Why park a timber prop horizontal?

[Guest Qwerty]

This is a genuine question. It doesn't make sense to me that "the water content in the timber will flow to the bottom blade and cause an imbalance". From my knowledge of timber, fluids and physics, I would suggest the the moisture content of a timber prop would be of the order of 5% - 12% and that this moisture will (for practical purposes) be evenly distributed and remain that way due to the interaction of the surface tension of the moisture, capilliary action and the diameter of the pores in the timber.

 

Can anyone tell me if there is a sound (proveable) reason to park timber props horizontally. It seems to me to make more sense to park a two blade prop vertically on a tricycle u/c a/c to reduce the hazard of walking into the the prop in the hangar. I park my props horizontally on my tail wheel a/c because I prefer the risk of walking into it over the risk (cost) of the a/c nosing over for whatever reason and damaging the prop.

 

 

[metanoia]

I asked my instructor about this (as it is the way that we park the jabiru). He said that it as largely an aesthetic thing in aircraft with nosewheels. With taildraggers, it is as you say, to prevent damage to the prop if the aircraft noses over.

 

 

[Ultralights]

in a tightly packed Hangar, it reduces the chance of damage to both the Prop, and the aircraft wingtips when aircraft are moved around

 

 

[Barefootpilot]

Yeah I've always been told that its to stop the prop getting inbalanced due to moister transfer. I guess it could happen if the aircraft was left for many months without flying but probably wouldn't effect the aircraft if it was flown every couple of days/weeks.

 

 

[Guest Brett Campany]

I've been told by a few old salts that it's so the residual sap within the wood doesn't slowly seep through the force of gravity and evidentially putting the blade out of balance.

 

 

[Bryon]

I prefer the s**tting bird theory, but want to know what happens if the prop is turned through 180 degrees whilst the birds are s#itting?

 

 

[Guest Qwerty]

I've been told by a few old salts that it's so the residual sap within the wood doesn't slowly seep through the force of gravity and evidentially putting the blade out of balance.

 

Yeah I've always been told that its to stop the prop getting inbalanced due to moister transfer. I guess it could happen if the aircraft was left for many months without flying but probably wouldn't effect the aircraft if it was flown every couple of days/weeks.

These are the stories that I am not convinced about. Cficare is right about the birds.

 

 

[Guest Walter Buschor]

It's horizontal so that the white ants can't get to it !

 

 

[flying dog]

From what I've been told:

 

Yes it is a "long term" thing, but if left for a long enough time and as the props are not 100% water proof, water can get in.

 

If the prop is vertical, the lower blade will get water at the tip, where as the upper one will get water at the root.

 

Thus: Imbalance.

 

Horizontal props get equal ammounts of water and so though they do have water in them, it is equal left/right side/"blade".

 

 

[skydog]

ols wives tales

 

One thing I have learnt is that " seemingly outdated aviation advice" as this wood prop question seems to have been taken, turn out to have very good sound reasons behind them, often obscure to some and nearly always me.

 

Best in my opinion is to go with the 'general experienced advice' regardless especially if the worst scenario is a bit of bird faeces.

 

I am sure water penetration of wood props has been decreased over the years with recent coating innovation and timber treatments but if you have to leave the plane outside then covers can be purchased for the blades which may help somewhat but if I had a wood prop it would be at the horizontal position.

 

I always leave my metal prop horizontal too.

 

Please make sure the master and mags are off before hand.

 

 

[Guest sirius]

Having owned an Auster years ago, (inline inverted Gypsy major), I can say the compression stroke for hand propping was with the prop in the vertical position. Hence if left in the horizontal position it would be safer from a faulty Mag perspective and unlikely to accidentally start if disturbed. I don't know how this compares with horiz opposed engines as nobody hand props much these days.

 

Prop's of old were mainly linen covered and timber was prone to moisture ingestion. I can speak from personal experience of what happens after a prolonged storage with expansion and contraction due to heat and cold and the prop is not torqued up. I can also believe, as water can get into a sealed 200 litre drum if not stored correctly, water can get into a wood prop with heat expansion and contraction.

 

Habit forces me to carry on the tradition.

 

Also I am not convinced prop covers are a good thing due to "hothouse effect".

