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Aft C.G. Issue is baffling me

toolmanmike

toolmanmike

Well Known Member
Friend
Weighted my plane today and yes, I have read the sticky thread on weight and balance and my issues just didn’t fit there so I thought I would add a new thread specifically for the RV-7.
First, I built a heavy plane. Primmed, painted, carpet, leather and full IFR with smoke. It’s just the way I want it and I do not want to change anything. BUT, IT HAS AN AFT C.G. from the start. And EVERYTHING you put in it- people, bags, smoke oil…. Are all aft CG! My plane with 2 adult passengers put me so far aft that I can only carry 25-30 lbs of bags. Forget making it into the aerobatic CG range. I heard some about this when I started but figured the big Lycoming with constant speed Hartzel would ballance me out. It did not.
There is absolutely NO WAY I can load the plane even close the forward CG.
The problem I have is that I can’t fix it. There isn’t anything I can take out of the empennage because there isn’t anything of substance back there. I can’t add anything to the prop or engine- I’m already at 1215lbs! Seems to me that the plane should have the wing attached 2” further back or a forward baggage compartment like the 8.
If I knew this ahead of time I may have looked into the forward engine mount, 3 blade prop, skipped the lithium battery, etc. however then I would have a 1250lb beast and would be up against the max gross.
I have run the numbers all sorts of ways and really don’t know what to do. I guess I’m just writing this for the next builder. THE TAILWHEEL 7 IS TAIL HEAVY. Plan accordingly.
 
My 7 came in at 1180 with a similar setup (including smoke), only I have a lighter prop. We're solidly good on CG and can carry the full 100 lbs of baggage. I'm curious how you got so far aft...
 
Was your hangar door closed when you weighed it, even a slight breeze can throw the weight off? Are you sure it was leveled properly? Just asking.
 
toolmanmike said:
Weighted my plane today and yes, I have read the sticky thread on weight and balance and my issues just didn’t fit there so I thought I would add a new thread specifically for the RV-7.
First, I built a heavy plane. Primmed, painted, carpet, leather and full IFR with smoke. It’s just the way I want it and I do not want to change anything. BUT, IT HAS AN AFT C.G. from the start. And EVERYTHING you put in it- people, bags, smoke oil…. Are all aft CG! My plane with 2 adult passengers put me so far aft that I can only carry 25-30 lbs of bags. Forget making it into the aerobatic CG range. I heard some about this when I started but figured the big Lycoming with constant speed Hartzel would ballance me out. It did not.
There is absolutely NO WAY I can load the plane even close the forward CG.
The problem I have is that I can’t fix it. There isn’t anything I can take out of the empennage because there isn’t anything of substance back there. I can’t add anything to the prop or engine- I’m already at 1215lbs! Seems to me that the plane should have the wing attached 2” further back or a forward baggage compartment like the 8.
If I knew this ahead of time I may have looked into the forward engine mount, 3 blade prop, skipped the lithium battery, etc. however then I would have a 1250lb beast and would be up against the max gross.
I have run the numbers all sorts of ways and really don’t know what to do. I guess I’m just writing this for the next builder. THE TAILWHEEL 7 IS TAIL HEAVY. Plan accordingly.
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You are sounding like you think every RV-7 ever built has a tail heavy issue.
That is not the case.
I suggest you start with listing some details of the process you used to weigh it, and evaluate the accuracy of the scales that you used.
 
Titanium Tailspring and lightweight tailwheel. Run some numbers for ballast at the very front of the engine.
Was the airplane perfectly level for weighting? Type of scales. Did you verify that all scales read the same with a test weight. Did you verify moment arms for mains and tailwheel with the airplane leveled As a last resort I would put some ballast at front of engine, whatever it takes and live with it.
With the round main gear the gear will move aft slightly with the heavier empty weight. Creating the moment arms for your airplane will compensate for any aft movement of the gear.
 
