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Constant speed prop puzzler

Ed_Wischmeyer

Well Known Member
So I finally decided to go from a fixed pitch Sensenich to a Hartzell constant speed on the RV-9A with an O-320. Took off the Sensenich fixed pitch metal prop and the 2 1/4" spacer (I measured it) but when we went to put on the constant speed prop, a surprise, and not a good one. The cowling and prop did not fit -- negative clearance.

If we remove the required 3/8" spacers between the prop hub and the spinner backplate, things just barely fit. The prop is an experimental prop but advertised as an HC-C2Y(L)-1BF, same as Van's sells. This prop shop says that this is the 502nd prop shipped for an RV.

Nobody around here or on any tech support line has any ideas, so it looks like I'll need a new cowling and some baffle modifications.

Any thoughts?

Thanks,

Ed
 
Call sam at sabre manufacturing and order a constant speed prop spacer at whatever thickness you want. I went from a catto to a whirlwind. My prop spacer went from the 4” long down to a 2” spacer. This gives me about .40” of spinner to cowling clearence. I do not like making them exactly to plans at .25”. My conical mount engine will move around a little and make contact under high g loads without the extra space.

06092601-958-D-41-D2-A83-B-681-A4-FA83031.jpg
 
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Any thoughts?

Make sure the new spinner back plate is flat.
It is fairly common for them to be slightly cup shaped (which isn't a problem when the get captured between the prop extension and prop) and then it gets worse when you cut the center out of it for the constant speed prop.

I doubt that this could account for the 3/8" delta that you say you have but it could easily be 1/8 - 3/16"

BTW, the 2 1/4" spacer was originally developed by Sensenich for Van's to account for the dimensional difference between a standard Hartzell propeller hub and the aft face of the fixed pitch prop hub, so it technically should match.

Since others have very occasionally mentioned a fit problem when doing a prop conversion, I measured one of our currently unused propellers just to re-familiarize my memory with the details and it measures exactly 2 1/4 inches from the aft face of the hub (where it interfaces with the engine) to the aft face of the bushing with one washer added as specified in the install manual.

A couple of other things that could account for some dimensional loss....

- If the doubler was riveted onto the rear bulkhead on the front side instead of the rear where it is supposed to be.
-If you accidentally installed the new prop with the ring gear support not installed.
 
One additional thought....


In the past, when a 4 inch extension was required for the old style cowl, two 2 1/4" spacers were stacked together.

Measure the spacer you removed and make sure it is 2 1/4 inches. It is possible someone got a hold of a prop that came with two 2 inch extensions and then just used one of them.

EDIT: Never mind.... I just noticed in your first post that you said you confirmed it was 2 1/4"
 
Prop Spacer

Thanks, but the problem is that there's not enough room for the prop with no spacer...

I think he's talking about a custom spacer between the ring gear support and hub (like a fixed pitch prop spacer) to move the whole prop forward, not the spinner back plate spacer...
 
I think he's talking about a custom spacer between the ring gear support and hub (like a fixed pitch prop spacer) to move the whole prop forward, not the spinner back plate spacer...

That seems like quite a project for a constant speed prop -- lots of stress analysis, funny bolt patterns, not to mention oil through the hub and O-rings. Unless I'm missing something...
 
Ed, before I had my oil pump issues, I was dealing with the same issue you are, although it's with a Whirlwind prop instead of the Hartzell. I had a FP prop with a Saber 2.25" spacer and it fit well with my cowl. I initially installed the WW prop without the spacer and had the same problem you do. Bottom line, for me, what I am having to do is have a custom spinner and back plate made by Whirlwind to mate with a 1.25" spacer I bought from Sam. The spacers work fine with CS props, so no worries there. Just let Sam know your application is for CS and he'll include an o-ring that provides a seal between your crank boss and the spacer. Yep, it's not cheap, but nothing is, it seems
 
Thanks, but the problem is that there's not enough room for the prop with no spacer...


You need this spacer at whatever length you determine that will give you the correct spacing. Sam will make you one to your requirements. It will move the entire prop and back plate forward to whatever dimensions you want. Bolts on with an o-ring and the prop bolts to the spacer. Short of rebuilding all your cowling, this is the only quick way to do it.

ACF87183-B5-FC-419-C-BE08-BA2-FCDF491-B9.png
 
Time to learn something new - what is a ring gear support, and where is there a picture of same? We haven't touched the ring gear. And then there's the issue of making sure the starter still engages properly.

The original builder did some "original" things, and there have been a few surprises already...

Thanks!

Ed
 
The ring gear is cyro fixed to the flywheel. The flywheel is normally dropped in a freezer overnight while the ring gear is heated in oil. When the ring gear is slipped over the flywheel and cools, it is fixed into place with an interference fit. Never heard of a ring gear support. The sabre spacer bolt directly to the crankshaft and drive lugs.

This is what it should look like.

75-F01-A43-71-CB-4380-A458-4-D7246-A399-EC.jpg
 
Time to learn something new - what is a ring gear support, and where is there a picture of same? We haven't touched the ring gear. And then there's the issue of making sure the starter still engages properly.

