likes to remain under tension and doesn't like pressure.
 
weilke said:
The blades dont remain in a single plane during operation to start with. Depending on angle of attack, speed, number of cylinders and amount of power deposited they are subject to considerable forces perpendicular to the axis of rotation.

On CS props you have to be careful not to impart a twisting motion on the blades against the direction that it is exposed to under power as there is some linkage thingy in there that likes to remain under tension and doesn't like pressure.
Click to expand...

on any CS prop we use in general aviation tell me where the prop is when at rest?

Is it:

fined off to the low pitch stops?
Coarsened up to the high pitch stops ?
some where in between , because there is no oil pressure to move it?
 
Last edited:
I've been moving my fixed pitch propeller airplane, by the propeller, for the last five years. My propeller hardly ever flies off my airplane in flight. I have not noticed any change in climb or cruise performance during this time either.

I don't know of any other way to move it by hand, by myself, that would not damage my bird.

John
 
From the Hartzell Owner's Manual 169

5. General​


B. Maintenance Practices​

(2) Never attempt to move the aircraft by pulling on the propeller.​


Also, pulling/pushing a plane by use of the propeller puts asymmetric stress on the crankshaft flange.​

Do people do it...sure.​

Is it smart? :dunno:​

I use a towbar.​
 
Steve said:
I use a towbar.​
Click to expand...

Do you have a push bar as well???

Most tow bars I've seen for light singles don't work very well pushing an airplane up a slight incline like the OP described.

If you are really concerned, you could do it like I do in the twin - straighten the nosewheel with the bar and then push on the inboard leading edge....pain in the behind because you have to keep going back and forth adjusting the nosewheel.
 
Steve said:
From the Hartzell Owner's Manual 169

5. General​

B. Maintenance Practices​


(2) Never attempt to move the aircraft by pulling on the propeller.​


Also, pulling/pushing a plane by use of the propeller puts asymmetric stress on the crankshaft flange.​

Do people do it...sure.​

Is it smart? :dunno:​

I use a towbar.​
Click to expand...

My towbar only works for pulling. If you push on it it just pops out and you have skinned knuckles.

As far as the asymetric stress argument, I think I'm going to have to go calculate the gyroscopic forces vs. the amount of torque I can exert on the prop flange. I'm thinking I can't generate even 10% of the torque that the prop exerts when rotating from takeoff to climb attitude.

Gonna have to dig out the old physics books.
 
COFlyBoy said:
As far as the asymetric stress argument, I think I'm going to have to go calculate the gyroscopic forces vs. the amount of torque I can exert on the prop flange. I'm thinking I can't generate even 10% of the torque that the prop exerts when rotating from takeoff to climb attitude.

Gonna have to dig out the old physics books.
Click to expand...
African or european?
 
Fearless Tower said:
Do you have a push bar as well???

Most tow bars I've seen for light singles don't work very well pushing an airplane up a slight incline like the OP described.

If you are really concerned, you could do it like I do in the twin - straighten the nosewheel with the bar and then push on the inboard leading edge....pain in the behind because you have to keep going back and forth adjusting the nosewheel.
Click to expand...


Mine has a rod that goes through the nosewheel axle. Works in either direction. I, too, have an incline in front of the hangar. And a plane with a 2200lb empty weight and 90 gallon fuel tanks. Do I push on the prop...yes, occasionally, but I try minimize it as I really don't like the idea of doing so. And I do occasionally have someone help by pushing on the leading edge as long they understand to push on a rivet line.

It's not like you have no alternative. Even taildraggers have options available to them.

The universal towbars such as those that are manufactured by Brackett work in tension and compression. They have a positive clamping action that will not disengage until you release it.

Several different powered towbars available use various installation schemes, but for the most part they involve some positive means of attachment or utilize the weight of the aircraft to maintain attachment and work in either direction.

I've written Santa several times to deliver a powered towbar, but, to date, he's not responding to any of my requests. One day my back will give out and then the issue will be settled.