My RV7 is definitely not tail heavy, but then again I don't have paint or interior yet. I have a big engine, but compensated by going with a whirlwind prop and earthX battery so I'd imagine the weight distribution is similar to a 180 hp with a Hartzell prop.

I'm currently at 1118 lbs with an EWCG of 79.1.
 
Unpainted RV-7, completed interior. I wight 170. I divided the CG range into fwd, mid and aft. I can not get fwd. full fuel, just me best I can get is mid range CG. My battery is up front on the firewall and I have a metal hartzell 2 bladed prop.
 
Thank you all for the replies. I weighed it twice. Got the same results. Did it by the book but I’m going to do it again. I have yet to hear of an RV7 that is FORWARD CG. I did not check the scales against each other first. That is a good suggestion. Also- I’m taking an old fashioned bubble level to the hanger tomorrow. I don’t trust the digital one I have used.
 
Just for reference purposes, my RV7 numbers:

LF 556 lbs
RF 547 lbs
Tail 63 lbs
Empty 1166 lbs
EWCG 78.22”

Extended hub Hartzell 2 blade
IO-390
Earth X
Full leather interior
Completely painted
Dual coms

IMG_1675.JPG
 
toolmanmike said:
Thank you all for the replies. I weighed it twice. Got the same results. Did it by the book but I’m going to do it again. I have yet to hear of an RV7 that is FORWARD CG. I did not check the scales against each other first. That is a good suggestion. Also- I’m taking an old fashioned bubble level to the hanger tomorrow. I don’t trust the digital one I have used.
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No offense intended, but doing something twice the same way when the first time was wrong results in the same wrong result the second time…
Your tail wheel weight does look a bit on the high side.
Hopefully, with some very precise leveling checks, you can come up with a lower number for that
I have done inspections on a lot of RVs that had errors in the final result not because of weighing issues, but because of measuring and math calculation issues.
Besides re-weighing it again I would recommend that you once again very carefully go through the process of establishing what the arm length for the main wheel and tail wheel positions are.
Just in case you didn’t, all of these measurements should be taken while the airplane is in level flight attitude… The same as when the airplane is on the scales.
I prefer to do all the measuring with the main wheels on the hanger floor because the scale pads get away and can affect accuracy. Then I put it up on the scale pads, re-level and get the weight values.
 
I got out my CX-3 and used my -7 data for wheel distances aft of datum and came up with 1215/82.14 weight/CG. My -7 was fully primed and built as day VFR O-360 FP PROP @ 1059/81.25.

The -7 was designed to have the CS prop out front. Without all the CS prop out front my CG moves back a couple inches. If you look at the pictogram of the -7, the aircraft CG sits right at the panel. Which means the full IFR panel is basically irrelevant to the CG calculation.

I think you will need to revisit the items like carpet, leather and smoke aft of the CG if you’d like to move the CG forward. Just comparing my numbers to yours. You have an additional 130ish pounds in your interior aft of the CG.
 
rockyfatcat said:
I got out my CX-3 and used my -7 data for wheel distances aft of datum and came up with 1215/82.14 weight/CG. My -7 was fully primed and built as day VFR O-360 FP PROP @ 1059/81.25.

The -7 was designed to have the CS prop out front. Without all the CS prop out front my CG moves back a couple inches. If you look at the pictogram of the -7, the aircraft CG sits right at the panel. Which means the full IFR panel is basically irrelevant to the CG calculation.

I think you will need to revisit the items like carpet, leather and smoke aft of the CG if you’d like to move the CG forward. Just comparing my numbers to yours. You have an additional 130ish pounds in your interior aft of the CG.
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Find some known 500 lb weights and check the scales for accuracy. Also, I've seen some RVs jacked up, and the scales slid under the wheels, which were then jacked down. But the wheel was pushing outward on the scale, and the bearings didn't handle it well. (although IIRC the scale read low.). Another question, were your fuel tanks really empty?
 