The original builder did some "original" things, and there have been a few surprises already...

Thanks!

Ed

Ring gear support is the flywheel. The ring gear is a separate piece that is installed with heat and shrinks itself onto the outside of the flywheel.
 
Time to learn something new - what is a ring gear support, and where is there a picture of same?

Never heard of a ring gear support. The sabre spacer bolt directly to the crankshaft and drive lugs.

This is what it should look like.

As Mike said, it is the official "Lycoming" name but what many people call Fly Wheel. It doesn't really have enough mass to warrant the Fly Wheel name, so I think Lycomings' name is more correct.... but when in a foreign country, I guess you need to speak the local slang to be understood;)
 
Figured it out - spacer is the way to do

Call sam at sabre manufacturing and order a constant speed prop spacer at whatever thickness you want.

Finally figured it all out in my old age...

First, consider the fixed pitch prop with a 2 1/4" spacer, and consider the front of the spacer as the datum for this discussion.
* For a fixed pitch prop, the *aft* side of the rear spinner bulkhead is immediately forward of the datum;
* For a constant speed prop, that datum is on the propeller hub. The *forward* edge of the rear spinner bulkhead is aft of the datum, moved further aft by two washers and a 3/8" spacer, for a total of about a half inch.

There's the problem.

Had a great talk with Sam just now (www.sabermfg.com/), and the thinnest spacer he can manufacture is 1.25". The easy solution appears to be to:
* Install the constant speed prop with the 1.25" spacer;
* Keep the present cowl, but extend the cowl right behind the prop with foam and fiberglass. Probably doesn't have to be all that stout because all the cowling loads are carried by the existing structure. The extension would be just shy of 3/4". And if I needed to, I could fly without the cowling extension;
* Install the new spinner.

$377 spacer, $45 bolts, $4 O ring. And he makes 3" spacers that pull 10Gs on Red Bull racers.

Yes, the present cowl would need touch up paint, but so would a new cowl -- and the new spinner will need paint, too.

Anybody want a brand new Van's cowl in Georgia for an RV-7/9, O-320?
 
Finally figured it all out in my old age...

First, consider the fixed pitch prop with a 2 1/4" spacer, and consider the front of the spacer as the datum for this discussion.
* For a fixed pitch prop, the *aft* side of the rear spinner bulkhead is immediately forward of the datum;
* For a constant speed prop, that datum is on the propeller hub. The *forward* edge of the rear spinner bulkhead is aft of the datum, moved further aft by two washers and a 3/8" spacer, for a total of about a half inch.

There's the problem.

As I wrote in my other post, the distance from the back side of the spacer bushing on a standard Hartzell propeller hub (with one thick washer installed as well), to the aft face of the hub where it interfaces with the engine is 2 1/4" so the spacer and washer are not reducing the 2 1/4" dimension.

The only variation in bulkhead position that should happen is the influence of the thickness of the bulkhead since the fwd face of it is now positioned against the 2 1/4" datum instead of the aft face.
As I mentioned before, if you installed the doubler on the wrong side of the aft bulkhead (It should be on the aft side), this would push the spinner bulkhead even further aft from where it is supposed to be.

You are not the only one to make a switch between fixed pitch and a constant speed. Many of them work fine. If it doesn't, it is like because something is nonstandard or not assembled as intended (or a bit of both).
 
Ed- be sure to do some W/B calcs before opting for the extension. Moving a 55# prop forward by an inch and a half is significant, and will amplify the negative impact of polar mass on handling and spin recovery. Could be fine, just know before you go, especially considering that you have to do some cowling work anyway. You should be able to use your old W/B data to predict CG impact of new installation pretty closely.

I’m grateful to Hartzell for having made a custom hub just for Vans with a built-in 1” extension for the composite CS. That bolts right on with the standard cowl with no mods.- Otis
 
Ed- be sure to do some W/B calcs before opting for the extension. Moving a 55# prop forward by an inch and a half is significant, and will amplify the negative impact of polar mass on handling and spin recovery. Could be fine, just know before you go, especially considering that you have to do some cowling work anyway. You should be able to use your old W/B data to predict CG impact of new installation pretty closely.

I?m grateful to Hartzell for having made a custom hub just for Vans with a built-in 1? extension for the composite CS. That bolts right on with the standard cowl with no mods.- Otis

Thanks for the reminder! Already did the W&B. The new prop is about 10# heavier than the fixed pitch, and the W&B change with the extra weight farther forward was, surprisingly, insignificant. The RV-9A has a generous CG envelope.

After doing all the math and such, the constant speed prop moves the spinner aft by at least 1/4", measuring from the aft spinner bulkhead. The current solution is to get a 1.25" Saber Mfg spacer, the minimum thickness, and then build up the leading edge of the cowling with foam and fiberglass. The structural loads will be handled by the existing cowling. And I'd have to do some fiberglass work and paint work with a new cowling, also.

Measure four times, change your mind five times...
 
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