RV7-A Weight and Balance

I did a search here to find this specific info but didn't find it, so forgive me if this has already been covered.
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I seem to recall several comments from different people about how the RV7-A can easily end up without enough weight up front for proper CG distribution at all loadings, even with a CS prop.

The configuration I have planned is an IO-360 w/FI and a Catto 3-blade FP prop. That's going to be considerably lighter at the nose than most CS installations, so I'm wondering if those who have gone before me can offer any comments on how their RV7-A's balanced out.

The problem is that even if the aircraft is built light, it's very possible that if there isn't enough weight up front, you won't be able to carry two souls plus the "rated" 100 pounds of baggage and full fuel without ending up out of CG when the fuel burns off.

What I'm pondering is if I need to plan ahead to install everything I can in front of the CG such as fire extinguisher, and even add a second full size battery just to get some weight up there to accomplish good CG balance at all loadings and fuel burn. Who knows; maybe even install a 20 lead weight at the inside bottom of the firewall if need be.

All comments welcome.

With an IO360, my guess is that you'll be fine on CG, even with the wood prop. If your CG does result in some baggage limitations, I wouldn't add built in weight, because you'll suffer that penalty on every flight.

Instead, consider alternative loading schemes:

With my wood prop 0-320 RV-6, I've found that I can put 5-10 pounds of stuff in the footwell against the spar on each side if CG is likely to be an issue. Also, I'm not hesitant to pack one easy to reach bag and have a passenger carry it in his/her lap for landing.

Using these methods can move quite a bit of weight forward 2 or 3 feet, which will have a real impact on the empty fuel CG...

KB

He might have the lighter 180hp IO-360. like I have and am wondering the same thing.

Dan C's Website has a listing of many AC with their configurations and W&Bs, including my 7A with 0-360 and Hartzell prop (N2447A). I'm sure you could get an idea how certain cofigurations will balance out. Mine is good for just about any loading you could imagine and be realistic.

Roberta

robertahegy said:

Dan C's Website has a listing of many AC with their configurations and W&Bs, including my 7A with 0-360 and Hartzell prop (N2447A). I'm sure you could get an idea how certain cofigurations will balance out. Mine is good for just about any loading you could imagine and be realistic.

Roberta

Click to expand...

Thanks Roberta, but from what I've seen on all the 7's and 7-A's, my "fears" are confirmed.
It looks like most of the CS aluminum prop airplanes have a CG less than 80" from Datum while most of the FP aircraft are over 80" rearward of the Datum.

And THAT's the problem for me more than gross weight. When I plug in typical full fuel, two (chunky) souls and 100lbs baggage, the CS planes with their heavy aluminum props plus governors come in fine while the lighter FP propped aircraft go beyond aft CG.
That may not be a problem for short hop scenarios if you want to leave some fuel behind, but my primary purpose for building the 7A is for long cross country.

The weight and balance computations from those on Dan's site doesn't change my plans for my chosen engine and prop, but it sure makes me start thinking that I need to install everything possible as far forward as possible.

Now if Vans would only offer an engine mount for the '7 that's just one inch longer and with a cowl to fit...
I would accept a (slightly) heavier aircraft if that's what it took to be able to fly with the 7's "rated" full loading and keep it all within the CG. I'm even wondering if a nicely machined and balanced "ultra heavy" crush plate (say 10 pounds) for my Catto prop might tip the CG scales in the right direction. A prop weight might even give a smoother idle although I have no idea what it might do to resonance issues at higher rpms.

The bottom line, though, is that based on the examples on Dan C's site, my concern does appear to be valid for those of us who are wanting to fly with a standard '360 and wood or composite FP prop in front of our RV7(A)'s.

I keep thinking the answer is simple and in front of my face but I just don't see it yet. But I'll keep looking...

W & B

Flame me if you like..... where I fly it gets up to 35 - 40 deg C in summer, so why not see if you could make an air-conditioned RV. You may loose a few HP but W & B will be good, and you will be cool on the ramp in summer.. just my 2cents.

jamie

Empty CG's more than 80" can limit RV-7/7A loading

Highflight said:

The configuration I have planned is an IO-360 w/FI and a Catto 3-blade FP prop.

