Rain On The Prop?

[sseeker]

Hi all,

 

It's been a question in my head for a long time. Does rain damage wooden propellors that have a fibreglass coating with polyurethane leading edge? (The Jab props, not sure about others) I've never flown a Jabiru in the rain so I've never really asked any of the instructors at my school. I've heard stories about having to reduce the power below 2000rpm, is this true? What's the story with the Carbon Fibre props? They feel weaker than the wooden props.

 

And I'm talking about inflight when the properllor is spinning, not when it's on the ground with the engine off

 

-Andrew

 

 

[Dieselten]

Heavy rain definitely not good for most props, metal excluded. Definitely a no-go for wooden (protected leading-edge, glassed, painted etc or not) and Woodcomp props.

 

Very light rain not so much an issue, but throttle back to reduce impact on prop from water droplets. One school of thought has it that "if you can see through it you can fly through it". I prefer to think of raindrops as being as hard as gravel at the speed a prop hits them. Would you fly through a shower of fine gravel?

 

I prefer to detour around rain wherever possible. Otherwise throttle back and get out of it ASAP.

 

 

[Guest davidh10]

An interesting question.

 

My aircraft's manual says that heavy rain can abrade the prop (carbon fibre) and that exposure to heavy rain should be avoided. On the other hand Boly's web site says their props treated with DuraTuff , which mine is, make waterborne operations possible."

 

I landed recently in reasonably heavy rain (approaching VFR minima) and also heard water splashed up by the wheels on the bitumen runway going through the prop, just after touchdown. No sign of any abrasion. I had only been in the rain for about 7 minutes, as it was a local shower pictured in the attached photo.

 

At normal cruise (60kt), the prop is doing about 1900rpm.

 

The next day, the seats were still rather wet and I had to go for a fly to blow dry the trike. :big_grin:

 

* Attached photo is Copyright David Hunt 2011.

 

 

 

[facthunter]

Rain abrasion/erosion of props. EVEN aluminium is eroded at impact speeds in excess of 400 Kts. This is not aeroplane speed it is prop speed, and takes into account the angle of impact, so mainly the leading edges are affected. As you go faster the energy is increased as the square of the velocity. I would definately stay out of rain with a wood prop. They do get wrecked in no time. If you get caught in rain exit as soon as possible and REDUCE engine revs to a minimum. The earlier types on Tigers and Austers etc have a brass inset leading edge. Some metal blade props run with tip speeds of about .9 Mach. The prop fitted to the the Alison on Electra's and Herc's was made with steel (Hollow) blades. Rather prone to cracking. But that's another story. Nev

 

 

[sseeker]

So probably a good idea to reduce power below 2400rpm in rain? (Possibly below 2200??)

 

-Andrew

 

 

[Guest Maj Millard]

All good advice above.

 

My experience with carbon fiber props in rain is that a small 'fuzz' will develope on the LEdge. This, on the Brolga at least, is easily removed with a light rub with 600 sandpaper. Obviously some loss of LE material is occuring. I run a LE protection strip on my current Powerfin CF prop, which is replaced periodically.

 

A wood prop will generally have some small damage spots on the LE from rock hits etc. These unsealed areas will quickly absorb moisture in rain, and can unbalance the prop in flight.

 

I do my very best to avoid flight in rain , however if you do get caught, a reduction in RPMs is a good idea to minimize damage.......................................................................Maj...

 

 

[bushcaddy105]

I have had no trouble with 450+ hours on my Duratuff edged Bolly blades in rain (seems lately that every time I fly somewhere, it rains on me!) The 3 blade prop is only doing 1920 rpm at cruise after the redrive, however. This obviously helps

 

 

[RFguy]

Back to light rain.

What is the collective wisdom on the Jabiru composite props in the rain ?

Anyway had the Urethane LE coating get shedded after a rain flight ?

 

Below - this 2019 updated document- is actually an EXCELLENT document for what props will do what results , and maintenance regimes, balancing etc.

 

especially section 3.3 onwards. I wholly encourage reading.

 

https://jabiru.net.au/wp-content/uploads/2019/08/JPM0001-6.pdf

[old man emu]

If you want to get positive proof of the effects of water at speed striking metal, just have a look at the propeller on an outboard motor. As Facthunter said: " is not aeroplane speed it is prop speed " Have you ever seen high pressure water cutting of steel?

 

Just think of the distance the prop tip travels. The Jabiru prop is 60 inches in diameter. That near enough to 1.5 metres. The distance the tip travels in one revolution is (pi x diameter) = (pi x 1.5) = 4.7 metres.

At 2500 RPM the tip will travel 2500 x 4.7 metres per minute = 11,750 metres. That is 11,750 x 60 metres per hour = 705,000 metres per hour (705 kilometres per hour), or (705000/3600) metres per second = 195 metres per second.

[Old Koreelah]

As others have said, any prop hitting enough raindrops is going to suffer damage and the wooden Jabiru prop needs TLC, even with its leading edge cover and layer of fiberglass.

 

Murphy dictates that your prop will encounter rain at some stage, so you need to reduce the impact speed as much as possible. It’s useful to know the minimum prop speed that can sustain flight.

Mine is around 2000 rpm, which gets me airborne and can sustain around 55kt in level flight. That means the leading edge gets nicely cleaned by drizzle, or quickly abraded by proper raindrops.