Kyle Boatright said:
What CG stations did you use? It would be good info so someone can check your math.
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Kyle beat me to it.

Show us the station numbers. There may be something wrong with the math. Hard to help without the info Scott asked for in post #6. Would be nice to see all the data and math.
 
My 7 MT weight was 1087lbs. Fully primed and painted, full Flightline interior, Odyssey battery on the firewall, O-360 PV with a FP Sensenich prop and a heavy Prestilote starter. No way to load it outside the CG and stay within the 1800lb gross weight. Easy to build the 7 tail heavy as many posts on the Forum mentioned so I did my best to keep the weight forward.

Your tail weight seems very high. A titanium tail spring, light weight TW, fork and chains could take a few pounds off but it would still be heavy.

As mentioned, station numbers would be helpful.
 
Here’s a my numbers if it helps.
IO-360 parallel valve, whirlwind 200RV prop with a 2.25” saber extension. Full paint, oversized tires. Classic Aero seats and front carpet.

right main 511#
left main 514#
tail 70#

total 1095# c.g=80.2" not too bad for a completed RV-7. Also, a pic on the scales. IMG_1324.jpeg
 
Thanks again to all who have given data and constructive criticism is welcome here. Main wheels were both 68.5} and tail came out to 250”. My math is correct. I may have an issue with level. I will follow up after I re-due tomorrow. I am praying that I did something wrong the first two time. Wouldn’t be my first mistake.
And to my recollection, until this thread I have never seen an Rv-7 with a cg less then 79 and change. I stand corrected.
 
I agree your tail weight sounds heavy... I'd at least stand on that scale and see if the number is particularly insulting! In lou of a callibration, I ran your provided numbers through a spreadsheet I built.

Like you said, your airplane is how YOU want it, so what does that leave you for utility....

I included several example loading scenarios. These weights are for your airplane, and the wheel stations are what you reported.

First: Empty Aircraft
Existing AC.jpg


Now, lets load it up...

Existing AC Load Examples.jpg

Not knowing what you or a partner might weigh, I threw the ol' FAA standard's in there. You'd still have 53lb of baggage, which isn't bad for an airplane carrying an extra 100 lb of equipment... For the solo aero, I eye balled a parachute and helmet, and figured you might have a backpack or tool kit strapped down in the rear. For dual aero... it's more of a "take the grandkid for a loop" kind of loading.... Interpolate how you wish for these.

Now, for the sake of "what can you do". Lets say you didn't already have a lightweight tailwheel (about 1lb savings) and you shed another 2 lb with some lightweight tailwheel components *cough cough* for a total of 3lb reduced tail weight.

Modified AC Load Examples.jpg

That'll set you up for an extra 12 pounds of baggage! And probably a few more years of flying aero with the grandkids...


**EDIT** After posting I saw your actual main/tw arm lengths. Updated the figures to include your stations/arm lengths.
 
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As others wrote, I think there is something wrong with your tail weight number. 92lbs seems way to high. did you put a level on the canopy deck (longeron)?, it must be level. A little bit tail down will drastically affect your tail weight reading.
 
For another data point I checked my -7 original numbers out of curiosity, in my original case Sensenich FP metal prop (44Lbs), no paint but full interior, the numbers were somewhat similar except for the tail which in my original configuration was 71Lbs so a 21Lb difference, seems a lot as there is not much extra you can put back there. At that time I had the stock Vans stinger and tail wheel assembly.
Figs
 
