Prop pitch

[Bill Mitchell]

I’m not sure where a question like this belongs but here goes. It seems like most of the top guys are using carbon props repitched for performance. I seem to recall Ted Fancher mentioning something about more pitch at the tips give a better turn or more power in the turn or something, I can’t recall exactly. However it seems most people are depitching their props which would give you less pitch at the tip compared to the middle of the diameter.

Which is preferred, to start with less pitch than you want to end up with,  thereby having a little more at the tip or to start with more pitch than you want to end up with and therefore ending up with less at the tip? I suppose the ideal would be to have true helical pitch across the blade.

Bill Mitchell

[Jim Pollock]

Bill,

I'm no scientist but I surmise that the extra pitch at the tip helps in pulling the plane through hard corners where the angle of attack changes so much.

Just how I see  what the extra pitch helps.

Jim Pollock

[Bill Little]

Hi Bill,
For the past however many years that we have been using CF props, all I have been accustomed to is pitching the last 1 1/2" or so of the blade.  Heat the root, and adjust there checking the pitch at the last couple of stations.  You can change the pitch all along the blade, even making the tips stronger or weaker by heat the end of the blade.  From all I have gathered, the last 1" to 1 1/2" is what really makes th emost difference.

[Alan Hahn]

Here's my 2 cents.

There are some nice prop articles in the Model Aviation Archives so I recommend to go pick them up.

There are all sorts of strange prop comments you hear about pitch etc. It is really confusing to say the least. I think if air was solid like a piece of wood, then maybe there would be some critical aspects to non-helical pitch.

However I think the plane is feeling a thrust given by a weighted average of pitch along the diameter. Ignoring the tips of the prop where things can get pretty complicated due to vortices, the total thrust is the sum of the air being accelerated by the prop. One inch of prop at a radius of 5 inches moves a lot more air than 1 inch of prop nearer the hub.
So when someone tells me they are pitching the tips less than the nearer the hub, I think they are saying the engine/plane really wants a prop with less total pitch. I don't think there is anything magic going on with the air in the last couple of inches which somehow pulls the plane thru that corner you just hammered!   However it is easier to repitch the tips than the (w)hole blade.


edited to fix my spelling mistake, since we are now taking notice!!

[phil c]

heh, heh, heh,  pitch on blade at 4 and the other at 5 and get the best of both worlds, maybe.

I agree with Alan.  All this talk of repitching props is just a way to throw away enough power in level flight to get the desired lap time and get the proper loading on the engine to get enough power to fly the plane in maneuvers.  Since the results are mainly a matter of "feel" do what works for you.  I suspect a lot of the tweaking pitch and adding it to the tips comes from the fact that measuring prop pitch is a very iffy process.

I did some experiments with the Master 10/7 three blade because it's cheap.  Depitching at the root(from the stock 5.+ pitch to 3.5+) worked pretty good.  It does make the tips way under pitched at something like 1 in.  Adding pitch to the tips to bring them up to 3.5+ did improve pull and flyability up high.  Now I can do little 20 ft. figure eights overhead.