I need a prop,

Now on my DH 2 thread i commented about the need for a prop, being a 30" x 12" propellor (if you want to be a bit anoracky here it is a propellor as it propelling "pushing" the aeroplane along from behind whereas one on the front of a plane is pulling it along and is an airscrew!!) but hey i need one for the back of the DH2 whatever its called!!

now there is nothing wrong with the one for it at present its just along with its refurb (and being a bit mad perhaps) i thought a nice new one would be good, now you dont just trip over these size props in your LMS, and to get one made is into 3 figures, and i dont mean £9.99, i thought in a Jeremy Clarkson moment, "cant be THAT hard surely" so orf i jolly well went.

what follows is in no way advocated as THE way to carve a prop, its just my self taught way, now there is a bit of maths to get your noggin around but its only simple trigonometry so no fear there then, BUT for my first attempt i cheated. i constructed a simple MDF jig to hold the prop i allready had, marked it out at intervals along its diameter and measured up from base to centre line of the blade all the way around and marked the hight in my little note book, the prop was then turned 90 degrees and an outline drawn nice and neatly around the blade, from this i made a template to use to draw the profile onto my lump of tree




after the profile was tranfered to wood lump it was swiftly cut on band saw, new blade essential here, then it was laid onto the jig clamped in place with the big silver nut you may see in some of the pics, at the marked points the sizes aquired from the original were marked on the blank and you end up with a line of dots you join up and end up with the centreline of the blade all around the blank
the lump of tree in this case being a laminated lump of beech and mahogony. in this pic you should be able to see the laminations and the transfered pencil marks.




do this on both blades, and when done its out with the wood removeing equipment

chris

now the wood workers out ther would be wise to look away now, as i got out the spokeshaves and scrapers and then put them all away and found just what an old engineer needed, a big file, well a nice sharp half round surform to be exact :whistle: and set about removing wood.
after roughing it out to a couple of mm either side of the centreline i then drew another guide line along the thickest point on the original blade at a distance measured at the same points but back from the leading edge






then set about refining it to start to give an aerofoil shape to the blade, still only rough here but a bit more accurate, the section is a basic clarke Y with a slight undercamber when finished, but for now this is where i am at, one blade roughed out and awaiting the time to do t,other one which will take a bit longer cos now i have to keep refering to the other blade and sizes to ensure i dont end up with a 30x12 by 30 x10 3/4 :woohoo:

so heres the leading edge shot of the first blade






thats all for now peeps.

well all except for a slight confession, i have done a test prop to learn the art a bit before commiting deadly surgery on a nice bit of wood, and ended up with a prop shaped bit of wood, but wrong in a few area,s but it looks nice on a wall :whistle: :whistle:

chris

CeeJayP wrote: ....if you want to be a bit anoracky here it is a propellor as it propelling "pushing" the aeroplane along from behind whereas one on the front of a plane is pulling it along and is an airscrew!!) but hey i need one for the back of the DH2 whatever its called!!

I believe it's otherwise called a "tractor prop" used for traction.

could well be, it was on a US site they called it an airscrew, for an AIRPLANE i s,pose

chris

Err, 'airscrew', I thought that's what you had to do to join the 'mile high club'!

I tried carving a prop, once, a long time ago, for a Keil Kraft elastic powered kit thingy. I ended up with a prop, BUT one blade was pulling, and one was pushing, so it wasn't very effective! :oops:

I did that too, twice when I was a kid. :lol: :lol: :lol:

Not just me, then!

I have carved a few wooden props for my scale RTP jobs.

The above is small 6x4(?) and for modest power (40W) so carved/sanded from laminated hard balsa to give a scale effect when stained. Very light it works well.

Ah HA!!

an expert!!!

chris

Well the lump of wood is slowley starting to resemble a useable prop, so just to back trac a bit here is a pic of the outline template i made from MDF 6mm thick





as seen before i hacked a lot of wood away to get it to roughly the correct shape then started to mark it out for the finishing bit, front of blade has the line of the thickest part of the section marked on,





back is cross hatched to enable me to see where i sand as the rear face will eventualy be undercambered very slightly





this is done with a purpose made sanding block with the curve on its face





chris

AND after a while and with me resembling a large flour grader we end up with a blade that looks like this

front



and back




now to do the other one to match...more tommorrow!!!

chris

Looks good, Mr McDougal! (or was it Homepride?)

Well then

Its finished, subject to final testing etc it will need makers logo,s and perhaps the "copper" leading edge painting on but i will see if it makes a draft first, finish sanded, checked tracking (its out less than 1mm) checked radius of each blade, cock on, and two coats of sealer sanded with very fine paper twixt each coat i ended up with this.

front




Back





and one assembled onto prop driver



..

cheers all

chris

missed a picture

noticed on the thread in the pics its hard to estimate the size of prop so i took this one





chris