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RV-4 engine mount

newrv-4

Member
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.
 
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,
 
Lets start with what you mean by "slightly". Have you calculated how far you would like to move it?
 
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.
 
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
 
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.
 
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
 
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
 
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
 
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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.
 
A couple of suggestions you may like to look at.
Tailwheel: Vans have a standard and a lightweight tailwheel, the less
weight right at the back the better for what you are trying to do.
Stinger: Try this link: http://www.irondesign-airparts.com/index.html
I understand this chap can make a titanium stinger, for a further
weight saving way down the tail end. I've not been able to get an
answer as to if it is a direct replacement for the vans stinger
though.
 
the CG problem is easy to solve, get out your credit card and buy a o360 and a constant speed hartzell. :D:D

bob burns
RV4 N82RB
 
We were chatting and he told me he had a 4" prop extension machined out of 4130 steel. He said it weighs 50 pounds...
50# at the crank flange would my my CG forward about 2". That's a lot in the right direction.
Moving my 250# O-320 4" forward would move the CG forward about 1". That's not a lot for the effort it would take. I'm not so sure I'd hang 50# at the end of the crank though.

As it is, my CG is creeping aft as I rebuild my panel and cockpit and a bunch of other stuff. I'm adding a Landoll balancer...maybe, a lightweight tailwheel, and if I can convince my supplier management/finance focal, a Catto 3 blade to replace my wood prop. That and a good diet should get me where I need to be.
 
Replacement gear

So now that I am modifying my engine mount I might as well change from the short gear to the long gear. I have read a lot of opinion about the two, but in the end I will let my friends fly it and I do not want a prop strike. So, the long gear it is. I have been trying to research the long gear but have come up short on information, such as the change in attachment angle at the engine mount.

While I was researching it I saw it mentioned that the max gross of the RV-4 is limited by the landing gear. While I have no intention of flying it over 1500lbs often, a little extra loading room would be great for cross countries. With a system similar to the safe air tanks I would be happy to turn that extra weight into fuel capacity. So my question is I does anyone know of where to get heavier duty RV-4 landing gear. The rocket gear is a little expensive. I saw some posts about RVs that are so equipped. The next question is that if I can not purchase it, What does the present long leg gear look like? Is it a tapered tube? Is it solid bend like the 8?
 
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fly over to 8A7

Fly over to 8A7 which is 10 miles south of INT and I will show you my long leg -4 as well as my neighbors F1 Rocket for comparison. I also have my plan set you can look at to see the difference in the mounts. I built mine, and can tell you about anything you need to know about your challenges. Should be a short flight for you. Do you ever go to Gilliam McConell for BBQ ?
 
While I was researching it I saw it mentioned that the max gross of the RV-4 is limited by the landing gear.
Where did you find this and can you share the info? I looked into it a looooong time ago and found something very different from a source that should be pretty reliable. ;)
 
I came across this information in the VAF forum archives. What have you read to the contrary?
Years ago I had a brief conversation with Van about what set the weight limit. I don't know if opinions or data have changed since then so I suggest a call to the factory might be in order. Suffice it to say that after that conversation I was comfortable in raising my TOGW to 1600 lb. My aerobatic weight is unchanged.
 
RV-4 Weight

I was unable to find the article mentioned in the VAF archives, but this is close http://rvbuildershotline.com/articles/grossweights.html While it does not say anything specific, it does make some obvious points.

" Several respondents stated that they had calculated G-load strength of their airplanes based on a ratio of standard category weight relative to aerobatic gross weight to arrive a higher gross weight.

For primary structural considerations, this is basically valid, but it does not take into consideration possibly safety issues related to higher stall speed, landing gear loads etc. Just because the wing won't break because of the higher gross weight doesn't mean that other bad things can't happen. "

Out of curiosity, what did Vans say to make you raise the gross to 1600?
 
