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06-05-2013, 11:17 AM
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Join Date: May 2013
Location: Raleigh, NC
Posts: 20
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RV-4 engine mount
I am looking to make my RV as light as possible. From experience flying a friend's RV-4 I know that with a 190+lbs guy in the back seat we are at the rear of the CG. The handling is not that desirable. I have been looking at Titan engines, Lycoming experimental series, light weight starters, alternators, batteries, and sam james cowl. All of these reduce the weight up front. While these mods will make the aircraft more of a pleasure for single seat flying it exacerbates the CG issue with my 190+ friend. As I have an older kit with the short legs, I know I need a new mount for upgrade to the longer ones. Has anyone extended the motor mount slightly to improve the CG? I know that will require some welding, but that is not an issue.
My goal is to keep it light and at an optimal CG, what I find the 4 is lacking.
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06-05-2013, 11:50 AM
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Join Date: Feb 2006
Location: George West, TX
Posts: 567
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Heavy Prop Extension
There is a gent with a RV-4 that has recently been commuting in & out of 8T6. (George West, TX)
We were chatting and he told me he had a 4" prop extension machined out of 4130 steel. He said it weighs 50 pounds; and, when he used it he had no problem packing a 250 lb passenger in the back.
I hope to track him down and get some more info on that. Our conversation was interrupted with RV-4 admirers. A constant speed prop helps in that regard. I'm thinking getting a prop extension machined might be easier than extending the engine mount. However, Russ McCutcheon can provide us with some good info.
Cheers,
__________________
Deal Fair
RV-4 (N34CB)
George West, TX (8T6)
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06-05-2013, 11:53 AM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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Lets start with what you mean by "slightly". Have you calculated how far you would like to move it?
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06-05-2013, 12:35 PM
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Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,213
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move/add battery
I have a -4 with O-320/Sterba WFP, and built fairly light. I have had the 200 lb backseater, and know the feeling. I mounted my battery per plans, in the tunnel, and plan to add second "back-up" FWD of the firewall. I have all electric (no mechanical fuel pump) and the back-up is a good option for me to add some usable weight up front....in the mean time, backseaters are usually kept below 180. Im really trying to improve what kind of bagage I can load for overnighters with my BIB, whom isnt a weight factor. I also have the 3.5 gal Smoking Planes system mounted in bagage compt.
__________________
Bill E.
RV-4/N76WE
8A7 / Advance NC
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06-05-2013, 08:22 PM
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Join Date: May 2013
Location: Raleigh, NC
Posts: 20
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I have not performed any calculations yet. That's why I am asking if anyone knows if this has been done before. I hope to only move the engine forwards a few inches. Right now I am trying to gather information on engine accessory weight to make a calculation. If anyone knows what firewall forward forward costs in weight that would be a great place for me to start. I want to make the changes as minimal as possible and do not want to redesign everything. If I have to move the engine forward I plan on making a female mold of my cowl and extending the back of it as necessary. If I move it too far forward I will have to raise the firewall maybe .5" or less to keep with the smooth transition lines from cowl to aluminum. The bigger the changes the greater the headaches.
I understand the theory of adding weight forward, but I want to avoid any unnecessary ballast to keep with the theme of building it light. If I can not find a better option than that is the route I will take.
As my plans are for the short leg gear and I am planning on the long legs, I will be in the needs on some more modern engine mount plans. Does anyone have plans for a long leg version? Are they different for an o-320 vs an o-360? As always, thank you all for your replies. -Ed
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06-05-2013, 11:14 PM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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I think you will find that you will have to move it a lot. It is a heavy mass yes, but still just a small percentage of the whole. I bet you will see a ratio of about 4:1 for (engine move)/(cg move). I did some very rough calcs based on some serious guesstimates of mass and moment arm and that was the resulting ratio.
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06-06-2013, 06:43 AM
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Join Date: May 2013
Location: Raleigh, NC
Posts: 20
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The 4:1 ratio is not too far off what I was thinking. As the RV-4 sits it is within limits, but with no comfort room. A little bit of forward CG will make a big difference. The added benefit of moving the engine forward is it will open more room forward of the firewall. My friend's RV has the battery aft of the fire wall because of space issues. If I could make space for it up front it will further help to move the CG.
