What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV-7A engine choice vs CG

H

harrisonaero

Member
For a stock RV-7A built to plans, which specific Lycoming 320 or 360 engine is considered most ideal from a CG standpoint? IOW which one gives the most usefulness for pilot, pax, fuel burn, and luggage without having to resort to moving things like the battery?

Or to put it another way, does this aircraft tend to be nose or tail heavy which means it may benefit from a lighter or heavier engine model?
 
It is my understanding that one can build the 7 design and wind up on either end of the CG envelope.

To be more specific, I think a 7 with small (meaning light) motor and fixed pitch prop and thick paint job will tend towards rear CG which can limit baggage.

A 7A with large motor, constant speed prop and polish finish will tend towards forward CG which affects nose gear loading and theoretically top speed due to higher tail loading.

There have been lists of RV weights but I'm not sure they are still available on the web. If you build to plans and minimize fluff in the cabin, you should be OK both for empty weight and CG.

The only hard data point I can give you is my own plane which is a 7A with IO-360-M1B, Hartzell, bare cabin and polish. 1109 lbs which is in the middle of the spec range for 7A. Forward CG (sorry I don't have the numbers with me) but to give an idea, it is out of range forward until the pilot sits in the seat. It is impossible for me to load the plane out the rear of CG if I stay within the 100lb baggage limit.
On the other hand, the nose gear loading is high (still within limits) with the worst case being solo with no fuel :eek:.

With that, my assumption would be that an angle valve installation would require some thought on mounting avionics in the tail (i have none in the back) such as AHRS, ELT to help with balance to avoid a lead weight in the tailcone. Maybe a 7A angle valve owner could speak up?
 
Last edited:
Its easier to live with a fwd CG, so my choice would be a parallel valve (I)O-360 with a Hartzell c/s prop. It will be difficult to get that combination out of CG rearwards. The penalty will be slightly higher stick forces when manoeuvring solo. By all means use light engine accessories - starter & alternator. Selecting the lightest engine & prop will mean taking care with baggage loading to remain within aft cg. I would take care to install nothing aft of the seats unless it was essential - eg EFIS magnetometer (can also go in wing), autopilot pitch servo, aft position/strobe light. If you find your cg is too far fwd it will straight forward to move stuff aft (lots of space), it is difficult to move stuff forwards.
Just my thoughts.

Pete
 
My -7A is a good example of the variety you can expect. I have a typical, plans built airplane (except I didn't bother with the lightening holes in the VS Spar). I have an O-360 but a Catto prop, which is very light. My CG is right under 81 inches (If I recall correctly). Max forward is about 2 inches forward of that.

But, that being said, I've never had a problem even being at the back end of the CG limit. Stalls just fine, handles just fine. My preference would be to get those 2 inches back, but it turns out to not be that big of a deal.

In my situation, a 20 lb. Landol weighted prop spacer would fix my issue for a lot less money than a C/S prop.
 
Catto

Not trying to hijack the thread, but I'm interested as well.
Lostpilot28,
Catto three blade or two and pitch for climb or cruise?
I want to build a similar config.
Any issues with max load?
Larry
 
I think the RV7/7A was basically designed with O360 and CS prop combination. This puts the CG up front and allows 2 good size passengers, fuel fuel and full baggage with no CG issues. With a light engine/prop you will likely have a weight limit on baggage, and not that I've ever done it, but it's nice to have a little "buffer" in the CG for things like trips to OshKosh where you come back with more weight than you left with :eek:
 
If Jamie Lee see's this he might jump in with numbers, he built light and has a Cato prop. We had to add a lot of weight in the crush plate to get the cofg right.

Catto I think supplies them too.

IO360 and a Hartzell CS s the go!
 
Walt said:
I think the RV7/7A was basically designed with O360 and CS prop combination. This puts the CG up front and allows 2 good size passengers, fuel fuel and full baggage with no CG issues. With a light engine/prop you will likely have a weight limit on baggage, and not that I've ever done it, but it's nice to have a little "buffer" in the CG for things like trips to OshKosh where you come back with more weight than you left with :eek:
Click to expand...

Mine is also built with the O-360 CS prop, nose gear, heavy but sweet Oregon Aero seats, and a loaded panel. Remember an A model will automatically be about 60 lbs heavier due to the nose gear. Have one PC680 battery up front with 8 lb back up under the panel and 10 lb back up alternator. Everything else typical. Total weigth 1187 (for real) and it is front loaded.

With only one person in front, and loading the plane up with all the camping gear, ice chests, clothes, tents, etc required for extended camping ...... the CG is right on the back edge ..... and it flys that way. I would not want any less weight up front than I have.

Bill S
 
Last edited:
It is good that you are thinking ahead but my opinionated opinion is that you should install no less than an O-360 variant. While an O-320 performs well...compared to a Cessna 172, you will always be at a disadvantage compared to O-360 RVs.