The towbars such as those supplied by Cessna and Piper are too flimsy to be relied upon in my experience. I've had them disengage while in use due to their susceptibility to slippage, distortion and breakage even when only used for pulling. But a lot of people use them.

As far as flight loads vs. ground handling... how many fatigue cycles is your flange good for? What forces are the flange and front main bearing designed for? It comes down to risk management and I find it more comfortable to minimize unanalyzed conditions. Others may feel differently and that is their prerogative.
 
Last edited:
One of the popular events at the Grumman owners' group (American Yankee Association) gatherings is the contest to see who can push his or her airplane backwards along a prescribed path most accurately, without a towbar.

It ain't easy, folks. :D
 
Tom-D said:
on any CS prop we use in general aviation tell me where the prop is when at rest?

Is it:

fined off to the low pitch stops?
Coarsened up to the high pitch stops ?
some where in between , because there is no oil pressure to move it?
Click to expand...
The props on my Baron rest on the anti-feather stops. The spring/gas in the hub pushes them towards feather and those stops are all that keeps the blades from feathering all the way.

From what I've seen most single engine CS props will be sitting against the low pitch stops at idle and likely remain there or close to them after shutdown.
 
Steve said:
From the Hartzell Owner's Manual 169

5. General​


B. Maintenance Practices​

(2) Never attempt to move the aircraft by pulling on the propeller.​


Also, pulling/pushing a plane by use of the propeller puts asymmetric stress on the crankshaft flange.​

Do people do it...sure.​

Is it smart? :dunno:​

I use a towbar.​
Click to expand...
Gyroscopic forces on the crankshaft that occur when making a rapid pitch or yaw movement are significantly greater than anything you could generate pushing or pulling a blade near the hub. AFaIK the reason the prop manufacturers advise against using the prop to move the plane is that they think pilot's are too stupid to apply the necessary force near the hub rather than further out on the blades.

Edit: Oops I see CoFlyBoy said pretty much the same thing first.
 
Last edited:
gismo said:
AFaIK the reason the prop manufacturers advise against using the prop to move the plane is that they think pilot's are too stupid to apply the necessary force near the hub rather than further out on the blades.
Click to expand...

I suspect they have data to support that assertion.
 
Pushing on the spinner will kill it, or the spinner bulkhead plate REAL quick. Applying force to the inner part of the prop just outboard of the spinner should not cause any damage at all. Pushing on the prop tip is another story and will possibly bend the blade. Just go to a prop shop and watch how they 'tweak' the blades to increase/decrease pitch. In am very sensitive to my prop mainly because it is a Carbon Fiber three blade unit and you cannot imagine how badly it distorts when doing a ground runup at full power.:hairraise::hairraise:. With that said, I always push on the inner part of the prop to roll my toy back into its home. The tow bar just steers the plane into its resting place. IMHO.:cheerswine:

Ben.
 
Last edited:
gismo said:
The props on my Baron rest on the anti-feather stops. The spring/gas in the hub pushes them towards feather and those stops are all that keeps the blades from feathering all the way.

Just for fun, try pulling the prop and mixture at the same time see what happens.

From what I've seen most single engine CS props will be sitting against the low pitch stops at idle and likely remain there or close to them after shutdown.
Click to expand...

the common C/S prop will have an engine driven governor that is set to low pitch/high RPM setting prior to shutting down. After the engine stops there is the no oil pressure to move any thing plus the speeder spring in the governor has the fly weights and poppet valve in the decrease RPM position because the governor is sensing a underspeed condition.

So, the prop is on the low pitch stops and trying to lower, and is locked in that position.

How are you going to harm the prop with the blades locked at the low pitch stops?

all the warning from the manufacturers are a simple CYA statement, they know some Dork will hook a truck to the blade and try towing it that way.
 
The early Beech Bonanza manual had a "Don't push on the prop" warning with no explanation. That lore seeped throughout the small but effective owner world and has become "common knowledge."

