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Adjusting RV-7 CG

Brantel

Brantel

Well Known Member
I have a RV7 with a Metal FP Sensenich and an O360. The crank has lightening holes around the flange. Empty weight right now is 1066lbs.

I need to shift my CG fwd after paint.

Considering a Landoll CG adjuster ring which is 12lbs.

Also thinking about the prop spacer. The one I have is the standard Sensenich one. How much weight difference is there between the Sensenich one and the Sabre Steel one?

Is it a good idea to put that much mass out front?

Comments, opinions?

This is an annoying feature of the RV7!!!!
 
Prestolite starters are cheap and effective. They also add 7-8 pounds well forward.

I'm surprised you have a CG concern given the metal prop.
 
What is the weight diff between a metal Sensenich and a Hartz. CS/gov. ?

The airplane fly's fine, the numbers tell a tail of aft an CG and severly limit the baggage.
 
I think a Hartzell weighs in ~50#. The governor could not be more than a couple of pounds.

Its an expensive upgrade, but you will get rid of the aft CG problem. The take off performance increase is substantial.
 
hartzell=58#, governor=4#. plus there is the cable and governor line. not sure what your Sens weighs.

for comparison: a WW200RV weighs 38#
 
Something to give a thought to is fixed ballast. In aircraft manufacturing there is often fixed ballast such as lead or stainless steel plates fastened into the plane to help with CG issues on aircraft. On the side-by-side model you can mount this ballast between your feet with a few nutplates. Fix ballast is a cheaper alternative with the same results as installing a heavier component, which is all just extra weight.
 
I read that a FP metal Sensenich weighs around 41 lbs. This correct?

21 more for the Hartzell/gov would make a big diff...
 
Hey Brian,
How far back is your CG? Mine is 81.5 empty. I'd prefer it to be further forward, but I have yet to see a problem with it. The only limitation that I have is that with myself and a very heavy passenger (255 lbs), I will fall outside of the aft range with 1/2 baggage.

I suppose if I needed that extra utility, I will go down the route that you're going through. I've given this thought before and always come back to the 20 lb. Landol ring. Until then, I'm just going to live with it.
 
Saber make a steel 14 and 19 pound crush plate for $175, you may need longer bolts at and extra $96. Im looking into this option now as I also have an AFT C of G.
 
I'm using a Sabre 22 lbs crush plate with my Catto... two 205 lb'ers and the baggage area full to the top with no issues. Empty weight 1059. Last I talked with Mark Landoll he stated the harmonic dampeners were no longer in production, just the 12 lb inertia ring for flywheel mounting. Sabre is awesome to work with! :) Pick up a couple of bags of lead shot and play with some different weights.
 
Saber is offering me a heavier prop extension and a custom steel plate to add to the front of the fwd spinner bulkhead.

The cost is high for adding 18-19lbs (his estimate) to the front.

Not sure I like their way of bolting an extension to the crank with separate bolts. I see no way to safety those bolts.

Does anyone know how much more the Saber extension weighs over the standard aluminum Sensenich one?
 
Brantel said:
Saber is offering me a heavier prop extension and a custom steel plate to add to the front of the fwd spinner bulkhead.

The cost is high for adding 18-19lbs (his estimate) to the front.

Not sure I like their way of bolting an extension to the crank with separate bolts. I see no way to safety those bolts.

Does anyone know how much more the Saber extension weighs over the standard aluminum Sensenich one?
Click to expand...

The 2.5 inch extension is solid metal and with bolts, weigts in around 6.5 pounds. I'm putting a SJ cowl and needed to move the prop forward to accommodate the longer cowl needed for the forward facing induction.

As far a moving the CG forward, I'm looking forward to it since my 7A is a bit tail heavy when fuel is low.

The CS MT metal prop is also getting moved forward so that should help.
 
When I replaced my sensenich with the catto I weighed the sensenich prop and spacer and it was 42lbs. I have the landoll and the saber 14lb crush plate and I could use about 6lbs more on the nose to keep the cg in range with full baggage and min (5gal) fuel. The saber folks are good to work with. In my current config I have to limit baggage to about 90 lbs if I will be arriving with low fuel. I also moved sone of my travel gear forward of the flap rod to which helps.
 
I found the official weight specs on Sensenich's website and they say average that a 72FM8S9 prop and spacer weighs 40lbs. Not sure if those who have posted weights included the bolts or not. They would be good for at least a couple pounds I bet. I cannot get an answer on how much just the Sensenich spacer weighs.

