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Propeller Weight Guidance

I am looking for some guidance. I currently have a RV 7 with a O-320-D2J engine with a Catto 2 blade prop with a 19.2# weight bolted on the front of the prop. (See attached Photos) I am getting a lot of vibration especially at lower RPMs so I have the prop removed to get it static balanced with the weight. I have a feeling the weight may be the issue. So I am looking at options as we are flying some formation but this set up is hard to slow down and stay in place.
Options:

1. 3 blade fixed pitch. What would be options for this and to naturally get more weight up front? I know Catto make one but it is really on 1# heavier than what I have.

2. The O-320-D2J engine does have a plug for a constant speed prop and a pad for a governor. I think this option may be cost prohibitive unless you know of a good option here.

3. Something between the 2 options above. A electric prop??? Basically a take off pitch then you can change it to a cruise pitch. I know nothing of this as it was just mentioned.


I am not in a hurry as I am currently putting some updated avionics in it but I need to fix this vibration issue and get more weight up front as we are always aft CG.
 

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A constant speed Hartzell would solve both issues - weight on the nose, and acceleration/deceleration, and might help with vibrations you are experiencing. How important is the formation forgiveness to you ($$)? I fly formation with a fixed pitch Sensenich GA prop now. Most of my RV flying has been with constant speed props, and they make it much easier in formation. With a fixed pitch, you just need to be thinking a little further ahead and be proactive. I have no problem with that, especially considering the costly alternative. I don’t think a fixed three blade would make enough of a difference to be worth the change. The third blade creates a little more drag, but the prop diameter is less, and unless it’s pitched for climb, I don’t think the difference will be that great. A GA Sensenich or Whirlwind would be much less $ than a Hartzell + gov + crankshaft plug work, and you could change pitch for climb/formation when flying mostly formation. You already have the spacer and extra weight if you need it.
As far as the vibration issue, maybe that’s what you should figure out first. Maybe take the weight off, make sure W&B is OK and test fly.
 
I have a 19# crush plate also. Requires care in assembly otherwise the weight pulls it off center as you tighten bolts. Install prop and plate with just finger tight bolts. Then push up on bottom of CP to neutralize the weight and center it, then tighten a bolt with other hand. Be sure not to push too hard or will offset it in the opposite direction. May take a couple tries to get rid of vibes. WIth that much mass, it doesn't need to be off a lot to create vibes. WIth the bolts loose, you can see how much it moves. Prop doesnt do this as it has a tight fit on the lugs. However, no such mechanism for the CP. You can get a machinist to scribe a circle (just slightly larger than CP) on the spinner plate that is concentric with the bolt circle and this will help you align it better, but not a big help as the holes in the spinner plate are not too tight of a clearance, but better than nothing .
 
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To lr72

It isn't designed and machined to stay concentric when installed? Wow, that is surprising and certainly problematic.
 
Using a dial indicator to confirm / adjust the concentricity of the weight/crush plate as you torque the prop bolts might help a lot.
 
I'm puzzled why you have a 19 lb plate on the front of a 7.

I've got a 6A, Catto, just a normal crush plate, also an o-320, and my CG is good.
Generally the CG of a 6 tends to be a bit more aft than the 7...

So with comparable equipment ... why such a big weight up there?
Seats from a Cadillac Escalade?
 
I'm puzzled why you have a 19 lb plate on the front of a 7.

I've got a 6A, Catto, just a normal crush plate, also an o-320, and my CG is good.
Generally the CG of a 6 tends to be a bit more aft than the 7...

So with comparable equipment ... why such a big weight up there?
Seats from a Cadillac Escalade?

Because an RV-6 and an RV-7 are not comparable equipment.
There is actually an extended engine mount that is recommended for use on an RV-7 with an O-320. Especially important if a fixed pitch light weight prop is used but probably still not enough, hence the added weight.
 
It isn't designed and machined to stay concentric when installed? Wow, that is surprising and certainly problematic.

Lycoming uses long bolts to hold the prop on. They specifically use lugs, and not just bolts, to give a very tight and concentric way to hold the prop in the proper position. However, both lycoming and the prop maker follow the same SAE standard for doing so, therefore all parties on the same page and things fit. I doubt they ever contemplated a 20# chunk of steel as a crush plate, nor did the prop maker. If you think about how thread engagement works, the bolt cannot keep from tipping a few degrees in any direction. Add 7" of arm to this and you get a decent amount of movement. You need a different method for centering. Unfortunately without redesigning the prop, there is no method to center the crush plate. If you want the benefits of this much weight up there, you need to take the steps necessary to get it centered yourself. This is NOT an issue for the standard 1/2 pound alum CP normally used, just the specialized 20 pound CP that that aftermarket uses. Saber, who makes these, does specify that care must be taken to keep it centered and that you can't just let it hang while tightening it.

