Interesting Technical Question

**BrendanS** · 28-09-05, 02:37

ermmm, I can't believe a prop designer would give such a simplistic answer for a variety of reasons. Does he design helicopter props or boat props, and in which case mobo or raggie.

Reason for asking, is that drag varies according to design, so no prop designer would ever give a cut and dried answer, in much the same way a scientist like me would add bits onto any answer like, yes, no, but.

A helicopter has huge diameter blades, with not a lot of total surface area, but huge loading on each blade. A mobo has far lower diameter blades, but with massively increased % of area covered for the diameter. Many raggie boats have slim chorded 2 blade props etc etc.

Each have different characteristics. The drag of each, will have similarly disparate drag co-efficients. Hence my question about what sort of prop your friend designs, as the statement that fixed props equates to less drag always, is just not true

**BrendanS** · 28-09-05, 02:49

Lets put this in laymans terms

Downsize a helicopter blades to a size you can put on the back of a boat. How effective do you thing that would be.

Upsize a typical duoprop to helicopter size. Do you think it would fly?

Now do you think they are comparable?

**Petrolia** · 28-09-05, 04:59

Sorry to butt in cause I'm only new here but I don't get the locked prop offering less drag either.
If a helicopter losses power it can auto-gyro down safely because the rotor has huge momentum so is effectively still powered and can therefore still create lift.
I'm afraid I have to side with JFM and the US Navy.
I believe a free rotating prop creates less drag.

**Joe_Cole** · 28-09-05, 05:44

The auto-gyro effect is NOT due to momentum of the rotor. If it were then the blade would stop quite quickly and the helicopter would fall out of the sky. They don't.

**BrendanS** · 28-09-05, 05:51

Helicopters can land, for exactly the same reason an autogyro can fly. Nothing to do with momentum, purely down to the aerofoil of the blades. Try putting them behind a boat in water tho?

**Gludy** · 28-09-05, 06:55

Brendan

"ermmm, I can't believe a prop designer would give such a simplistic answer for a variety of reasons. Does he design helicopter props or boat props, and in which case mobo or raggie. "

His answer is that for all props (non collapse types) the drag when not fixed will always be greater than the drag when fixed - that is an understanbable answer because of the nature of what is causing the drag.
He is a boat prop designer.

"Reason for asking, is that drag varies according to design, so no prop designer would ever give a cut and dried answer, in much the same way a scientist like me would add bits onto any answer like, yes, no, but."

I am a chartered engineer and I can give you equally simplitic answers to equally simp;otic questions. This prop designer is stating a clear principle and is providing figures to me by emailo. He requested the dimensions of my props so he could be specific to mine but that does not change the fact that when rotating without power they offer greater drag than when fixed.

"A helicopter has huge diameter blades, with not a lot of total surface area, but huge loading on each blade. A mobo has far lower diameter blades, but with massively increased % of area covered for the diameter. Many raggie boats have slim chorded 2 blade props etc etc."

Makes no difference to the principle at all - you are getting lost in the fog of detail. sure the exact prop etc matters when you calculating the exact effect, the exact drag but to state that for a given prop when fixed is a lower drag situation is understandable and logical.

"Each have different characteristics. The drag of each, will have similarly disparate drag co-efficients. Hence my question about what sort of prop your friend designs, as the statement that fixed props equates to less drag always, is just not true "

You are right each prop is different but taht has no effect at all on the principle that it offers greater drag when left to rotate - gain I will be posting further details. I am not inventing this ..... anyone with skill in the subkect would tell you the same.

**Gludy** · 28-09-05, 06:57

"Lets put this in laymans terms

Downsize a helicopter blades to a size you can put on the back of a boat. How effective do you thing that would be.

Upsize a typical duoprop to helicopter size. Do you think it would fly?

Now do you think they are comparable? "

clearly YES because its the principle - the eact drag values make no difference to that at all. Explain why if a helicopter freezes it baldes it drops like a stone - if the engine stops it can glide to the ground - that is the principle we are talking about here. It makes no difference wether it is air or water, large or small prop.

**Gludy** · 28-09-05, 06:59

When a helcopter loses power the engine has no momentum at all - it can gyroglide to earth only if the prop is rotating - ask any helecopter pilot. Sorry but you are wrong on this.

**Gludy** · 28-09-05, 07:00

Totally true.

**Gludy** · 28-09-05, 07:02

Spot on brendon, its down to the gyro effect of the blades and the same principle applies to any prop in any medium. The actual size/shape make no difference to that principle.