Click to expand...

WHY A CATTO? Really give the Sensenich 2-blade Aluminum FP another real good look. It is faster (2-blades are faster than 3-blades, metal blades are thinner and more aerodynamically efficient), maintenance is less with metal and the Sensenich is tested and made for the RV. Many people have had erosion guards come off, Catto, MT, Whirlwind, and Aero Composite. Metal is much more "hearty". Also price is the same. You will have more issues with a wood/fiberglass prop. A metal prop is a bolt it on and forget it deal. I guarantee that a composite prop will need more attention and repair. A metal prop can be worked on by any prop shop. A Catto has to go to Catto.

You have a metal airplane for a reason, it is better. Why do you want a glue and string (composite) prop. I know the Catto uses wood, but when you are talking about a natural material like wood (God's composite) you always have the chance of quality issues. I am sure Catto makes a fine prop, but there has been at least one structural failure that I know of.

To double-check you numbers, you are right 80" is the magic empty CG. From Dan C's W&B data base, a sample of high, low and middle of the road CG's.

Engine HP Prop Make Blades Mat'l Type EmptyWt
IO-360 180 72" Hartzell 2-blade Aluminum CS 78.69" 1026<(3)
IO-360 200 72" Hartzell 2-blade Aluminum CS 78.8" 1109.4
IO-360 180 74" Hartzell 2-blade Aluminum CS 80.11" 1071.1<(2)
IO-360 200 72" MT 3-blade Composite CS 80.54" 1142
O-360 180 72" Sench 2-blade Aluminum FP 81.88" 1131
O-360 180 68" Other 2-blade Wood FP 83.14" 1058<(1)

Arms
Fuel 80.00"
Pilot/Pax 97.48"
Bags 126.78"

Weights and Limits (RV-8)
Gross Weight 1800 Lbs
Aerobatic Gross 1600 Lbs
Forward CG 78.80 inches
Aft CG 86.82 inches
Aerobatic aft CG 84.50 inches

(light fuel, heavy pilot/pax weight and any baggage shifts CG aft)

EXAMPLES
Take the worst case RV above 83.14" / 1058 lb<(1)
Assume 2x 190lb pilot/pax, 6 gal fuel, no bags
Total 86.71" / 1454 lb (inside aft normal CG limit, .11")
(To meet Aerobatic aft CG limit, must be solo and have 35gal fuel min)
Note: The 83" is a killer aft empty CG, no BAGS!

Take a typical RV above: 80.11" / 1071.1<(2)
Assume 2 x 180 lb pilot/pax, 6 gal fuel, 90 lb bags
Total 86.82 " / 1557.08 lb (at aft CG limit)
Note: Slight limit of baggage; without baggage you can make Arco CG with two +180 pounders

Take the best case CG RV above 78.69" / 1026<(3)
Assume 2 x 215 lb pilot/pax, 6 gal, 100 lb bags
Total 86.81" / 1592 lb (over 200 below gross, and .01" inside aft cg)
With the above but no bags (Total 84.13"/1492 lb) within Acro cg / wt
Note: empty CG of 80" or less (fwd) will give good W&B loading

So you are right, light prop, light engine (O-320) could be a problem. However with a typical O-360/180hp and metal prop (fixed or constant speed) the empty CG should be around 80" range.

The Median of the 26 RV-7/7A's in Dan's list was 79.56", which would produce good payload W&B. The only one that was well aft of 80" was the wood FP prop.

If using a lighter fixed pitch prop give consideration of equipment and heavy paint, which can shift CG further aft. Also use of heavy starter may help. A Sensenich will almost assure you will have no problem. I know it is not sexy like three blades but the Sensenich is a better prop in every way. I really feel strongly that you are passing up an obvious prop choice. Van tested the Sencenich prop and it was one of the fastest, just as fast as some CS props. Forget Van making a special engine mount and cowl just for people using a Catto or wood FP prop, it will never happen. The CG problem can be solved with 10-15 lbs of lead. G

You're taking a slight step off topic with your comments about composite props, but your comments about them (in general) are well taken and should be considered by those who have yet to decide.

