Nose bowl jigging

[Luke68]

I have a question about squaring the nose bowl with the fire wall. I just finished building the plywood hub that is used for holding the nose bowl to the crank, when I clamp a straight edge on it to check squareness to the firewall the sides are perfectly even at 41 3/4” on both sides but the vertical plane is way off, 42 3/4 on top and 40 1/4 on bottom. Am I missing something?

Engine is an O-540 with type 1 mounts

[jim.mclaughlin924]

I think you are ok, no need to be parallel to the firewall.

[robcaldwell]

I did not vertically square mine either. I detailed my installation on my blog. https://bearhawk4place.blogspot.com/...e-cowling.html

[Luke68]

Thanks for the quick response
The paragraph in the builders manual that says to make sure it’s square to the firewall in the vertical plane is what made me wonder . I really don’t want to build the cowling more than once.

Luke

[jaredyates]

Luke, can you help me find that so that I can fix it please?

[Luke68]

Jared
On page 50 the last paragraph in the jigging the nose bowl section.
My manual is form 06 so that might be my problem

[jaredyates]

Yes, the updated manual is at https://bearhawk.tips/kit
Let me know if you need help with getting access.

[Battson]

Having built my cowling twice, 540 engine....
I think the most important thing is getting it level with the wings, per se. That is the split between the halves and the air intakes, level.
Second priority is getting the nose bowl gap matched to the spinner as nicely as possible.

Overall, it's a visual thing more than a functional thing, provided there is no interference...

[Rollie]

The nose bowl needs to be evenly spaced from the spinner. It's a bit of a complex process but it would be a lot more complex if you are trying to set it in relation to the firewall too.

[Luke68]

I thought the same thing, it would be next to impossible to get everything square

[aerolite]

And don't forget to account for the engine sag on it's mounts, after a year it will drop slightly. The Lords sag the least, I usually set the cowl so it's about 1/8 to 3/16 lower, (depending on engine), than the spinner.

[Luke68]

What is the best way to get it level?
This is what I came up with since I don’t have a digital level like Rob mentioned in his blog, I just set the aluminum channels on top of the pushrod tubes and measured from the inside of the air intakes.

[Battson]

Nothing wrong with that!!

[svyolo]

Off topic, thanks for the pic of the top mounted prop governor cable. I don't remember seeing one before. Looks easier than going underneath and then back up.

[davzLSA]

I am in the process of doing this now and my process is as follows, I have my nose bowl clamped to my prop hub at eyeball level and the instrument panel mounted, then I have strings going from the top of the nose bowl to the top of the instrument panel, that gives me the height of my fire wall. I have the fire wall attached now and it is inline with my nose bowl and the instrument panel. If the top of the firewall is not exact then you could cut the flange off the fire wall and install a new one that lines up with the top of the instrument panel and the nose bowl. Now having said all of that I discovered that the bottom flange of the fire wall contacts the bottom tube of the front of the fuse so that limits how far up the fire wall can go. I'm not going to permanently fasten anything until everything is aligned. However I did rivet the tunnel to the bottom flange of the fire wall.

[John Bickham]

Just want to help avoid a mistake I made. Probably only happen to me. Once you get nosebowl jigged in place, keep an eye on it throughout the whole process. Somehow I bumped or rotated the nosebowl slightly during the fitting of the parts. A little bit makes a big difference!!

[robcaldwell]

Oh yes. You're not alone, John. That happened to me several times. Good point!

[svyolo]

I don't have a prop and spinner yet, but want to start on the engine cowl. I have not ever inspected a prop or spinner, but I did find some pictures of Hartzells hubs. Can I just use the face of the flywheel as if it is also the rear of the spinner, or does the bowl of the spinner extend aft of the face of the flywheel?