toolmanmike said:
Weighted my plane today and yes, I have read the sticky thread on weight and balance and my issues just didn’t fit there so I thought I would add a new thread specifically for the RV-7.
First, I built a heavy plane. Primmed, painted, carpet, leather and full IFR with smoke. It’s just the way I want it and I do not want to change anything. BUT, IT HAS AN AFT C.G. from the start. And EVERYTHING you put in it- people, bags, smoke oil…. Are all aft CG! My plane with 2 adult passengers put me so far aft that I can only carry 25-30 lbs of bags. Forget making it into the aerobatic CG range. I heard some about this when I started but figured the big Lycoming with constant speed Hartzel would ballance me out. It did not.
There is absolutely NO WAY I can load the plane even close the forward CG.
The problem I have is that I can’t fix it. There isn’t anything I can take out of the empennage because there isn’t anything of substance back there. I can’t add anything to the prop or engine- I’m already at 1215lbs! Seems to me that the plane should have the wing attached 2” further back or a forward baggage compartment like the 8.
If I knew this ahead of time I may have looked into the forward engine mount, 3 blade prop, skipped the lithium battery, etc. however then I would have a 1250lb beast and would be up against the max gross.
I have run the numbers all sorts of ways and really don’t know what to do. I guess I’m just writing this for the next builder. THE TAILWHEEL 7 IS TAIL HEAVY. Plan accordingly.
Click to expand...
well my -7 EW is 1065 and 75.77"....painted , 180hp Hartzell prop, light interior ....
 
Re-weighed the plane today. It is still heavy and still aft cg but not as bad as I originally found. I paid particular attention to the level attitude and that got the numbers in the ballpark a bit more.

Main-567
Main-561
Tail-87
1216 total. Cg is 81.59. Wish it was 80 and I wouldn’t have the aft issues but there really isn’t anything in the tail to move. Thanks for the input.
 
toolmanmike said:
Re-weighed the plane today. It is still heavy and still aft cg but not as bad as I originally found. I paid particular attention to the level attitude and that got the numbers in the ballpark a bit more.

Main-567
Main-561
Tail-87
1216 total. Cg is 81.59. Wish it was 80 and I wouldn’t have the aft issues but there really isn’t anything in the tail to move. Thanks for the input.
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I plugged your latest numbers into my CX3 and came up with 81.39. [email protected] and [email protected]. These data points are off my -7 w&b sheet. Which isn’t nearly as bad as where you were at.

The next question is what starter and alternator are you using? If you are using a lightweight unit, there is room to swap it out for a HEAVY unit and shift things forward. The other option is to bolt some weight to the motor. An additional 20 pounds up front could shift your CG forward by 2 inches.
 
Something still doesn’t seem right. I saw a spreadsheet on VAF with the measured weights for dozens of RV8’s. Except for a few outliers they all were within +/- 30 pounds. Just a cursory look at some of the -7 data shows the same thing suggesting yours could be maybe 100 pounds over the average. Any idea why?
 
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rockyfatcat said:
I plugged your latest numbers into my CX3 and came up with 81.39. [email protected] and [email protected]. These data points are off my -7 w&b sheet. Which isn’t nearly as bad as where you were at.

The next question is what starter and alternator are you using? If you are using a lightweight unit, there is room to swap it out for a HEAVY unit and shift things forward. The other option is to bolt some weight to the motor. An additional 20 pounds up front could shift your CG forward by 2 inches.
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On an airplane that is already about 100 lb over nominal... bolting dead weight to the front is the last option I'd consider. Hey most even try to avoid dead weight on a lightweight airplane!

I'm skeptical of the scales accuracy. Between your two measurements, you have 5lbs that shifted from the tail, to only one side of the main gear.... That's not what I'd expect from just raising/lowering the tail. I know it's a pain, and you must be eager to move on, but what happens if you reposition the scales to different spots... Same numbers?