I agree with whoever responds, "That prop is under a lot more stress under normal flight conditions than I can ever impose with my hand..."
 
dmccormack said:
The early Beech Bonanza manual had a "Don't push on the prop" warning with no explanation.
Click to expand...

those were a beech roby prop, they had composite blades, and had a history of ejection a blade in flight, they were not held in the hub all that good.

There is an AD for inspecting for blade security.
 
dmccormack said:
The early Beech Bonanza manual had a "Don't push on the prop" warning with no explanation. That lore seeped throughout the small but effective owner world and has become "common knowledge."

I agree with whoever responds, "That prop is under a lot more stress under normal flight conditions than I can ever impose with my hand..."
Click to expand...


So am I to interpret that to mean that any additional stresses I might put on the propeller manually is ok? If not, then what is the bounding case?
 
Steve said:
So am I to interpret that to mean that any additional stresses I might put on the propeller manually is ok? If not, then what is the bounding case?
Click to expand...

Pushing on the tip is probably bad practice because the load in flight isn't so localized and uneven.

Still, if the prop can't take you pushing near the hub (the thick part) --- can you trust it to to pull you and an airplane aloft?
 
dmccormack said:
Pushing on the tip is probably bad practice because the load in flight isn't so localized and uneven.

Still, if the prop can't take you pushing near the hub (the thick part) --- can you trust it to to pull you and an airplane aloft?
Click to expand...

You've just made my case by pointing out that there are conditions whereby one can possibly affect the propeller (and the crankshaft) integrity by hand. My propeller is not marked with "safe to push/pull" areas so to minimize the risk I try to avoid doing so. I feel without having evaluated what stress I might be subjecting the assembly to there is the probability that I am reducing the margin of safety by an unknown amount. It's a matter of to what degree does one want to challenge that margin.

Lycoming recommends not using the crankshaft flange for supporting the engine during shop overhaul to avoid the possiblity of distortion, in fact, in the factory they use jigs that mount directly to the case. Certainly the engine components comprise much less weight than an airframe. And a propeller blade is a somewhat large lever to be pulling on.

Dynamic air loads are a different animal from a static force, particularly on rotating machinery. Granted, the inflight loading can be substantially greater in magnitude, but the manner in which those are applied to the propeller/crankshaft assembly are not representative of ground handling activities. Turning a blind eye to it doesn't change anything, imo.
 
Steve said:
.
Dynamic air loads are a different animal from a static force, particularly on rotating machinery. Granted, the inflight loading can be substantially greater in magnitude, but the manner in which those are applied to the propeller/crankshaft assembly are not representative of ground handling activities. Turning a blind eye to it doesn't change anything, imo.
Click to expand...

I remember from 'introduction to finite element analysis' that 'the vector doesn't care'. Whether a load is dynamic or static matters for fatigue analysis, if you look at max forces at the blade-root/hub/flange it makes no difference. You can either create a greater moment at the blade-root or you can't.
 
weilke said:
I remember from 'introduction to finite element analysis' that 'the vector doesn't care'. Whether a load is dynamic or static matters for fatigue analysis, if you look at max forces at the blade-root/hub/flange it makes no difference. You can either create a greater moment at the blade-root or you can't.
Click to expand...

Have you ever watched a prop shop change the blade angle on a prop?

or repair a blade tracking problem?

it's called brute force and it's not pretty to watch.

It's amazing how far the blade must be pushed/twisted to gain the desired bend, or the spring back in heat treated aluminum will negate any effort you applied.

I will not mess with any pilot that can apply that kind of force
 
Tom-D said:
Have you ever watched a prop shop change the blade angle on a prop?

or repair a blade tracking problem?

it's called brute force and it's not pretty to watch.

It's amazing how far the blade must be pushed/twisted to gain the desired bend, or the spring back in heat treated aluminum will negate any effort you applied.
Click to expand...

That is what I am saying. If the prop doesn't get into the range of plastic deformation from the forces active during flight, my scrawny self won't get there without using some sort of mechanical advantage.