However, you can see for yourself that even aluminum FP props usually push the CG aft of 80" so the issue isn't specific to composite props, but has more to do with the the difference in weight between CS and FP props and the additional weight of CS props due to a governor and supporting hardware.

There are many of us who just can't afford their way into CS props. And we don't have to since overall performance is close (in cruise) between them. Plus, if I can still climb twice as fast as a comparable GA aircraft with a FP prop on my '7A, I'm quite happy.

So even if I went with an aluminum FP prop, I'd still be dealing with the issue.

What it looks like to me is that anyone who is going to build a '7 and use a 180hp/360 w/FP prop needs to do more than pay attention to keeping the aircraft light in general; you need to carefully consider every gram of weight you intend to put behind the cockpit and ask yourself first if it is possible AT ALL to mount that item forward.

ELT's, fire extinguishers, strobe power supplies and magnetometer sensors are typical of things mounted in back; sometimes WAY back. Yes, you have to also consider potential electronic interference (e.g., from the strobe power supply), but if you can put it up front and not affect electronics, do it.

For me personally, I've already decided to use a two battery system because of my all-electric panel. So that second battery may add weight, but it will be a full size battery on the firewall which should be a good start toward pulling the CG in front of the magical 80" mark.
I also like your point about using a heavier starter rather than one of the superlight starters. That's a really good place to add another pound of much needed ballast in the right place.

Vern,

This CG thing is one of the reasons I am going with a big engine and a constant speed prop. Actually, one of the reasons I decieded on the RV-7 was that it worked favorably with a big engine and prop.

Go big! Dan also cites better economy too!

CJ

Heavy Up Front

I second Vern's statements. Install every thing you can forward, and keep the drive train heavy. It's real easy to load an RV way aft, but near impossible to load it forward, so kept the empty c.g. forward.

Our -7 came in at 79.3 with a O-360/Hartzell. Cross-countries involve heavy loads in the baggage area and I can tell you that hours spent balancing a ball on the tip of a cone requires strict attention. At max aft loadings, moving your torso, reaching, or even inadvertant pressure on the stick when distracted by some minor task will send you soaring or plummeting if not instantly corrected. I would characterize aft c.g. pitch stability as divergent because, in testing, the nose was still going up past 45 degrees before I decided enough was enough and ceased waiting for the airplane to pitch down. If it had, I'm sure it would have blown right through red line. The plane is not trying to get away from you when trimmed, but you must actively pilot it; about any bobble will set if off. This aft c.g., is, by the way, within the envelope.

So the current -7 project is going to have the ELT and strobe supply in front of the spar. Screw preserving the ELT in a crash; I won't be around to care if I crash that hard. The strobe power supply is not an EMI problem if you use proper ground techniques. I run the carburetored 360 20 degrees lean of peak, it's smooth enough, and reliably get 7.2 gph @ 195 true between 11 and 13,500. That's great mpg in my book, and overwhelms any cost reductions a 320 might offer at the expense of aft c.g. bias (and less power available to haul that load off).

Priming the tail feathers with Akzo-Nobel epoxy adds two pounds way back there. I weighed the skins before and after priming and made a calculated adjustment for the spars/ribs surface area.

Considering all the RVs can do, even as haulers, I think they're remarkable aircraft.

John Siebold
Boise, ID

gmcjetpilot said:

A metal prop is a bolt it on and forget it deal.

Click to expand...

I don't think so.

(A bit off topic but it is a good story and one we should all think about.)

Here is a good example of what can happen if 17" of a propeller decides it doesn't like to fly along with the rest of the prop. In this case the prop was an all aluminum Curtis Reed fixed pitch prop. The picture was taking in 1995. The plane is a Waco UPF-7 piloted by Ed Delrosso. Upon losing the engine he dove for the parking lot, bounced it off the berm in the background (note the weeds hanging from the right wheel), landed, and swerved between the light polls without damaging the plane. The only thing holding the engine on the airframe was the batter cable and tack drive. About 15 minutes after this picture was taken the engine fell to the ground.

The airplane was flying the next summer with a new engine, engine mount, cowlings, and wood prop. Ed's comment about the flight, "I felt s a small vibration, pulled the throttle back, looked up and realized the engine was gone, it happened that quickly. The Waco was a little draggy with the engine hanging down there like that." or something like that.