I know of a per-plans RV-14A that was weighed recently and deemed unflyable with full fuel and one passenger (as a result of heavy nosewheel weight). Knowing something was in error, the airplane was reweighed on another set of scales which totaled to the same weight, but was distributed much differently. Further investigation showed the original set of scales were totally out of whack. The surprising thing is they summed correctly, they just didn't distribute properly.
 
toolmanmike said:
Weighted my plane today and yes, I have read the sticky thread on weight and balance and my issues just didn’t fit there so I thought I would add a new thread specifically for the RV-7.
First, I built a heavy plane. Primmed, painted, carpet, leather and full IFR with smoke. It’s just the way I want it and I do not want to change anything. BUT, IT HAS AN AFT C.G. from the start. And EVERYTHING you put in it- people, bags, smoke oil…. Are all aft CG! My plane with 2 adult passengers put me so far aft that I can only carry 25-30 lbs of bags. Forget making it into the aerobatic CG range. I heard some about this when I started but figured the big Lycoming with constant speed Hartzel would ballance me out. It did not.
There is absolutely NO WAY I can load the plane even close the forward CG.
The problem I have is that I can’t fix it. There isn’t anything I can take out of the empennage because there isn’t anything of substance back there. I can’t add anything to the prop or engine- I’m already at 1215lbs! Seems to me that the plane should have the wing attached 2” further back or a forward baggage compartment like the 8.
If I knew this ahead of time I may have looked into the forward engine mount, 3 blade prop, skipped the lithium battery, etc. however then I would have a 1250lb beast and would be up against the max gross.
I have run the numbers all sorts of ways and really don’t know what to do. I guess I’m just writing this for the next builder. THE TAILWHEEL 7 IS TAIL HEAVY. Plan accordingly.
Click to expand...
I'm guessing paint made it tail heavy. A good friend did his own paint job, beautiful, and had a heavy aft situation.
He ended up putting a heavy flywheel type weight on his spinner back plate
 
How much are you willing to redo to fix this? A sam James long cowl with a 2.5" extension would move the CG forward about 0.16" without increasing weight. A lightweight tail wheel and fork will save you ~1.5 lbs directly on the tailwheel, giving you another 0.2". A custom turned titanium tail spring will save you another pound and get you another 0.12". Those 3 things get you 0.5" with no added weight.

Is your smoke tank behind your seats? My empty smoke system tank with pump weighs 12 lbs. Moving that 24 inches forward from behind the seats to behind the firewall gets you another 0.24" of CG empty and even more when full.

Did you go with leather interior or the fake leather? According to classic aero, a full synthetic interior saves you 14.5 lbs over the real leather.

Going from an aft mounted prop governor to a fwd mounted one would get you another 0.1".

If your ELT is behind your luggage bulkhead, consider installing under the cargo floor or a seat. Would get you another 0.1".

If your AHRS is in the tailcone, mount it on the sub panel for another 0.1-0.2".

Who painted your plane? How many gallons did you go through? Did you weigh it before/after paint? Ours is a heavy paint job, but done by a professional and we gained 73 pounds between paint and interior and the cg moved aft 0.6". interior is 25 lbs which leaves 48 pounds for paint, most of it behind the CG. A heavy handed job might cost you double that in weight and pull the cg back over an inch. Suggest buying a cheap paint thickness gauge and getting an average thickness reading around the plane. Compare that to the recommend thickness for whatever paint you used and see if it was sprayed on too thick.
 
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agent4573 said:
How much are you willing to redo to fix this? A sam James long cowl with a 2.5" extension would move the CG forward about 0.16" without increasing weight. A lightweight tail wheel and fork will save you ~1.5 lbs directly on the tailwheel, giving you another 0.2". A custom turned titanium tail spring will save you another pound and get you another 0.12". Those 3 things get you 0.5" with no added weight.

Is your smoke tank behind your seats? My empty smoke system tank with pump weighs 12 lbs. Moving that 24 inches forward from behind the seats to behind the firewall gets you another 0.24" of CG empty and even more when full.

Did you go with leather interior or the fake leather? According to classic aero, a full synthetic interior saves you 14.5 lbs over the real leather.

Going from an aft mounted prop governor to a fwd mounted one would get you another 0.1".

If your ELT is behind your luggage bulkhead, consider installing under the cargo floor or a seat. Would get you another 0.1".