Even if you wanted, you cant make very tight clearance holes in a wood prop. You must leave a good amount of clearance to deal with the natural movement of wood and this extra clearance allows the bolts to move and therefore cannot hold the CP concentric.

It is problematic, like many things where we step outside the boundaries of normal, and are left to do the engineering and application ourselves.

Larry
 
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Thanks for the explanation. I guess I'm just as perplexed as to why they don't design it to bolt up inboard of the prop itself, like the landoll balancer does. It seems to me from a balancing standpoint, you would want it closer to the prop hub than further away. Especially since concentricity is a concern.

It's not just a casual concern on my part, I might be doing something similar to this when I switch to an aerobatic prop so I've been following this thread.
 
Thanks for the explanation. I guess I'm just as perplexed as to why they don't design it to bolt up inboard of the prop itself, like the landoll balancer does. It seems to me from a balancing standpoint, you would want it closer to the prop hub than further away. Especially since concentricity is a concern.

It's not just a casual concern on my part, I might be doing something similar to this when I switch to an aerobatic prop so I've been following this thread.

Saber has a product like that. It is a 2 1/4 spacer made from steel instead of aluminum. Not sure how much it weighs though. It indexes to the lugs on the flange (like a prop does) and has additional lugs to keep the prop concentric. That solution will not have the concentricity problems the CP has due to the lugs and bores giving precise alignment, but also gives up 4" of arm. W&B is a function of weight plus arm.
 
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Because an RV-6 and an RV-7 are not comparable equipment.
There is actually an extended engine mount that is recommended for use on an RV-7 with an O-320. Especially important if a fixed pitch light weight prop is used but probably still not enough, hence the added weight.

In other words: the 7 runs toward more aft CG than the 6, and so you'd better install a heavy engine and prop. If you don't you'll need to hang 19 pounds of dead weight up front. ???
 
In other words: the 7 runs toward more aft CG than the 6, and so you'd better install a heavy engine and prop. If you don't you'll need to hang 19 pounds of dead weight up front. ???

It didn’t turn out to be tail heavy and need a heavier engine, and or propeller, it was designed specifically to accommodate a heavier engine or propeller than the RV-6 was.

When the RV-6 was originally designed, its configuration was optimized for lighter engines and propellers. Propellers in particular, because at that time, the only fixed pitch propeller that could be used that had sufficient pitch for the performance range of the airplane was one made out of wood.

When the design was updated to become the RV-7, the design was optimized towards what most customers wanted by that time, which was larger engines, and constant speed propellers. So the overall configuration of the airplane was designed to account for that, so the finished airplane CG position would be correct.
Because of that, if a lightweight engine, or particularly propeller is used on an RV-7, things need to be done to compensate for that. That is why an extended engine mount is available, but that in itself, I don’t think is enough to compensate for a much lighter propeller weight.
 
Scott,

I don’t want to hi Jack this thread but I’m changing from the Hartzell CS aluminum prop to an MT9 three bladed prop because of a prop strike last summer at Airventure. I understand the MT prop is around 15 to 18 lbs lighter than the Hartzell. Should I be concerned about my aft cg because of this change?
 
Scott,

I don’t want to hi Jack this thread but I’m changing from the Hartzell CS aluminum prop to an MT9 three bladed prop because of a prop strike last summer at Airventure. I understand the MT prop is around 15 to 18 lbs lighter than the Hartzell. Should I be concerned about my aft cg because of this change?

It is possible that your baggage compartment payload capacity could be impacted, but the only way to know for sure is to do some weight and balance calculations on your airplane. Particularly if and it’s current configuration with the typical passenger loading that you have when carrying a heavier baggage load, you are at the aft CG limit with the fuel tanks at their minimum level for normal flight.
If that is currently, true, you will likely lose some baggage payload capability to avoid exceeding the aft CG limit.

Do a theoretical re-calculation of your CG by subtracting the worst case 18 pounds from the arm position that relates to the propeller and see where your new CG position would be. Then do some sample load conditions that would match Your typical flight conditions, and see how it looks.
 