 

 

[flying dog]

The thing is with a carbon prop is that they can not be put on a factory job.

 

And factories are not willing to fork out the $$$ to get that particular prop ok'ed for the plane.

 

So really: If the plane is a factory job and it has a wood prop, it has to keep that.

 

Same with all other parts too - which is a bit of a in lots of ways.

 

But, on another point, I also agree that although there are "wives tales" about things and why they are done, the reality is there is truth behind a lot of it - ever heard of the games "Chineese whispers"?

 

Over the years people's version of the facts change but there is usually a solid fact behind the original story/method.

 

We shouldn't discount something of old simply because it sounds stupid or has been proven to the contrary. It is possible that what we are hearing and what was originally said are two very different things.

 

 

[FlyingVizsla]

Zenith Air (May 1999 bulletin) recommend a propeller sock be used if the prop is exposed to the elements - moisture proof but breathable. Prop should be stored horizontal, rotated periodically and waxed.

 

Sensenich airworthiness (cf-69EK7 Rev.F) says - horizontal position - "This will keep the moisture in both blades evenly distributed."

 

A very interesting research article on moisture in wooden propellers can be found here http://www.fpl.fs.fed.us/documnts/fplrn/fpl_rn309.pdf

 

If I have time I will look through my engineering books to see if there is any mention of sap migration. The boss (plane builder and owner of several wooden props) is out pushing muck up hill in the pouring rain, but I am sure he will have something to add when he returns. If only to point out that wood is far superior to the metal I have on the nose of my plane.

 

Sue

 

 

[Captain]

Sensenich recommend that their carbon fibre props also be stored horizontal.

 

I assume to minimize or stop moisture/condensation from running down into the hub thru the openings in the spinner.

 

 

[flying dog]

Well, that's another thing:

 

If the prop is vertical and water gets on the spinner, RUST may happen.

 

Not a good thing.

 

 

[Captain]

Well, that's another thing:If the prop is vertical and water gets on the spinner, RUST may happen.

 

Not a good thing.

G'day Doggie,

 

With a timber prop I'd certainly be concerned if it is vertical and rain or condensation gets inside the spinner & down in and around the hub to swell and get into the prop around the attachment bolts, and also encourage rust on the face of the hub.

 

Regards Geoff

 

 

[Flashheart]

Not strictly about timber props. But for any variable pitch props, it's supposed to keep the water out of the seals.

 

 

[Guest Qwerty]

I think that the following responses have merit

 

1 Horizontal, to stop rain running down the blade into the hub area where it can enter the timber and cause swelling.

 

2. Horizontal, to stop rain running down the blade of a CSU prop and possibly enter/damage a seal.

 

3. Horizontal, to positively ensure that the engine is nowhere near a fireing point, usually after vertical and befor 45 deg (to allow for a strong pull-through).

 

4. VERTICAL, from obad, store the prop vertical (non magnito engines) so that he doesnt walk into it in his shed.

 

These (IMHO) are valid, reasoned approaches, I can't wear the migrating moisture one though.

 

Keep em coming, Cheers Qwerty.

 

 

[flying dog]

On that: When I am pre-flighting the plane, I put the prop vertical to reduce the possibility of prop stike.

 

 

[dunlopdangler]

I used to dress metal props horizontal, wooden vertical (mainly because of moisture balance and minimise heat expansion of the different layers of prop timbers glued together if the aircraft was left outside). These days however, I leave the prop where the engine leaves it unless I go anal and do the horizontal and vertical thing...and for three blades = one blade up and two blades down:ban me please:

 

 

[Guest Qwerty]

Qwerty, I'll do a test for you (& to satisfy my own curiosity). I have had my replaced Allsize prop laying horizontally on the floor of my rumpus room since replacing it in July. I just weighed each end & got 1162 grams one end & 1174 grams the other. I will now store it light end down in a vertical position & weigh it weekly, still in the same room (which is subject to air conditioning on hot days). If nothing changes over a few weeks I will put it outside on my verandah so that it can absorb any air moisture there & see if that makes a difference. Stay tuned.Cheers

Dexter

Excellent job Dexter. If there is a change though, we have a real problem (and potentiall an opportunity). We will then have to find the reason for the change and my scientific training won't allow me to accept, "the weight changed when it was stored vertically therefore is it due to the moisture flowing to the bottom of the propellor."