If your AHRS is in the tailcone, mount it on the sub panel for another 0.1-0.2".

Who painted your plane? How many gallons did you go through? Did you weigh it before/after paint? Ours is a heavy paint job, but done by a professional and we gained 73 pounds between paint and interior and the cg moved aft 0.6". interior is 25 lbs which leaves 48 pounds for paint, most of it behind the CG. A heavy handed job might cost you double that in weight and pull the cg back over an inch. Suggest buying a cheap paint thickness gauge and getting an average thickness reading around the plane. Compare that to the recommend thickness for whatever paint you used and see if it was sprayed on too thick.
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Excellent post.
 
All these ideas are good and very doable on a build that is NOT FINISHED. This is the reason for this thread. I’m not about to spend thousands of dollars and months of build time on my completed airplane just to move the CG by 1” to 1.5” even though that would make it much more user friendly. The plane is very usable where it is, just not as I expected. Let this be a lesson to all the 7 builders out there to think ahead about where they put things and what effect it will have on the end result.
I knew I didn’t want a stripped down rocket of an airplane. My wife and I wanted the heated seats, leather interior, etc. And we are not big people so it still works for us. But, I was concerned more about the heavy engine and prop being a CG issue for my 7 due to previous posts then a title extra paint on the tail.
Not blaming- just saying for those of you still in the build process.

Just because vans says 100lbs is your max baggage weight doesn’t mean you will be able to carry 100lbs in you “fat” Rv.

And I know, every plane is different.

I’m going back to finishing now. Merry Christmas all.
 
Step on all three scales for a quick accuracy check.

Also, where’s the smoke tank?
 
You say fully primed and painted…both inside and outside…? Paint can add a lot of weight and this is really sounding like your only variable behind the c of g from a lot of other aircraft. I did extensive testing of paint weight and thickness before priming and deciding on a non chromate interior primer. Rattle cans went on around .15 to .2 thou and weighed nearly nothing ….mere ounces and my primer including primers like akzo are put on around a dry film finish of .6 to 1.2 thou. Most of mine around .9 thou. Still only ounces but more than the wash primer vans uses or the rattle can stuff. This all depends on how it’s put on. But I’d fail to see that paint could make 100 pounds of difference to your build behind the CG! So for that reason you would need to look carefully again at 1. The calibration of the scales and in fact try different scales….moment arms….and flight attitude when measuring….lastly math. I know you have done this over and over but something is missing here….either your paint is super super thick and heavy or you have something placed behind the c of g but the numbers don’t equate to a lot of 7’s as noted in this thread. Granted they are an aft c of g aircraft anyway compared to an 8 with both forward and aft baggage holds…..so building needs to be thoughtful.
 
toolmanmike said:
All these ideas are good and very doable on a build that is NOT FINISHED. This is the reason for this thread. I’m not about to spend thousands of dollars and months of build time on my completed airplane just to move the CG by 1” to 1.5” even though that would make it much more user friendly. The plane is very usable where it is, just not as I expected. Let this be a lesson to all the 7 builders out there to think ahead about where they put things and what effect it will have on the end result.
I knew I didn’t want a stripped down rocket of an airplane. My wife and I wanted the heated seats, leather interior, etc. And we are not big people so it still works for us. But, I was concerned more about the heavy engine and prop being a CG issue for my 7 due to previous posts then a title extra paint on the tail.
Not blaming- just saying for those of you still in the build process.

Just because vans says 100lbs is your max baggage weight doesn’t mean you will be able to carry 100lbs in you “fat” Rv.

And I know, every plane is different.

I’m going back to finishing now. Merry Christmas all.
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I’m sure you are annoyed by all the silly “fixes”. Your weight is what it is. The only thing you can possibly fix is an inaccurate W&B measurement.
At least share some pictures of the two seat interior you have that weighs more than an IKEA couch.
 
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