Since it appears that the Saber extension is aluminum like the Sensenich one is, I don't think I will gain much weight from the spacer swap but as far as I can tell, to add their 2.2" x 6.0" steel crushplate to the front spinner bulkhead will require the use of their spacer since to do otherwise would require very long and expensive if not hard to get bolts.

Solidworks calculates a 2.2" x 6.0" carbon steel crush plate to weigh 17.53 lbs. A 2.2" x 6.5" weighs 20.57 lbs.

How does one safety the Saber extension to crank flange bolts?
 
Brantel said:
How does one safety the Saber extension to crank flange bolts?
Click to expand...
They're not safety tied. The bolt heads sit in recesses on the extension and are almost flush with the face of the extension. Once the spinner plate and prop are placed over the extension, there's no place for the bolts to go if they tried to back out. Then, of course, you safety wire the prop bolts normally. Saber does recommend putting some blue loctite on the extension bolts for good measure but, again, it's not really what is keeping those bolts in place. Is this an issue? I can't speak with any authority, so take this FWIW.
 
No issue that I am aware of. It just seems weird to not safety them. I see they offer an option to machine in safety wire ability for $80 more.

Anyone ever had to remove a stock Sensenich extension from an existing prop? Mine went on real tight originally...

alpinelakespilot2000 said:
They're not safety tied. The bolt heads sit in recesses on the extension and are almost flush with the face of the extension. Once the spinner plate and prop are placed over the extension, there's no place for the bolts to go if they tried to back out. Then, of course, you safety wire the prop bolts normally. Saber does recommend putting some blue loctite on the extension bolts for good measure but, again, it's not really what is keeping those bolts in place. Is this an issue? I can't speak with any authority, so take this FWIW.
Click to expand...
 
When I sold my sensenich prop I tried to remove the spacer to make it easier to ship (the spinner backplate is between the spacer and the prop). I tried with a 5lb hammer (which I used to install the spacer) and a large pin punch and made little progress except to make my ears ring from the tuning fork sound of the prop each time I hit the pin punch. Finally gave up.
I suspect a press might be the best approach.
 
Question for those in the know...

At what point does someone need to be concerned with how much mass he is hanging on the end of his crankshaft? I don't worry about adding up to the weight of a CS Hartzell (~58lbs) but beyond that worries me greatly.

Anyone got any knowledge here????
 
Good Question

Brantel said:
Question for those in the know...

At what point does someone need to be concerned with how much mass he is hanging on the end of his crankshaft? I don't worry about adding up to the weight of a CS Hartzell (~58lbs) but beyond that worries me greatly.

Anyone got any knowledge here????
Click to expand...

I also would like to know what the engine builders think on this.
 
moving engine forward on RV-7

It seems like many RV-7's come out a bit on the tail-heavy side, except for those using constant speed props.

Has anyone thought about putting spacers between the firewall and the engine mount to shift the engine forward an inch or so? Some thick stainless steeel spacers would be easy to make, and then just use longer engine mount bolts too.

You would probably have to do this during the build process, since the baffling will need to fit the cowl, and the cowl itself will be longer. There's plenty of excess material supplied with the cowls to lengthen the nose an inch. It would make the lines a bit sleeker too. I suppose this could be done to an existing airplane by scarfing on an inch of fresh glass on the back edge of the cowl. That would not be difficult. That way, the existing cowl preserves its location relative to the engine, and everything fits as before.

this would shift the landing gear forward an inch.....that might not be such a good thing. For the tricycle gear, it would be fine.
This would not be a problem if the spacers were placed between the engine mount and the engine.

Thoughts?
 
Engine mount for a O-320 is 2" longer

Hello Steve

The RV-6 had the same engine mount for the 320/360. All the 320 have problem with the CG.

On the RV-7, they made a separate engine mount for the 320, which is 2" longer. This to work out the aft CG problematic.

The gear is on both mounts on the same position, only the 4 engine mounting points are more forward.

Regards,

Dominik
 
0-320 engine mount

Thanks Dominik,

So someone building a light RV-7 with an O-360 but using a FP prop could just use the O-320 engine mount and thus shift the engine forward 2". Thats great news.

I think this should be better publicized so that builders can make the choice early on. But its a good long-term fix even for those flying, if you don't mind some fiberglass work to extend the cowl.