Does anyone have a breakdown of their RV-4 weight and balance? -Ed
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06-06-2013, 09:31 AM
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Join Date: Mar 2006
Posts: 9
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I had a stock -4 with a light starter and alternator and a wood prop. it was easy to get tailheavy with a passenger. I switched to a metal prop and the solid prop extension and it helped a great deal. That change moved the empty CG forward 3/4", which is a lot.
so, the CG was good, but I still would run out of nose down trim on long, high speed descents from high altitude. Increasing the trim tab travel doesn't really help, (I think it has diminishing returns at some point because the tab may be stalling out?).
After experimenting with a fixed tab on the other elevator, I decided to raise the incedence on the horiz stab. I added about 1/2 to 3/8" under the spar. This made the airplane much more solid flying with a passenger, had lots of trim available, and maybe picked up a few knots in cruise. Highly recommended.
Marc
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06-06-2013, 09:43 AM
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Join Date: Jul 2009
Location: Niceville, Florida
Posts: 434
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Drop an e-mail and I'll send you a spread sheet, or you can access the weight and balance worksheet that Van's provides on the company website ( http://www.vansaircraft.com/pdf/RV4wb.pdf). About 450-475 for each main weight and 45-50 lbs at the tail will give you a reasonable average to start with. A basic operating weight (everything except bodies, baggage and usable fuel) of 975 or so will give you some flexibility on loading and remaining below designer's recommendations for max gross. There's enough information on the drawing to derive approximate station information for the propeller/flywheel locations for a cocktail napkin WAG. Van's recommended datum for the -4 is 60 inches ahead of the wing leading edge, so all moments are positive. Empty weight varies widely, and it's not uncommon to see a -4 that exceeds 1000 lbs empty. Assuming a 1K lb airplane, 192 lbs of fuel aboard, 2 standard FAA 170 pounders, you've already exceeded the 1500 max gross recommendation. Even a single 170 lb pilot with full fuel is close to the aerobatic gross limit of 1375--just about there with a 13 lb parachute (if one is worn).
There is only so much leeway for manipulating RV-4 weight and balance (hence the RV-8!). I'm a "light nose" fan for handling and performance qualities, but it does limit how big a person I can reasonably put in the back. I'm just fine with that, but I can also understand that limit may be unacceptable to some. Like you said in your original post: Built-in lightness--the key to overall performance...A heavy nose can increase the CG range, but has a detrimental affect on gross weight--there is limited trade-space.
A couple of accepted solutions, shy of modifying the engine mount/cowling are to utilize a mass ring bolted to the starter flywheel on the engine or to have a custom made crush plate fabricated that puts weight right at the propeller. Saber manufacturing, the folks that make the fixed pitch propeller extensions can fabricate crush plates to your weight specification. One example of a mass balance on the flywheel is a Landrol hydraulic damper--they will pop up from time to time in the classifieds. There is also quite a selection of engine/propeller combinations that can help you manipulate CG (e.g., light engine, heavy prop, etc.). I think it was Van that said Lycoming never built the same engine twice! Add to the mix all of the "after market" engine combinations/components (as you mentioned!), and there is a wide band of suitable powerplants and propeller combinations.
Moving the engine sounds interesting, but there may be some unintended consequences. It would be interesting to look at the data after you're done testing! Keep us posted on your progress.
Best of Luck,
Vac
__________________
Mike Vaccaro
RV-4 2112
Niceville, Florida
Last edited by Vac : 06-06-2013 at 10:08 AM.
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06-06-2013, 10:06 AM
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Join Date: May 2013
Location: Raleigh, NC
Posts: 20
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The spread sheet will be a great place for me to start. The goal is to not add weight while moving the CG.
I spoke with Titan engines the other day. The rep mentioned something about the impulse of the higher compression engines being too much for heavier props. I was hoping it said more in the info he sent me, but it did not. He recommended a light weight composite prop. So basically no allowance for a large mass forward of the engine. At least that's the plan for now.
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