The picture was taken by one of the NYC news agencies. I can't remember which one and the picture I have is a proof print (which explains the poor quality) from the paper but it isn't labeled as to who took it.

For the record, I too am going with a Catto prop on my O-290-D2 powered -9, which I ordered this past week.

Ps

The NTSB took what remained of the prop for analysis. Their conclusion, the failure was due to intergranular corrosion, not a nick.

In other words, there were no visible signs of an impending failure.

Enginemount

Hello all

The motormount for the O-320 is 2" longer, to help with the aft CG problem

Regards,

Dominik

All good points

Highflight said:

You're taking a slight step off topic with your comments about composite props, ...............................

There are many of us who just can't afford their way into CS props.

So even if I went with an aluminum FP prop, I'd still be dealing with the issue.

ELT's, fire extinguishers, strobe power supplies and magnetometer sensors are typical of things mounted in back; sometimes WAY back. Yes, you have to also consider potential electronic interference (e.g., from the strobe power supply), but if you can put it up front and not affect electronics, do it.

For me personally, I've already decided to use a two battery system because of my all-electric panel. So that second battery may add weight, but it will be a full size battery on the firewall which should be a good start toward pulling the CG in front of the magical 80" mark.

Click to expand...

My comments about props was weight related. FP props can and do give great performance, especially since the Sench props have come out. I and really impressed with their performance. I realize that a Sench prop is not as heavy as a CS prop, but it will still go a long way in getting the CG fwd.

Also you have the right idea about mounting the 2nd battery and other equipment fwd of the spar.

The bottom line is the point you started, watch you empty weight CG and weight. The RV-7 was designed to use heavier props and engines, unlike the RV-4, RV-6. The RV-7 has great utility and can be loaded fully and be well with in W&B limits with empty CGs fwd of 80". Further you make a good point that using a light prop can cause CG issues, and every RV-7 builder can benefit from mounting and much equipment fwd in the plane. G

PS
NOW OFF THE TOPIC:
RV7ator: "I run the carburetored 360 20 degrees lean of peak", John Siebold

How the heck can you get lean of peak operation with a carburetor? Lycoming says it is impossible.

well that probably shoots my plan to carry worldwide Jepps and all my build records with me all to hell.

guess i'll just have to use it as a hat rack.

goofin' around

I'm mounting my spare on the right side to counter some weight on the left side.

I wonder where George is mounting his?

If I'm tail heavy I'll keep my speed up and not stall it any higher off the ground than I want to drop it.

Hey Mark!! Is that Drywall???

Roberta

Lean of Peak

Well, GMCJETPILOT, I'm taking a bit of liberty with the purist definition of lean of peak operations. And since this is off the original thread topic, I'll start another thread on the Traditional Engines forum. Maybe kick up some dust.

John Siebold

What?

N941WR said:

I don't think so.

(A bit off topic but it is a good story and one we should all think about.)

Here is a good example
of what can happen if 17" of a propeller decides it doesn't like to fly
along with the rest of the prop. In this case the prop was an all aluminum
Curtis Reed fixed pitch prop.

Click to expand...

Aluminum Curtis Reed fixed pitch prop? What is that. What year what it made?

I am not sure what routine maintenance has to do with it, but we all know
you need to rub your had along the leading edge of the prop on preflight. A
little file action is all that is needed to fix a ding. As far as inter granular
corrosion ( I wrote a report about it engineering school) well that is true of
aluminum. However what are all those bonds and dissimilar materials doing in
a Catto 10, 15, 25 years from now. I would trust the aluminum prop more.
Wood has great properties but now that we have a great aluminum FP prop
made for RV's I think it is a better choice. In the past all we had was wood
FP props.

Now your Catto has delimitation? The abrasion strip comes off or gets loose?
How are you going to repair that? You are not. You will have to unbolt it and
send it off (shipping cost, time). That was my point. G

Aluminum used in the modern forged blanks they make Sench props out of is
not beyond failure but they are super reliably and the quality control very
good. Again how old was that prop on your friends plane? I guarantee you will
be more likely to see a 40-year-old Sench than a Catto in 40 years later. G