It didn’t turn out to be tail heavy and need a heavier engine, and or propeller, it was designed specifically to accommodate a heavier engine or propeller than the RV-6 was.

When the RV-6 was originally designed, its configuration was optimized for lighter engines and propellers. Propellers in particular, because at that time, the only fixed pitch propeller that could be used that had sufficient pitch for the performance range of the airplane was one made out of wood.

When the design was updated to become the RV-7, the design was optimized towards what most customers wanted by that time, which was larger engines, and constant speed propellers. So the overall configuration of the airplane was designed to account for that, so the finished airplane CG position would be correct.
Because of that, if a lightweight engine, or particularly propeller is used on an RV-7, things need to be done to compensate for that. That is why an extended engine mount is available, but that in itself, I don’t think is enough to compensate for a much lighter propeller weight.

I was aware that the 7 was 'upgraded' to support the heavier engine option, but I was under the mistaken assumption that this was primarily structural analysis/changes - to handle the loads. I need to stop advising people to keep it simple, keep it light when discussing the 7 since keeping it light up front (prop/engine) doesn't actually subtract weight when ballast is then needed on the nose. If someone actually wants a light 7 (320, FP) then they need a custom long engine mount and probably a custom cowl to fit it.
 
I was aware that the 7 was 'upgraded' to support the heavier engine option, but I was under the mistaken assumption that this was primarily structural analysis/changes - to handle the loads. I need to stop advising people to keep it simple, keep it light when discussing the 7 since keeping it light up front (prop/engine) doesn't actually subtract weight when ballast is then needed on the nose. If someone actually wants a light 7 (320, FP) then they need a custom long engine mount and probably a custom cowl to fit it.

If a design is modified for a heavier engine and prop., part of that process has to include compensating for the influence on C.G. The RV-7 has a slightly longer fuselage than the RV-6 for that reason.
Your advice of building light still is good advice, and as relevant for the RV-7 as it is for any other model. Just not in the context of engine and prop. A lighter engine is fine with the extended engine mount as long as a light prop is not used.
BTW, the longer mount is only slightly longer (1 inch I think?) and the standard cowl has enough extra length to be used.
 
I have an RV-7 with IO-360-M1B and Catto 3 blade. Empty weight is 1055# and CG is 80.46". It fly's well as far as CG and no noticeable vibration. Holes in the Saber CP are close, with bolts in finger tight I really couldn't feel the CP move with push up on the bottom when tightening bolts. Happy with the set up. Would be nice to have the CS but $$$$$.
 
Scott,

Thank you for all the good information as it helps me determine my path forward. I looked at getting a CS prop but that is not in the cards right now.
I am now convinced that this would solve my aft CG issues. I am looking for a 3 blade fixed pitch option hoping I can get a little more weight up front and get rid of the 19 pound spinning weight. I am also looking at the weight of the starter as mine is giving out. Any other ideas on how to get some forward weight would be appreciated.




If a design is modified for a heavier engine and prop., part of that process has to include compensating for the influence on C.G. The RV-7 has a slightly longer fuselage than the RV-6 for that reason.
Your advice of building light still is good advice, and as relevant for the RV-7 as it is for any other model. Just not in the context of engine and prop. A lighter engine is fine with the extended engine mount as long as a light prop is not used.
BTW, the longer mount is only slightly longer (1 inch I think?) and the standard cowl has enough extra length to be used.
 
Scott,

I don’t want to hi Jack this thread but I’m changing from the Hartzell CS aluminum prop to an MT9 three bladed prop because of a prop strike last summer at Airventure. I understand the MT prop is around 15 to 18 lbs lighter than the Hartzell. Should I be concerned about my aft cg because of this change?

Jim,
Bet you are looking forward to getting back in the air. Crummy story last year, sorry that happened. When you get that 3-bladed prop mounted, make sure and post a pic or two, that’s going to look great! Hope you are doing well!
 
Scott,

Thank you for all the good information as it helps me determine my path forward. I looked at getting a CS prop but that is not in the cards right now.
I am now convinced that this would solve my aft CG issues. I am looking for a 3 blade fixed pitch option hoping I can get a little more weight up front and get rid of the 19 pound spinning weight. I am also looking at the weight of the starter as mine is giving out. Any other ideas on how to get some forward weight would be appreciated.

Mark Landhal(spl?) that used to sell the balancing ring that mounts on the ring gear support also made a solids steel ballast weight that mounted at the same location. If you could get your hands on one of those it would help.
 
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