[svyolo]

The two posters that also did videos were you and Nev. I looked through both. Nev has a 3 blade, yours is the same as the one I am going to buy. Nevs flywheel flange looks about 1/2" or so recessed from the outer edge of his nose bowl. Yours I assumed had a couple inch prop extension, but after looking further I couldn't identify the extension in the video. The two blade hup is longer? Do you have an extension? Your build website plus the video makes me think your nose bowl is 2 or 3 inches farther forward than Nevs.

I am a babe in the wilderness on this. Hartzells website isn't much help, but they do show some adjustability fore and aft of the position of the spinner. But it is mostly useless for what I am looking for. I did email them earlier today.

I completely understand the engine has to be mounted. I am much less certain that you need the prop and spinner to build a cowl. I think you need an accurate fore and aft measurement from the aft edge of the spinner to the flywheel face. If each prop and spinner was unique, including the same model and type prop,n you would have to build a new cowl if your prop had a problem and you replaced it with another of the same type.

[svyolo]

Once again, thanks for the videos and such to those that do them.

[robcaldwell]

I really don't know why my prop hub was longer. It seems like some are, and some are not. I do not have an extension.

[svyolo]

Hartzell did get back to me with an exact measurement, but off of Robs' video, it appears way off. There is a spot in one of Robs videos that you can get a pretty good measurement, but not good enough to build a cowling off of. It looks over 3 inches from Robs videos, and I think I will wait to get a prop. Hartzell said 1.19 inches, or .875 inches with a spacer on the spinner mounts.

[Mark Goldberg]

John, I thought I had answered this yesterday and maybe I did but on another thread. I suggest you wait until you have your prop & spinner to make your cowling. It is hit or miss if you get the cowling right starting without the spinner to position the nose bowl. Mark

[N942VT]

You really only need the spinner back plate, they may be able to send you one long before a prop shows up. Pic 140.jpg

[svyolo]

Looking at the Spinner Rob used, the spinner backplate isn't captured between prop and flywheel. It looks like it is bolted separately to the hub, 3 + inches forward of the hub/flywheel, or at least that is what I saw on his video.

There was one fleeting moment on his engine mounting video that I could fairly accurately measure the distance, but not close enough to build a cowl off of. Hartzell did say that it can vary a lot based on the type of spinner used. The one he quoted 1.19 inches of distance was the one they normally spec for a BH, which is 13.7 inch diameter.

I did manage to mount the nose bowl at the distance spec-ed by both Hartzell and off Robs video. I can't build a cowl off of it but I can check for clearances to mount the exhaust hangers, and probably cut the top of the baffles. I especially wanted to make sure I didn't have any exhaust clearance issues with the Vetterman exhaust. I have plenty of room at both distances.

[Bcone1381]

The Hartzell spinner I have attaches not to the flywheel, rather is indeed mounted to the propeller hub. My prop will be different from your prop. (O-360) It does indeed have generous adjustment (in-out adjustment) potential.

[svyolo]

Yeah I saw the spacers on Hartzells website. I suppose that makes a lot of sense when you want to adapt a single hub to lots of airframes. I wonder if their is an aerodynamic advantage to having the prop as close the the cowl as possible, or farther out?

[alaskabearhawk]

Hey, question for those that have their engines installed for awhile. Eric Newton and others suggested using 1/8th inch as a measurement to compensate for engine sag for the spinner/nose bowl location. Is that about right? Doing my cowling now.

Also, I have had my split nosebowl on the shelf for years (early, maybe some of the first production run?) and I have to admit I'm not impressed with the fit and finish of it. With the spinner location made flat, the joggle for the split and where the front cowling lies isn't close to lining up. I'll probably be doing some glasswork to even that out.

[svyolo]

This is my first run at building an airplane so I am not the burning bush. My late model nosebowl fits really well. I did do a lot of research into a lot of things. The sag you are talking about, I heard it is more. As much as 1/2 inch. I am not the burning bush, so I have no way of verifying this. My engine sat on its' mount for two years, to a modified HF engine stand. I am going to guess that 50-75% of the sag has already happened.

That is my guess. I am willing to listen to experience, rather than my guess.