Shorter prop faster or longer prop slower...

[Aerosan]

HI Guys

Here is the scenario:
Engine 100 HP
Reduction- to suit prop
Prop diameter- under discussion


So useing the above as an example where would you go. Ill start; (JUST AN EXAMPLE TO STIR THE POT )

In my opinion it is better to run say a 72 inch prop at 2600 P RPM because you will be moving more air that equates to torque, rather running a smaller say 68 inch at 3200 RPM.

so what is your opinion.....

[Duck Rogers]

It's simple...........a good big one will always beat a good little one. Period.
Now go and work that out........

[Jean Crous]

Give Oom Piet De Necker a call, ........there you will get the right advice.

@ Duck Rogers........ Skaam jou Oom Eend, om so iets vir die jongspan te se
Groete
Jean.

[Duck Rogers]

[Aerosan]

HI Jean

No worries here I have to speak to Oom Piet but I thought it would be interesting to see what comes out of a nice clean (non violent!) debate.
It seems however that we all agree on 2 things-
1- the Duck aint too bright
2- bigger is better
that said is bigger always better on any aircraft or is there merit in saying that a sleek aircraft like a Samba (example) will benefit from a smaller higher pitched prop?

[Kingfisher]

Dan maak ons in Januarie so 'n blik wurms oop, net vir die sports.
Klink vir my amper soos "What is the best two stroke oil for my 503 or 582"
Raak 'n laaaang storie.....

[justin.schoeman]

2- bigger is better
that said is bigger always better on any aircraft or is there merit in saying that a sleek aircraft like a Samba (example) will benefit from a smaller higher pitched prop?

Unless it is a VP prop, the bigger props run out of pitch speed quite quickly. A really slick plane will need either a shorter, higher piched prop, or a big VP prop. That said, bigger props will almost always give you better climb, so it does depend what you want - climb, or cruise.

[Aerosan]

Mmmmm Justing that is starting to make sence....

Ja en terwyl jy die saak aanraak, wat is die beste olie

[WhiteEagle]

The greater the pitch on the prop, the more drag it enduce, somewhere there is a cut-off point where it is of no use altering the pitch to a coarser value. This is also applicable to altidude, meaning the closer you get to MSL, the finer the pitch may be, lesser drag on the prop and the more trust you can get with more prop speed, it also depends on the Hp the enjin produce, coarser pitch = more Hp needed. What is interesting is that the reason for reduction gearboxes is the following. Any propeller's speed at the wingtips may not exceed the speed of sound, even thow the last couple of centimetres of a prop produces no lift at all, only from the centre to more or less 4/5 to the tip, the lengt of the prop is thus limited so that the tip travel to just under the speed of sound. We all know what happened to Chuck attemting Mach 1, as soon as he reached this, everyting worked in reverse. Not that the prop would suddenly starts to revearse, but it creates its own problems, hence the reason why no aeroplane fitted with a prop will ever be able to reach the sound barrier. It's not the plane that limit it, but the propeller's tip speed. O ja, three blade does NOT give more thrust that a 2-blade prop, it just run's quieter.
Best oil? Not that much proven difference between mineral and synthetic on the lube side, but definitly less carb build-up on synthetic. I only use Castrol products. Just my pennies worth.

[Jean Crous]

There`s more to prop dynamics than we normal blokes understand. An important aspect is also the cord of the blade.......this has to do with power available, and with climb. A shorter faster turning prop, with a wide cord "pumps" the same volume of air as a longer slower turning prop ( this is very loosely described). The thickness of the prop blade, the "wing profile " so to speak also plays a role. I am on prop no 5 or 6 with my Bushbaby, and have now much better performance than initially.
Again i say.........speak to Pieter De Necker, he KNOWS props.

[Aerosan]

Jis Jean your are so right, I have just had a thought, what would we have done if we didnt have Oom Piet Prop

I will speak to him this weekend.

Cheers

[Kingfisher]

One day NASA will come up with a variable pitch/variable cord/variable length/bi directional prop with the perfect
profile and everybody will be happy....... for two days & find something else to talk about.
Still, aircraft are in all aspects interresting engineering at its best.

And.. the best oil is the one that keeps you from falling off the sky

[Jean Crous]

That just about sums it up

[Boet]

Mmmmm Justing that is starting to make sence....

Ja en terwyl jy die saak aanraak, wat is die beste olie

Doos olie, want het jy al gehoor van n piel wat bearings geslaan het?

[Boet]

Oom Piet het vir my errie n prop gemaak uit 3 bome se hout. Dis n moerse ding, en VREESLIK mooi. 72"x 64"......en dit op n Rattex 582?? . Ek het bedenkinge gehad oor die prop, maar hy sit jou terug in die kussing met die wegvlieg, en klim soos n poep uit n bad, en die cruise is ook nie te sleg nie!! Jaaaa, Oom Piet ken.