RV-4 Weight

I still can not find a limiting factor for MGTOW in the RV-4. I did come across some additional information. Many Rv-4's in the US appear to use 1800 as the MGTOW. That is beyond what I would ever desire. As far as I can tell there were no additional airframe modifications.

"John Johanson told us about how he rebuilt his world spanning RV-4 to have a "wet wing" before his Antarctic adventure. The gross weight of his airplane at take off with 1000 liters of fuel (264 US gallons) was approximately 3000 pounds."

I do recall that Vans issued an airframe modification to add a larger aluminum bracket aft of the firewall to avoid buckling due to rough landings.

I have performed a little math on the Titanium landing gear and have not seen anything that would cause me to back off the project. From what I can determine I would only need to drill 4 holes to prepare a tube. Nothing too scary. Two where it meets the engine mount and two at the bottom to attach to an axel fitting that I will mill. Its either that or I just buy a set of long gear from vans. I wonder that they sell for and weigh. Changing the gear may not be worth the effort.
 
I still can not find a limiting factor for MGTOW in the RV-4. I did come across some additional information. Many Rv-4's in the US appear to use 1800 as the MGTOW. That is beyond what I would ever desire. As far as I can tell there were no additional airframe modifications.

"John Johanson told us about how he rebuilt his world spanning RV-4 to have a "wet wing" before his Antarctic adventure. The gross weight of his airplane at take off with 1000 liters of fuel (264 US gallons) was approximately 3000 pounds."

I do recall that Vans issued an airframe modification to add a larger aluminum bracket aft of the firewall to avoid buckling due to rough landings.

I have performed a little math on the Titanium landing gear and have not seen anything that would cause me to back off the project. From what I can determine I would only need to drill 4 holes to prepare a tube. Nothing too scary. Two where it meets the engine mount and two at the bottom to attach to an axel fitting that I will mill. Its either that or I just buy a set of long gear from vans. I wonder that they sell for and weigh. Changing the gear may not be worth the effort.
You can't just change the gear legs, the angles are not the same, therefor the engine mount is not the same, the long legs are also larger in diameter and wont fit the old mount anyway so you buy a new mount with the gear legs drilled for you buy Van's.
 
I realize this is an old thread but the challenge still exists. I just purchased a flying RV4 last week and after getting transition training with a 200+ # instructor in the back seat, I have a keen understanding of the aft cg characteristics. My thinking was this, move the battery to the firewall and reconfigure the former battery box area for covered storage. Since flying pax will normally be sightseeing or more importantly on a trip with baggage, I was thinking of making a weight(with appropriate security) to fit in the stock battery box area to further shift the cg forward. Normal use of this area would be to have somewhere to store stuff like tie downs etc. What do you guys think? By the way, the backseat rudder pedals from ACS worked great in my pax foot-well equipped -4 and was happy to have them during my transition now and aerobatics later.
 
Terry, take a look at the pictures of the smoke tank we built over in the smoking RV-4 thread. 33lbs full with an odyssey battery mounted on the firewall. We haven't done the W&B yet, but we are hoping this will help with the passenger cg problem.
 
RV-4 CG

My brother and I built and fly both an RV-7A and an RV-4. We anticipated the rear CG bias on the RV-4, so put the battery on the firewall (PC680), turned the old battery location into a luggage area with a hinged cover and added the optional under panel luggage box. We also made the two boxes right in front of the spar behind the front foot wells into small storage places for oil, tools and chocks. It has a 160 hp, O-320 E2D ( converted from 150 by STC) with a Senseniche carbon prop. We still had to add the Saber Engineering, 12 pound ring to the starter ring gear and returned to the 18 pound starter we had taken off. We now can fly someone a bit more than 200 pounds without problems. The plane is a tick over 1000 pounds empty. I also added a lightweight tail wheel and replaced the screen door springs with the lighter cables from Van's for a loss of 1 1/2 pounds which is the = of adding 6 pounds to the nose CG-wise.
 
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