Plus you will have a plane that most RV pilots would not buy (opinion again) if you have to sell it.

Given that engine, you have plenty to play with to work on the CG. I suggest getting actual Weight/balance data and use of the software programs that quickly allow you to assess CG based upon projected loadings.
 
Good stuff all, thanks for the advice. Looks like a parallel valve O-360 with CS prop is what to shoot for as far as CG is concerned.
 
No...

BillSchlatterer said:
...
Remember an A model will automatically be about 60 lbs heavier due to the nose gear. Have one PC680 battery up front with 8 lb back up under the panel and 10 lb back up alternator.
...
Click to expand...

Bill, can you confirm these numbers? How did you obtain them? Did you weigh the parts?

Pardon my skepticism. I haven't weighed them myself. But from having built a -7A, I don't think the entire nose gear assembly weighs anywhere near 60 lb, never mind even offsetting weight associated with a tail wheel. And looking at the specs on Van's web site, they list the typical empty weight ranges for both -7 and -7A, and they show the -7A being only 16 lb heavier than the -7, not 60 lb. That sounds much more believable to me.

Also, what kind of backup alternator do you have that weighs 10 lb? A B&C SD-8, probably the most common backup alternator found on RV's, weighs less than 3 lb. Even a Plane Power AL12-EI60, which is Van's standard issue 60 A main alternator, weighs less than 7 lb.
 
roee said:
Bill, can you confirm these numbers? How did you obtain them? Did you weigh the parts?

Pardon my skepticism. I haven't weighed them myself. But from having built a -7A, I don't think the entire nose gear assembly weighs anywhere near 60 lb, never mind even offsetting weight associated with a tail wheel. And looking at the specs on Van's web site, they list the typical empty weight ranges for both -7 and -7A, and they show the -7A being only 16 lb heavier than the -7, not 60 lb. That sounds much more believable to me.

Also, what kind of backup alternator do you have that weighs 10 lb? A B&C SD-8, probably the most common backup alternator found on RV's, weighs less than 3 lb. Even a Plane Power AL12-EI60, which is Van's standard issue 60 A main alternator, weighs less than 7 lb.
Click to expand...

I agree, the 60 pounds of difference sounds too much. As a point of comparison, I converted my 9A to a 9, and the weight difference was -25 pounds. Replacing the sensenich (40) with a catto (15) got me an extra -25 pounds.

Greg
 
wirejock said:
Not trying to hijack the thread, but I'm interested as well.
Lostpilot28,
Catto three blade or two and pitch for climb or cruise?
I want to build a similar config.
Any issues with max load?
Larry
Click to expand...

Hi Larry, it's a 3 blade. I asked Craig Catto to pitch it for best of both worlds and it seems to be more for cruise...although it climbs well, too. I top out at 207 mph at 8500 ft.

Regarding the aft CG and baggage weight, I do have a restriction set in my POH that if I have 2 heavy guys in the airplane, then I'm limited to 60 lbs of baggage. Although it would put me just outside the envelope to load it to 100 lbs, I think it would fly fine. I don't want to fly that way...but, as I mentioned, a 20 lb. prop spacer would offset the CG and give me my 40 lbs of baggage back loaded in the heaviest possible way. I just don't ever have a need for doing that, so I haven't added the spacer (yet).
 
roee said:
Bill, can you confirm these numbers? How did you obtain them? Did you weigh the parts?

Pardon my skepticism. I haven't weighed them myself. But from having built a -7A, I don't think the entire nose gear assembly weighs anywhere near 60 lb, never mind even offsetting weight associated with a tail wheel. And looking at the specs on Van's web site, they list the typical empty weight ranges for both -7 and -7A, and they show the -7A being only 16 lb heavier than the -7, not 60 lb. That sounds much more believable to me.

Also, what kind of backup alternator do you have that weighs 10 lb? A B&C SD-8, probably the most common backup alternator found on RV's, weighs less than 3 lb. Even a Plane Power AL12-EI60, which is Van's standard issue 60 A main alternator, weighs less than 7 lb.
Click to expand...

Hey Roee, you might be right! I said 60 and that probably is high. My airplane is about 60 heavy and I sort of had that on the mind. I have a gear leg and I think it's every bit of 20 pounds plus tire and nose fork but I will weigh it and get back to you. Right on with the SD8 which is 3.7 plus a little for the regulator and capacitor. Sooo.... I may have been high on the 60 but I do know for a fact that on calibrated digital scales my airplane weighs in at 1187 and there isn't much extra other than what I have mentioned.

I withdraw the 60 subject to fact finding and should have put a pencil to the numbers before posting. Mea Cupa!

Good call on your part... don't let bad numbers stand.
 
You must log in or register to reply here.
Back
Top