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RV-4 Heavy elevator /stab trim

danweseman

Active Member
Hi Guys. I now have 5 flights on the -4 (see second first flight from last week) flown By myself and two other more experienced RV -4 Flyers. Both felt the elevator force is very heavy. It has no substantail static drag in the system and is balanced Per Van manuel. In cruise the elevator balance arm front tips were 3/4ths inch above H stab. It could trim all the way into stall with flaps but ran out of trim at 175 or so. it had no shim under the front H stab mount so I added a .0125 shim under the front mount and flew it again today. the forces seemed a little lighter(maybe?) ,and i could trim to over 200. I could also trim to under 70 mph easily on the bottom end with full flaps. the arms are now approx 1/2 inch above the H stab at 2700 rpm which was 189 or so indicated at 500ft. I was thinking of adding another shim to get the arms and trim tab aligned in cruise. the cg for all flights was 71.14 ,pretty far foward. Any thoughts on the heavy elevator and also on the balence arms position would be help full. Also any info on speeds/rpms from others with 0-360 and 85 inch pitch sensnich prop would help. Thanks Dan Weseman
 
Consider this a long shot, but there is a bellcrank back inside th tail cone that needs to be in a neutral position when the stick and elevator are neutral. This will give the maximum mechanical advantage at the stick. I think it would have to be pretty far out to throw things outta whack though.
 
Before doing too much more I'd load it up with some weight in the back. It will be a totally different airplane and you may want to tune it so it works in both situations.

For what it's worth... My airplane runs out of down trim at 180 IAS with a bubba in the back and low on fuel. 160 HP metal fixed prop. Mine always has small amount of down elevator deflection at cruise no matter how it's loaded.

Have fun and be safe.
 
How do the radii on the elevator trailing edges compare to the RVs that are reputed to have lighter stick forces? I suspect the stick forces will be higher if the radii are smaller than other RVs.
 
More info

Both Freinds have flown several -4s. One has a aft cg ,ive flown it from the back and and... with me in it was very light. The other has a 360 with Hartzell CS and has a more foward CG than me. The trailing edge radius on mine is on the small end of the plans specs. I didnt build this -4 or surfaces.I will compare tomorow. I checked the stick lengths and bellcranks and all are per plans I will verify the elevator horn length tomorow. Also the stick travel seemed the same for all 3 aircraft but will measure this also to verify geometry. Ive read of some increasing the TE radius some how, has any one done this and give me some pointers. squezzing them is easy. My last plane a sonex has light elevators and heavy ailerons. This has nice ailerons but.... a really really stiff elevator , a 3 g pull at 180 mph is very hard to do, even at VA its very stiff.I will come back with more info tomorow. Thanks Dan
 
more data

Hi Guys I couldnt help it ,i walked out to the hangar and checked the elevator horn lenght . it is 4.5 inches per plans.the reversing bellcrank in the baggae compartment cannot be installed upside down without a large hole in the lower skin. the total stick travel with 30 degrees up and 25 down is 10.25 inches. the stick clears the bottom of the per plans panel by 3/4ths inch.
the trailing edge radius per plans should be 3/32. mine look good and are basically flat right to the radius. the skins are .016 and are not perfect but have no major oilcans dips etc? I wouldnt mind increasing the stick travel 2 inches if the the stick interconnect push rod has enough clearence. but would rather figure this out and correct the real differance. I will compare to the examples around here and post back tomorw. Thanks for any thoughts Dan
 
Sounds like everything is good. I still think it's CG issue. Put someone in the back and see if the stick forces don't go down by more than half. if they do, you know it's probably just a CG thing. You may want to move the battery if that's the case. The -4 is much more sensitive to this than the -8.
 
A long shot.

Remove the rudder and then compare the trailing edges of the elevators against each other and make sure they are perfectly in line with each other. You can have a twist in the elevators to where they match at the leading edge but are off at the trailing edge which can substantively increase the stick force.
 
maybe better

Hi guys thanks for all the thoughts. i flew again today . i departed my airpark and flew 20 miles to get fuel. I meet up with two RV-freinds and flew formation home and did some tail chase. when i landed i put the plane away and realized i never noticed the stick forces on the way home. I walked out and looked at the G meter... 4 gs. I must be getting used to it? in retrospect it is still heavy at 180+mph , but around 140 (manuvering)it feels much better. The only thing i did was try and squezzed a few spots where the skin wasnt straight right to the TE bend. I did check the alighnment of the elevators to each other and could find no twist. clamped the elevators and used my digital level to to check then at various spots. almost exactly the same. I will try it with some one in back next. Im also going to add another shim under the stab. Any more thoughts would be great. Thanks Dan
 
Dan, my view is that this is how -4 are. In roll my -4 is only arguably stable. It is very quick as a result and feels great.

In pitch though I always find the stick forces very high solo, but put even a lightweight in the back and it becomes very light. I dont think from the sound of it you have a problem.

Will be interested to hear your next comment.
 
another Data point

Hi Guys. I flew my freind Chris Smiths new RV -4 Today (see post below) It flies Great. His control feel is exactlly what i expected out of mine(but dont have Grrrrr). His is lighter in roll and pitch. I will not be happy with mine until i figure out how to make it have stick forces like his! I read in the RVator about a guy doing hard copmetiton acro in his -4. he mentions some kind of tab on the elevator to lighten the forces.Any one have any more info on that? could i add several layers of tape to the TE of the ailerons and elevator to increase the radius to see if it helps? Any thoughts /ideas welcome. Also we will weigh his plane and redo CG and compare. But his is still lighter than mine even with a 175 lbs passenger? Thanks Dan
 
More info i found a few items to change...

Hi Guys. I went over my -4 again today with a critical eye.
I found two things that may help. First Stick length.
the plans show 20 inches with 4 from the pivot to the control rod hole,and 16 above. so a 4-1 ratio.
Mine has a fancy fighter style stick grip . to the top it is 15 inches . But it has a "hat" on the top to keep you hand in place. with my hand as high as possible my top finger is 14 inches above the pivot. The effective stick length is 14 inches!! thats 3.5-1. I will remove this stick and install my full length one from the HRII project and see how it feels
my ailerons have a mild stick bump with hard aileron use. Its only one way. I started measuring the fairing gap and hinge ratios on the left side. all was close to plans. on the right side i notice the gap totally closed with full up aileron??? i found the lower wing skin is not trimmed flush with the rear spar. it hangs over by almost 5/16! dont know if this would cause the stick bump or heavy ailerons but im gonna trim it per plans tomorow and find out!I will report back soon Dan
 
stick length.

Ok guy . i installed the stick out of my HRII project and a new stick grip . It is full factory lenth. It made a noticable difference in both pitch and roll feel. It reduced my total force "feel" by about 10%. Im going to try a few more things to get the rest of the way to "light feeling" control forces(approx another 10%. I also checked and my aileron "bump" is only to the right. The right side lower skin extends past the rear spar by 5/16ths a inch and with full aileron travel totally seals off the gap. I have no bump to the left. Im going out to trim the skin flush now and will report tonight.
 
No Bump!

Hi Guys I had some succes. I trimmed the rear lower wing skin and did a test flight. No Bump either way at 150 mph. a slow speeds i can force a "nibble" but its very minor and i have to really jam the stick over. I also Increased the TE radius on the elevator approx .020 by using a block of wood and tapping with a small hammer. It did lower the control preassure some. Its getting closer to what i want. Next i will increase the aileron radius and elevator radius a bit more. I will report back... Dan Weseman
 
Good info Dan. Can you be a little more specific on how you increased the TE radius on the elevators?
 
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