How does the c.g. come out for those RV-7's with O-320/FP? Does the 2" shift completely solve the problem for them, or are they still a bit on the tail-heavy side?
 
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Yep it stinks to build a simple, frugal and light airplane only to have issues with CG.....

I just heard from Saber that the max recommended spinning weight on these engines is 65 lbs. I would hope they know what they are talking about but people can decide for themselves.

I am going to weigh the airplane again after double checking the arms of the gear and tw and after checking the scale calibration.
 
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Just got word back from Sensenich on the weights of their standard spacer and bolts. This can be good info for someone that goes thru this in the future..

Bolts and Washers: 2 lbs 7 ounces
Spacer and pins: 4 lbs 4 ounces

They supply these bolts as standard for O-360's with the 1/2" bolts: AN8H-66AS 6.844" long with 5.300" grip
 
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Should shift CG ~2/3rds inch forward

scsmith said:
Thanks Dominik,

So someone building a light RV-7 with an O-360 but using a FP prop could just use the O-320 engine mount and thus shift the engine forward 2". Thats great news.

I think this should be better publicized so that builders can make the choice early on. But its a good long-term fix even for those flying, if you don't mind some fiberglass work to extend the cowl.

How does the c.g. come out for those RV-7's with O-320/FP? Does the 2" shift completely solve the problem for them, or are they still a bit on the tail-heavy side?
Click to expand...

The engine, prop and attached accessories represent ~ 1/3 the weight of an empty RV-7, therefore CG shift ought to be ~ 1/3 x 2" = 2/3" forward.

LarryT
 
scsmith said:
.....So someone building a light RV-7 with an O-360 but using a FP prop could just use the O-320 engine mount and thus shift the engine forward 2"..........
Click to expand...

Chad Jensen used the longer mount on his -7. I think it worked out great for him.
 
Added Saber crush plate, gained baggage

Before Oshkosh I added a steel Saber crush plate and longer bolts to my Catto 3-blade (21 pound increase in gross weight) and gained 18 pounds of baggage capacity. 1077 lbs empty weight, RV-7. YMMV
 
Spindrift said:
Before Oshkosh I added a steel Saber crush plate and longer bolts to my Catto 3-blade (21 pound increase in gross weight) and gained 18 pounds of baggage capacity. 1077 lbs empty weight, RV-7. YMMV
Click to expand...

Adding a steel crush plate won't increase your gross weight.

Do you mean that you added a 21 pound crush plate? If yes, then your empty weight would have increased by 21 pounds.

I did not run the numbers, but you should have ended up way more than an 18 pound increase to your baggage area.......................:eek:
 
Unless he hit max baggage weight

gasman said:
Adding a steel crush plate won't increase your gross weight.

Do you mean that you added a 21 pound crush plate? If yes, then your empty weight would have increased by 21 pounds.

I did not run the numbers, but you should have ended up way more than an 18 pound increase to your baggage area.......................:eek:
Click to expand...

If he was previously at 82 lbs., then he would have maxed out @ 100. You are certainly correct that the moment arm from empty CG to crush plate is much greater than moment arm from baggage compartment to CG.

Hopefully Chad J. will pipe in here. I know he had figured out that the 320 mount was the way to go with 360 & FP prop as the previous poster suggested. The only issue might be packaging if the 320 cowl would be too tight for the 1" wider (1/2" per side) difference between the 320 and 360.

LarryT
 
cjensen said:
'Nuther data point for you...

Right main 480#'s
Left main 492#'s
Tail 73.5#'s

Rounded up weight 1046 empty, cg 81.59"

Full Flightline interior, O-360, three blade Catto prop with AL 4" spacer AND the 2" longer O-320 engine mount.
Click to expand...

Here is Chad's posted numbers sans paint on most of the plane. He had a SJ cowl.
 
Numbers for RV-7 (A) with O-320

Hello Steve

I do not have the numbers handy from my buddy (RV-7/O-320/Sensenich FP).

Mine still need a year until I can fly it (RV-7A) and another buddy about a half year (RV-7A).

All that I know is, that the RV-6 and RV-6A with the same setup of Motor prop are a bit tail heavy when the fuel is flown off.

I hope that Vans really callculated those 2" and it is enough to bring it into a good range.

If you go this route, then you have to buy a RV-9 cowling and cut the scoop off and reglue a 360 scoop. But this only if you go for a vertical injection.

Regards,

Dominik
 
New data!

