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CS vs FP and the Effect on CG

Nihon_Ni

Well Known Member
I've researched the CS vs FP question a lot, but have not been able to find an answer to the question that bothers me the most. I know both props have their pros and cons, but I want to limit this discussion to each prop's effect on CG.

I'm building an RV-8 with Grove gear, planning to install a Lycoming IO-360-M1B. I want to be able to load 2 bubbas (200 lbs ea), ~42 gals of fuel, and be able to max out each baggage area without running into CG issues. I know that load will likely put me in the 1900 lb gross weight neighborhood, but I could either accept a higher GW or off load fuel.

My big concern is if I install a Catto three blade, will that limit the amount of baggage I can put in the aft baggage hold? If I need the heavier CS prop on the nose for this max gross mission, I'm willing to carry ballast around on solo flights, which probably won't be that often. My wife and I typically fly together or I'll fly with a friend, so it's rare that I go up solo. My goal is to have the best envelope for cross country flying.

To guys with Catto and Hartzel props, do you find yourself running into CG issues at max gross, or is this something I really don't need to worry about?

Thanks,
Rob
 
To guys with Catto and Hartzel props, do you find yourself running into CG issues at max gross, or is this something I really don't need to worry about?

Thanks,
Rob

In short - no, Your decision on CP or FP prop need not factor in CG issues. The caveat is assuming you don't do something weird like moving a lot of weight aft.

Here is my RV-8A data point:
- 180 HP IO-360
- Hartzell CS prop
- Two Odyssey PC-625 batteries mounted aft of the firewall (one in the baggage compartment well and one in the same spot on the left side of the fuselage).

Heaviest cross country was a 180 pound guy in the back and he brought enough luggage to fill every nook and cranny (forward compartment biased to the heavier stuff). CG was well inside limits. Solo and low fuel is the most forward CG - but that is still within limits. I did carry a 10 pound bag on the aft baggage compartment shelf (oil, tools and such) but that was with or without passengers and baggage.

Carl
 
I would say that with your defined mission listed above, I would go CS for sure. I have a 0-360 with Catto and it's PERFECT for solo aerobatics, but 2 up with baggage it's hard to stay in CG range if you load up the rear baggage area.

According to my numbers if I load 250/250 in the seats I need to load about 90lbs in the front compartment to balance out about 60lbs in the rear. and loaded like that I would have a bit of a long ground run, longer than I would ideally like.

I can fly big passengers, but not with lots of baggage.
 
Thanks for all the replies, this is really helpful. I hate to give up the light weight and simplicity of the Catto, but it sounds like I need more weight on the nose.
 
You can always add a weighted crush plate from Saber to add fwd CG. I expect to have to do this on my -7A with O360-A4M (solid crank) and Catto 3 blade setup.

A friend with a -8 and Catto 3 blade limits his back seat to 225 (more if countered by fwd baggage). The reason, he says, is that the tail does not have enough lift when landing unless you keep the speed above normal.
 
You can always add a weighted crush plate from Saber to add fwd CG. I expect to have to do this on my -7A with O360-A4M (solid crank) and Catto 3 blade setup.

A friend with a -8 and Catto 3 blade limits his back seat to 225 (more if countered by fwd baggage). The reason, he says, is that the tail does not have enough lift when landing unless you keep the speed above normal.

I've thought about that, but if I'm going to add weight to the nose I'd rather have it be used for performance instead of just ballast. I've been thinking about building the plane light at every step, so the adding weight to the nose in either CS or crush plate is hard for me to get my arms around.
 
Would anyone else care to join the convresation? It would be nice to have some more data points to help me make a decision.

Thanks,
Rob
 
Over 200 lbs. in the back seat and I put 25 lbs. of sand in the front baggage compartment. My empty weight is 1067 with a Superior IO360 and 3 blade Catto. 1067 and 250 lbs. of fuel and 400 lbs of people leaves 83 lbs. for bags, most of which could be put up front and not be outside CG. I would never exceed the designer's gross weight. Gross weight is an engineering /design limit which you likely have insufficient information to make an informed decision to alter. Changing it would certainly have a negative affect on handling.
 
CG and handling qualities

Agree that exceeding gross weight limits should not be a planned loading option. Another thing to consider is the handling qualities of fwd vs. aft cg operations; my 8A handles SO much better in the aft cg range that I load 50 lbs in the rear baggage when flying solo, even with FP whirlwind prop. This makes the plane much more of a delight to fly, take off, and land as compared to forward range cg loadings. With empty weight of 1055 I bust gross weight before aft cg limits when loading a Clydesdale passenger + full baggage.
 
I have the IO-360 M1B in my 8 with a pc680 installed on the firewall and a Whirlwind RV200 constant speed on the nose. This prop is about 15 lbs lighter than an aluminum constant speed. I'm also running around without paint, which will move the cg aft when I get it done.

I find the combination to have much utillity, but I'm a bubba. With me solo, full fuel and nothing in the baggage compartments my cg is pretty far forward but ok. I prefer landing the plane at a more forward cg, but it really just likes to wheel land in this configuration. Normally, for solo flight I keep about 15 lbs in a tool bag in the rear baggage compartment and that's my favorite. I can 3 point it if I'm coming into a short strip or wheel land it otherwise.

9gvith.jpg


I've also flown it a bit overweight and at pretty far aft cg. (I've placarded it at 1900) The airplane gets much more pitch sensitive in that condition (which is completely normal) and it really prefers 3 pointers, but I will add that I don't like that as much and I'll throw 30 pounds or so in the front baggage compartment. I'd rather be a bit heavy than too far aft of course.

33ot7vt.jpg


Once the airplane is painted I suspect I'll be carrying more in the front baggage compartment. Moving 20 lbs between baggage compartments I can put the cg in a comfortable place regardless of loading.

I'm guessing the extra weight of the constant speed prop works really well for me because of the bubba factor. I suppose if you and your normal gib go around 150 lbs each that the lighter fixed pitch prop would give you better loading utility.
 
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