VAF Forums

VAF Forums (https://vansairforce.net/community/index.php)
-   RV-8/8A (https://vansairforce.net/community/forumdisplay.php?f=6)
-   -   Prop trailing edge clearance (https://vansairforce.net/community/showthread.php?t=93845)

edsong 12-05-2012 01:40 PM

Prop trailing edge clearance
 
I'm mounting my cowling and it looks like the propellor trailing edge will be very close to the intake scoops. I have 3/8" clearance for the spinner to cowling and 1/4" from the prop trailing edge to the front of the cowl. It's the new composite prop from Hartzel that I bought from Van's and I'm also using the smooth cowl. I used air pressure(150psi) in the front governor fitting to pitch the prop to coarse(you can hear it hit the stops) so I'm confident it won't pitch any further. I was just wondering if this is to close?
http://farm9.staticflickr.com/8346/8...eac10236_c.jpg

[ed. Edson, it looks like you closed the thread when you created it (so I re-opened it). Assuming since you are asking for input you want people to be able to reply. br,dr]

Thanks dr
Eddy

Sparky 12-06-2012 07:44 AM

Eddy,

I too have the Hartzell composite prop on my RV 7 (not flying yet) and have similar 1/4" clearance.

A while back I received a letter from VAN's telling me to check the clearance from the trailing edge of the prop blade to the cowl with the prop at coarse pitch before first start because someone with the same setup (in South Africa, I believe) reported that the prop struck the cowl when they cycled the prop. VAN's suggested that if I had the same interference I could send the prop to Hartzell to have the coarse pitch stop adjusted to provide more clearance.

I suspect that the composite blades will flex a little, but upon power application, if anything, they should flex away from the cowl. When braking they could flex toward the cowl, but then they should be in fine pitch with plenty of clearance.

I'm thinking 1/4" is enough, but I haven't started the engine yet, so I too would like to hear from the experts.

rocketbob 12-06-2012 08:11 AM

The high pitch stop is a bushing on the pitch change rod and its easy to get to without removing the prop. Normally they're set up to prevent the pitch change fork from bumping into the hub casting at coarse pitch, but there are also airframe considerations as is the case here. Its pretty easy to get to by removing the dome, but it requires a new dome oring and resealing the threads. The stop itself is just an aluminum bushing that one could easily duplicate on a lathe but longer to prevent the prop from interfering with the cowling.

JoeBlank 12-06-2012 10:50 AM

Quote:

Originally Posted by Sparky (Post 722766)
Eddy,

I too have the Hartzell composite prop on my RV 7 (not flying yet) and have similar 1/4" clearance.

A while back I received a letter from VAN's telling me to check the clearance from the trailing edge of the prop blade to the cowl with the prop at coarse pitch before first start because someone with the same setup (in South Africa, I believe) reported that the prop struck the cowl when they cycled the prop. VAN's suggested that if I had the same interference I could send the prop to Hartzell to have the coarse pitch stop adjusted to provide more clearance.

I suspect that the composite blades will flex a little, but upon power application, if anything, they should flex away from the cowl. When braking they could flex toward the cowl, but then they should be in fine pitch with plenty of clearance.

I'm thinking 1/4" is enough, but I haven't started the engine yet, so I too would like to hear from the experts.

Yes, this is what is going on. Vans did notify customers who had purchased the composite props about this a while back. While Vans has not tested this particular prop on all of the models, Hartzell wanted to offer it though Vans.

Contact Hartzell in order to have the course pitch stop reset. This is typically only a problem on the 6/7/9/14 cowls. No one has tried it on an -8/8A yet though. Works OK on a -4...

Do not start the engine until this has been rectified otherwise expensive noises will ensue.

Sparky 12-06-2012 03:56 PM

Eddy,

Here's what I found as airworthiness standards for transport category aircraft in the FAR's:

********************************

"Sec. 25.925 Propeller clearance.

Unless smaller clearances are substantiated, propeller clearances with the airplane at maximum weight, with the most adverse center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following:

(a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level takeoff, or taxiing attitude, whichever is most critical. In addition, there must be positive clearance between the propeller and the ground when in the level takeoff attitude with the critical tire(s) completely deflated and the corresponding landing gear strut bottomed.

