prop length

[miyu1975]

I know Vans site recommends 72 inch for TW and 74 or 72 for Trik... My question why the 72 for TW?.. I would think the ground clearance is more for TW than trikes..

[John Courte]

know Vans site recommends 72 inch for TW and 74 or 72 for Trik... My question why the 72 for TW?.. I would think the ground clearance is more for TW than trikes..

It is if the nose is pointed up or straight. The only reason I can think of is that with a nosewheel, it's less likely the aircraft will somehow be in a nose-down attitude on the ground, resulting in a strike with a longer prop.

Add to that the fact that the pivot point of the ship rolling on the main gear is a bit aft of the same situation in a nosewheel, which adds length to the (whatever engineers call one side of a lever) resulting in more vertical travel for the nose for a given angle of rotation.

[Daver]

I just don't understand prop stuff.

Ed Sterba cut me a 68" (72 pitch) prop for my 150 HP -9A.

It seems to work great & I'm happy.

Some have looked at my prop and commented that it looks too small (short). It does look kinda short.

I have about 12" of ground clearance and I have not picked up any rocks & dinged the prop so far. That's a good thing..

If a 68" prop is good, why would a longer(72") prop be better?

I'm just glad there are people out there that understand these things

Dave (electrical engineer ignorant about things aeronautical...)
-9A

[Neal@F14]

I think that if you're pitched so far nose down in a taildragger RV that you're counting on only an extra 1" of length on each blade to spare you from a prop strike, you're probably going to suffer that prop strike anyway. The RV-8 in my avatar has the 74" Hartzell blended airfoil prop and it is a fine fit on that aircraft. We've lifted the tail up in the hangar to see what the prop clearance is like, and the tail has to come up in the air to an astonishing height before the prop blade tip would ever touch. If you ever saw that kind of abnormal ground sight picture in the windshield while rolling down the runway, you would've already resigned yourself that you were already way too deep into the process of having a propstrike. And if you're hitting on the mains so hard as to compress them down enough to hit the prop without the tail being way up too high, then you weren't landing... you were already crashing anyway. Maybe with the springier main gear of -4, -6, -7, -9 this may be more of a factor to consider for deciding on the shorter prop, but the with the tall, heavy-duty rectangular main gear of the -8, I don't think the 74" prop poses any ground clearance problem at all.

[miyu1975]

thx. good feedback. anyone flying a 7 tw and a 74 inch prop?

[gciampa]

The primary reason for the shorter prop for the taildragger is for reserve distance between the prop and runway during a botched wheel landing.

In a normal taxi, takeoff and three-point landing confguration, the prop clearance in a tail dragger is greater than the tri-cycle gear.

In a tri-gear, one can reliably depend on the prop to ground clearance because of the nose-wheel and fixed distance between the ground and prop. Naturally, a severe landing or porpoised landing, all bets are off.

However, in a tailwheel, when doing a wheel landing, one intentionally pitches the nose forward, as soon as the mains grace the runway. In the case of an over-zealous pitch forward, the only thing to keep the prop off the runway is pilot skill. In a normal wheel landing, there is ample room to support a 72 or 74 inch prop on a RV7. But, a 74-inch leaves less reserve should one over-compensate in forward pitch.

You can check the static ground clearance by raising the tail and setting zero degree pitch attitude. But, that is a static stance, envision some gear flex on landing, with a 20-knot cross wind and bumpy grass runway, you might see a little PIO as the intrepid aviatior guides his craft to a uneventful stop.

I think....

[elippse]

One of the things that effects the efficiency of a propeller is the mass flow through the disc. Since thrust=mass-flow X deltaV, and deltaV is a loss, the more mass flow, the lower the deltaV, the downwash, and the more efficient a propeller can be. Since mass flow is the disc area X (forward speed + deltaV), the disc area goes up with the square of the diameter. So in going from a 68" to a 72" increases the area and mass flow 12%, and decreases downwash 12% which can allow the prop to be more efficient. However, a larger diameter prop will have a higher tip speed, and if the tip chord is wide, it will have a lot of tip loss. This is why, since the mass flow is also dependent on the number of blades, with a multi-blade prop you can have the smaller diameter for more ground clearance, lower tip loss, and still have good mass-flow.

[F1Boss]

"Keep your prop as long as you can, as long as you can."

The 72 might climb a bit better, but the 74 will be more efficient in cruise. Since you spend more time cruising than you do climbing,....

Paul can jump back in, but it might be that the 72 would be more suited to any SARL activities?

Carry on!
Mark

[elippse]

Originally Posted by F1Boss

"Keep your prop as long as you can, as long as you can."

The 72 might climb a bit better, but the 74 will be more efficient in cruise. Since you spend more time cruising than you do climbing,....

Paul can jump back in, but it might be that the 72 would be more suited to any SARL activities?

Carry on!
Mark

A shorter prop should give lower climb rate, rather than higher, all other things being equal. The reason for this is that the longer prop will have greater mass flow and so be more efficient. The same is true for multi-blade props because for a given diameter, a three-blade prop will have 50% more mass flow, so less energy gets thrown away in downwash. But all other things are not equal. Some propeller designs are more efficient than others. If your blades have a round tip or a wide tip is it less efficient than a blade that has a very narrow or zero chord tip. Do your blades have a clunky, non-aerodynamic shape in the root just outside of the spinner, with a flat, triangular area about 2"-3" long top and bottom? Then it is not as efficient as a blade that has the correct airfoil shape at the correct helix angle all the way to the spinner. Does your spinner have wide clearance around the blade or is it sealed up to the blade? Big holes make drag! All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!
This extends also to the back-and-forth about CS vs FP. Even though you can operate the engine more efficiently at high MAP and low rpm with a CS, if the CS blade has any of the drawbacks just listed, a well-designed FP can actually give more efficient cruise at a higher speed!

[hevansrv7a]

Please see embedded questions. Not arguing, seeking to understand.

Originally Posted by elippse

A shorter prop should give lower climb rate, rather than higher, all other things being equal. The reason for this is that the longer prop will have greater mass flow and so be more efficient. The same is true for multi-blade props because for a given diameter, a three-blade prop will have 50% more mass flow (Doesn't this assume equal blades area, shape, CL, etc., which would not be feasible?), so less energy gets thrown away in downwash.(But aren't longer blades better for induced loss, if all else is equal?) But all other things are not equal. Some propeller designs are more efficient than others. If your blades have a round tip or a wide tip is it less efficient than a blade that has a very narrow or zero chord tip. Do your blades have a clunky, non-aerodynamic shape in the root just outside of the spinner, with a flat, triangular area about 2"-3" long top and bottom? Then it is not as efficient as a blade that has the correct airfoil shape at the correct helix angle all the way to the spinner. Does your spinner have wide clearance around the blade or is it sealed up to the blade? Big holes make drag! All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!
This extends also to the back-and-forth about CS vs FP. Even though you can operate the engine more efficiently at high MAP and low rpm with a CS, if the CS blade has any of the drawbacks just listed, a well-designed FP can actually give more efficient cruise at a higher speed!