I will not mess with any pilot that can apply that kind of force
Click to expand...

Wouldn't that be a marketable skill ! :idea:On-site prop adjustment, just call 'Big Deals Prop Service':

big_deal_reading_480.jpg
 
Last edited:
Steve said:
You've just made my case by pointing out that there are conditions whereby one can possibly affect the propeller (and the crankshaft) integrity by hand..
Click to expand...

:confused:

Oh come on -- what are you pushing by hand? A P-51?

My airplane weighs 880 lbs empty. I push near the hub to get started. I'm sure plenty of pilots have pushed on the prop over 61 years, and somehow the shaft and blades remain true.
 
Pushing on the prop must be exempt from hand propping.
 
weilke said:
I remember from 'introduction to finite element analysis' that 'the vector doesn't care'. Whether a load is dynamic or static matters for fatigue analysis, if you look at max forces at the blade-root/hub/flange it makes no difference. You can either create a greater moment at the blade-root or you can't.
Click to expand...


so you're saying the thrust loads on a rotating propeller blade are equal to some one pulling on a blade on the ground as far as the blade and flange are concerned. if that's the case then i should be able to pull the plane around on the ground by the prop tips, yet i don't see that being the customary practice

i did a few stress/strain diagrams in school, too
 
Steve said:
so you're saying the thrust loads on a rotating propeller blade are equal to some one pulling on a blade on the ground as far as the blade and flange are concerned.
Click to expand...

I am saying that if the prop beating the air into submission creates a bending moment X at the flange and it doesn't create plastic deformation and you create a bending moment of less than X, you will not bend the prop or engine mount.

if that's the case then i should be able to pull the plane around on the ground by the prop tips, yet i don't see that being the customary practice
Click to expand...

How is one supporting evidence for the other ?




There is only one thing I could come up with where rest vs. rotation makes a difference. While the prop is rotating, the blade roots are under a couple of thousand pounds of tension and the needle-bearings that keep the blades from flying off are in compression. They may be more resistant to a bending moment while under that centrifugal load than at rest.
 
Last edited:
Tom-D said:
on any CS prop we use in general aviation tell me where the prop is when at rest?

Is it:

fined off to the low pitch stops?
Coarsened up to the high pitch stops ?
some where in between , because there is no oil pressure to move it?
Click to expand...

Depends... Most singles are intended to go fine with loss of pressure. Most props on multi's are intended to go coarse (feather) with loss of pressure (as an engine failure would cause)

But you could put either prop on either plane.... (we are talking theory, not legalities)..
 
Steve said:
so you're saying the thrust loads on a rotating propeller blade are equal to some one pulling on a blade on the ground as far as the blade and flange are concerned. if that's the case then i should be able to pull the plane around on the ground by the prop tips, yet i don't see that being the customary practice

i did a few stress/strain diagrams in school, too
Click to expand...


Do it by the tips and you'll bend the blades. Those blades are under terrific centrifugal forces when the engine is running, and that keeps them from bending forward beyond the yield point when generating thrust.

Same thing applies to helicopter rotors, though even more so.
 
Doggtyred said:
Depends... Most singles are intended to go fine with loss of pressure. Most props on multi's are intended to go coarse (feather) with loss of pressure (as an engine failure would cause)

But you could put either prop on either plane.... (we are talking theory, not legalities)..
Click to expand...
I see no problem with placing a full feathering prop on a single engined aircraft, but not the other way around.

the twin with a full feathering prop, has 1 of 2 ways to go to feather, first is a N2 charge that acts as a big spring pushing the blade up to feather opposed by engine oil pressure.

the other is a electric feather pump activated by the cockpit crew.
 
Last edited:
Tom-D said:
I see no problem with placing a full feathering prop on a single engined aircraft, but not the other way around.
Click to expand...

I thought the 'Rocket' conversion Mooneys use a feathering prop, basically the whole power package off a twin.
 
You must log in or register to reply here.
Back
Top