(b) Water clearance. There must be a clearance of at least 18 inches between each propeller and the water, unless compliance with Sec. 25.239(a) can be shown with a lesser clearance.

(c) Structural clearance. There must be--

(1) At least one inch radial clearance between the blade tips and the airplane structure, plus any additional radial clearance necessary to prevent harmful vibration;

(2) At least one-half inch longitudinal clearance between the propeller blades or cuffs and stationary parts of the airplane; and

(3) Positive clearance between other rotating parts of the propeller or spinner and stationary parts of the airplane."

********************************

And for normal, commuter, etc. category aircraft:

********************************

"Sec. 23.925 Propeller clearance.

Unless smaller clearances are substantiated, propeller clearances, with the airplane at the most adverse combination of weight and center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following:

(a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level, normal takeoff, or taxing attitude, whichever is most critical. In addition, for each airplane with conventional landing gear struts using fluid or mechanical means for absorbing landing shocks, there must be positive clearance between the propeller and the ground in the level takeoff attitude with the critical tire completely deflated and the corresponding landing gear strut bottomed. Positive clearance for airplanes using leaf spring struts is shown with a deflection corresponding to 1.5g.

(b) Aft-mounted propellers. In addition to the clearances specified in paragraph (a) of this section, an airplane with an aft mounted propeller must be designed such that the propeller will not contact the runway surface when the airplane is in the maximum pitch attitude attainable during normal takeoffs and landings.

(c) Water clearance. There must be a clearance of at least 18 inches between each propeller and the water, unless compliance with Sec. 23.239 can be shown with a lesser clearance.

(d) Structural clearance. There must be--

(1) At least one inch radial clearance between the blade tips and the airplane structure, plus any additional radial clearance necessary to prevent harmful vibration;

(2) At least one-half inch longitudinal clearance between the propeller blades or cuffs and stationary parts of the airplane; and

(3) Positive clearance between other rotating parts of the propeller or spinner and stationary parts of the airplane."

**********************************


I think I will have the coarse pitch stop adjusted to provide the 1/2" longitudinal clearance on my RV-7 for my own peace of mind if nothing else.

Toobuilder 12-06-2012 04:40 PM

Consider engine mount wobble as well. 1/2 inch may be more than enough close to the hub, but the further out the blade you go, the more relative movement you will have.

Sparky 12-06-2012 05:24 PM

If I'm not mistaken, engine mount wobble (wet dog shake) is mostly rotational. The rubber engine mounts don't allow much, if any, fore/aft movement. So while "wet dog shake" may be an issue with baffle or FAB clearance, I don't think it's much of an issue with prop blade clearance. I'm comfortable with 1/2" longitudinal clearance between the prop blade and the cowl.

Edit: Also, I believe "wet dog shake" is less pronounced with a low-inertia composite prop than with a heavier metal prop.

Toobuilder 12-06-2012 07:28 PM

Quote:

Originally Posted by Sparky (Post 722931)
If I'm not mistaken, engine mount wobble (wet dog shake) is mostly rotational...

The operative word there is "mostly"...

My Hiperbipe will set up a resonance with the spring gear at certain taxi speeds/RPM and the spinner wobbles very noticeably in relation to the cowl. This is a non concentric path that in extreme cases will contact the cowl with the spinner backplate. The mounts are admittedly getting soft, but the point remains - consider some angular displacement when setting up minimum clearance from a very expensive propeller blade.

Sparky 12-06-2012 07:56 PM

Quote:

Originally Posted by Toobuilder (Post 722971)
...but the point remains - consider some angular displacement when setting up minimum clearance from a very expensive propeller blade.

Point taken.

edsong 12-07-2012 07:47 AM

Prop clearance
 
I think the easiest solution will be to change the bushings in the prop. That way the cowling will stay where it's suppose to be. I'm more comfortable with 1/2" space between the prop and cowl too.

Thanks VAF and all of you for helping me on my project.

Eddy


All times are GMT -6. The time now is 05:24 PM.