 

Each time that you weigh it, would you mind weighing it three times each end as a simlistic check on your scales/method? I'm looking forward to the results.

 

Cheers, Qwerty

 

 

[Guest sirius]

Despite perceptions, water can be ingested into timber props no matter how well sealed. The following from the Sensenich website;

 

http://www.sensenichprop.com/sen_html/aircraft_cet/install/cf-a.pdf

 

Some selected quotes. The tip drain holes mentioned in the last paragraph is significant;

 

1) The main factor that leads to the loss of propeller

 

bolt torque is the variation of the wood hub thickness.

 

The hub thickness will vary with (a) wood moisture

 

content changes and (b) temperature changes.

 

Even though your propeller has been sealed and/ or

 

painted, changes in wood moisture content will occur

 

which can significantly change the thickness of the

 

hub. A one percent (1%) change in the moisture

 

content of a propeller (increase / decrease) will cause

 

a 0.010” change in hub thickness. As the required

 

compression for a typical 65 HP wood propeller is

 

0.021”, almost half of the required hub compression

 

has now been lost. Moisture content changes are

 

not immediate and can span several weeks or

 

months, depending on many factors such as

 

temperature, humidity, and operating schedules.

 

 

2) Inspect and check propeller attaching bolt torque

 

at least every 50 hours according to the Bolt Torque

 

Check Procedure and Propeller Maintenance

 

sections. More frequent inspection may be necessary

 

when climatic changes are extreme, such as change

 

of seasons.

 

When the propeller is not in use, place the

 

propeller in a horizontal position and if it is exposed to

 

the weather, cover it with a waterproof cover.

 

Do not use the propeller as a tow-bar to move

 

your aircraft.

 

Protect your propeller from moisture and UV

 

exposure by waxing with an automotive type paste

 

wax at least once a year. Keep the tip drain holes in

 

metal tipping clear.

 

 

[Guest Qwerty]

Ohhhhhhhhhh

 

The moisture content of the prop can not change if the prop is sealed. Sealed means that there is no route in or out for moisture. I suspect that what they meant to say was that it is difficult to maintain the integrity of the seal, and for practical purposes, the seal on all props can be considered to be compromised. I would expect that the moisture content of props with a decent coat of paint woudl be reasonably constant. Dexter's experiment will give some indication of this.

 

They say "place the propellor in a horizontal position" but don't give any reason. I'm not too good at doing what I am told, I need a reason.

 

What is this about not using the prop to move the a/c. On some a/c, mine included there are not that many strong points to push or pull the a/c. I use the root of the prop to move my a/c as a matter of policy. I wonder if there is a reason or if they are looking to find an out in court should it become neccessary. "Your honour he has been seen on many occasions pulling the aircraft by the propellor against the content of the user instructions....."

 

 

[skydog]

sealing of props

 

Sorry to disagree Qwerty,

 

Paint will not seal out moisture

 

Lots of other proprietry 'sealants' also will not seal out moisture.

 

Paint and other like sealants are permeable and allow the substrate to "breathe" this means allows moisture in and out in the right atmospheric circumstances.

 

Our own Skin will keep out rain but it is permeable

 

Dont let the paint ads on TV fool you.

 

 

[Guest Andys@coffs]

..... I'm not too good at doing what I am told, I need a reason.What is this about not using the prop to move the a/c. On some a/c, mine included there are not that many strong points to push or pull the a/c. I use the root of the prop to move my a/c as a matter of policy......"

Yeah, I to like to understand why and also cant see what the issue is with moving an aircraft around on the ground with the prop hub as per qwerty's post. I figure, if the engine produces 120hp and the crank, shaft, mount and prop can handle that then as a single human, perhaps at best some years ago capable of 1/4hp then what damage can I really do? especially as I own the wallet that paid for the aircraft and will pay for things that I break. As a result of that I hardly slam things around?

 

So what am I missing or is this another "tale" that has persisted through time from when it was relevant to now when I assume it isnt.

 

For other Jab owners, if you dont move the aircraft around from the prop, how do you ground move it? There are, to my mind, a lot of areas on a jab that I wouldnt want to use a lot of force on that seem far more fragile than the prop?

 

Andy