Well something was just not adding up right in the numbers I had after weighing my airplane after paint. I could not find many reports of a stock RV7 with a metal FP prop comming out as tail heavy as my plane did. Not only that but the people selling stuff to fix tail heavy RV's were shocked when I told them how much weight I needed on the nose to fix it.

I used the same set of scales borrowed from a friend that I used about 1.5 years ago to do the initial W&B. They are Intercomp SWII scales and are marketed as being very accurate.

I decided to contact the manufacturer and get the directions on how to do a user calibration on the scales and weigh the airplane again. I borrowed 550lbs of certified weights from my work and commenced to calibrate each one of the scales following the manufacturers directions. I used the entire 550 on each one to get the best accuracy knowing that the mains would have close to this on each one when weighing.

After performing the calibration, I went thru the process again of draining the fuel and getting everything ready to weigh the plane again. My friend Tim McGinnis helped me go thru this process again. It was about 100° in the hangar today and we both sweated gallons.

The end results are:

Left Main: 510lbs @ 68.45"
Right Main: 516lbs @ 68.45"
TW: 75 @ 249.25"
Total: 1101lbs
Empty CG: 80.77"

This is more normal for a plane setup and built like mine. I can now get almost full baggage at minimum fuel. Way better than before.

The Landoll Ring would now get me close to that magic 80" number...

I now suspect that my initial W&B was not as accurate as it could have been and now I have no real good way to know how much my paint job weighed.....

I feel much better with these numbers and feel much better about their accuracy since I did the calibration myself. I moved around 8800lbs worth of weight up down and all around in order to get em done. When the calibration was completed, I checked out each pad one at a time from 0 to 550 to check their accuracy and each one was +-1 all the way up. Yes I am sore today!



Just for the sake of comparisons:
My plane is a TU, Painted with base/clear, O-360 carbed, FP Metal Sensenich, one mag/one pmag, PP alternator, flyweight starter, PC680 battery on firewall, Dual Screen Legacy Dynon VFR (for now) panel, dual AP, AP74 & HS34, minimal interior painted with front carpet, most of the inside skins are lightly primed, Strobe PS, Dynon remote compass and ELT right behind baggage bulkhead.
 
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what is a light tail wheel?

i am working on a 7 that will be fp from this post i learned i need to think about getting the cg forward what can be done?
1. light in tail as possible, how, not much can be done?
2. get the o-320 mount.
3. the propeller, is the fp metal the best because of the weight?(why is the starter of this theard removing the metal prop?)
4. can the battery be moved.(heavy item)(any thing else to potental move?)
5. I don't want to add weight unless i get some benefit.and if the cg. is only with baggage could enough baggage be added to cabin floor (some type of tie down in floor , elsewere forward of mainspar?)
6. anything else i'm missing to think about?
 
Yep, good to think about this at that stage instead of later.

Suggestions for keeping the CG inline:

1. Don't prime the inside of any skins aft of the cabin.
2. Don't use a heavy conduit for the wires running to the tail
3. Not sure which is lighter in the tail (manual or electric trim) but go with the lightest option, I went electric the first go round, if the manual is lighter, use that.
4. Go with a light weight tail light
5. Go with a light weight tail wheel/fork
6. Go with single stage paint
7. Use minimum filler on the emp tips/don't fill the seams.
8. Don't put the strobe PS or ELT in the tail.
9. Don't put any antennas on/in the tail
10. If you go FP use a metal prop (I am not removing my metal prop)
11. Battery on the firewall, go with a larger one than stock if it will fit.
12. Use a heavy starter instead of a flyweight one
13. Use a heavy alternator (this may be hard to fit in the cowl)
14. Come up with some other way to mount and connect the AP Pitch servo to allow it to be mounted more fwd. (Careful with this one)
15. Don't finish out the baggage compartment with a fancy interior

I am sure there are more ideas out there....


bill v said:
i am working on a 7 that will be fp from this post i learned i need to think about getting the cg forward what can be done?
1. light in tail as possible, how, not much can be done?
2. get the o-320 mount.
3. the propeller, is the fp metal the best because of the weight?(why is the starter of this theard removing the metal prop?)
4. can the battery be moved.(heavy item)(any thing else to potental move?)
5. I don't want to add weight unless i get some benefit.and if the cg. is only with baggage could enough baggage be added to cabin floor (some type of tie down in floor , elsewere forward of mainspar?)
6. anything else i'm missing to think about?
Click to expand...
 
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