[facthunter]

OME It's a helical path . The forward speed is added as a vector. The long side oft the right angled triangle 60x48 sides.. Nev

[old man emu]

It's a helical path . The forward speed is added as a vector

 

I understand what you are saying, but I was just trying to get the message over in the simplest possible way. Perhaps I should have said that I was describing the simplest possible case - where the aircraft is stationary - like during an engine run up.

 

Also you need to define the frame of reference being used. Are we using the propeller's of the raindrop's? You can see that the calculations get more and more complicated as factors such as the forward velocity of the propeller, and/or, the size and velocity of the raindrop are taken into account.

 

The absolute simplest case is to consider the problem in relations to the raindrop's field of reference, and take it that at the instant of impact, the velocity of the raindrop is zero. Therefore it is the raindrop that is being struck by the propeller moving at a calculable velocity.

 

Which ever way you approach the calculation, experience tells us that a propeller moving through rain will have its surface eroded over time. That's why propellers used to have sacrificial brass strips on teh leading edge.

[RFguy]

Well in that case I'll also point out that rain drops fall vertically (usually) and at some non -zero velocity and might as well be included in the resultant vector. so the prop side on the up will hit the raindrops differently to the down side.

[facthunter]

The max velocity is the major determinant and which relative angle it meets the prop. Decides the area most affected. Above 400 knots even metals are eroded with water. I don't know what the terminal velocity of a raindrop is but it's not that high when you compare it's effect with what happens at say 6o mph on a motorcycle on your face or arms. Nev

[RFguy]

back to the original question :

Anyone had the Urethane LE coating on their composite Jab prop get shed after a rain flight ?

[facthunter]

Ask Jabiru. You don't have windscreen wipers also on these planes, either. so landing in rain is not such a good idea.. The stuff you put on motorbike visors does help but you shouldn't really be there on a regular basis. Nev

[old man emu]

Anyone had the Urethane LE coating on their composite Jab prop get shed after a rain flight ?

 

Sorry to have dragged a red herring across your thread.

 

I have seen the leading edges of Jabiru props shedding. Whether those examples were due simply to flight through rain, or simply expected wear and tear. Don't forget that prior to a flight through rain, a propeller would have normally suffered damage to the integrity of the urethane seal simply by having been in contact with the pollutants in the air. Have a look at the blades of a room fan to see how much muck they accumulate.

 

So if the integrity of the seal is already compromised, then hitting it with anything else will accelerate the shedding. That anything else could be rain, or dust.

 

I'll also point out that rain drops fall vertically (usually) and at some non -zero velocity and might as well be included in the resultant vector. so the prop side on the up will hit the raindrops differently to the down side.

 

Two points - Both blade halves will go up and down the same number of times so the average amount of damage will be the same at the end of an the flight through rain.

 

Secondly, the energy transfer (hence amount of "damage") in a collision between a raindrop and propeller can be calculated using the Law of Conservation of Momentum.

 

HOWEVER, as RFguy and Facthunter have correctly indicated, the raindrop and propeller have velocities before they collide. They will also have velocities after they collide. The hard part of the calculation is determining the angles of approach to the point of impact and the angles of departure afterwards. Unless someone wants to get bogged down in pages of Vector additions, seeking an actual mathematical answer to the question is far beyond the needs of this discussion.

 

This is about the best answer for here:

I prefer to think of raindrops as being as hard as gravel at the speed a prop hits them. Would you fly through a shower of fine gravel?

 

There are two solutions to saving the prop:

1. Gently sand back the leading edges until they are smooth, then recoat with polyurethane

2. Shape and fit metal sheeting to the leading edges as sacrificial materials to be replaced when worn. (Yes, and balance the prop. And Yes, get an engineering order for the modification)

[Tasmag]

Superglue, with the addition of baking soda for dings, makes a good sacrificial layer on my Jab prop.

[APenNameAndThatA]

An interesting question.

 

My aircraft's manual says that heavy rain can abrade the prop (carbon fibre) and that exposure to heavy rain should be avoided. On the other hand Boly's web site says their props treated with DuraTuff , which mine is, make waterborne operations possible."

 

I landed recently in reasonably heavy rain (approaching VFR minima) and also heard water splashed up by the wheels on the bitumen runway going through the prop, just after touchdown. No sign of any abrasion. I had only been in the rain for about 7 minutes, as it was a local shower pictured in the attached photo.

 

At normal cruise (60kt), the prop is doing about 1900rpm.

 

The next day, the seats were still rather wet and I had to go for a fly to blow dry the trike. :big_grin:

 

* Attached photo is Copyright David Hunt 2011.

 

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I'll stick my neck out and say that that photo is of rain from convection and that abrasion of the prop is the last thing you should have been worried about. If you want to fly through rain, fly through rain from stratus clouds.

[SSCBD]

With amphibian beavers and metal props we used metal files to take the water damage out (smooth the dings) of the leading edge. Huge damage on takeoff on water after a number of hours.

[old man emu]

With amphibian beavers and metal props we used metal files to take the water damage out (smooth the dings) of the leading edge.

 

Ah! Veterinary dentistry in the wilds of Northern Canada. Another example of the Hudsons Bay Company looking after natural resources.

[SSCBD]

Ah! Veterinary dentistry in the wilds of Northern Canada. Another example of the Hudsons Bay Company looking after natural resources.

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No try the Whitsundays flying out to the reef. Lost soooo many pairs of ray bans falling off the floats in open sea swell