Cg for 6a too nose heavy- how bad is it

[erikpmort]

Hi guys investigating a 6a, flew in it yesterday- the weight and balance shows my suspicion confirmed. It's nose heavy. 180hp lycoming w cs prop- not surprising considering setup. 1110 EW

Check it out- 2 inches forward of forward limit. 68.7- 76.8 normal limits, his is 66.4

How much does this hurt ? The place I noticed it the most was landing. It kept hauling butt down the runway and wouldn't slow down- finally it stalled and settled the last couple feet and he was good at jerking back on the stick to compensate. Infact the only damage history on the plane is a bent nose fork from a hard landing- I guess is also due to the cg. He also said stuff like the pitch trim is sensitive and it's hard to slow down- something I don't hear from owners with cs props.

How much could this effect cruise and top speed ? We went to 10,500 turned on AP went full power and saw 2680 rpm 21" mp and 161 ktas. I thought that was slow considering the setup. Am I right or wrong ? I know nothing about cs props and don't know what mp to expect. Any analysis welcome.

 

[Kyle Boatright]

The speed is reasonable, depending on how well rigged and finished the aircraft is.

As far as the CG goes, I'd break out the scales and a calculator and verify those numbers. People don't always do the initial weight and balance properly.

If that w/b is correct, the owner needs to look into a lightweight starter, alternator, etc if he hasn't already done those things. Then he needs to consider relocating some items like the ELT and battery if that is possible. A CG that far forward is a bad thing and stands a good chance of causing another nosegear problem.

 

[L.Adamson]

My Hartzell C/S 6A, with an empty weight higher than that, was excellent as an air brake. The choice was to drop quickly & flare at the precise moment, or use a bit of power. It never had a tendency to float Below 60 kias, the airspeed would drop off very quickly. If you didn't watch it, it would just fall right through the flare, onto the runway. It was easy to be downwind at 120 kias, and still drop to 70 kias while on a steep final descent.

 

[L.Adamson]

If that w/b is correct, the owner needs to look into a lightweight starter, alternator, etc if he hasn't already done those things. Then he needs to consider relocating some items like the ELT and battery if that is possible. A CG that far forward is a bad thing and stands a good chance of causing another nosegear problem.

I installed the ELT & strobe pac, towards the tail. Battery was aft of firewall, as with original plans. Required at least a light weight pilot, to be in CG.

 

[DanBaier]

If it were mine, I'd reweigh it and get a really good starting point. Then, if (with pilot aboard) it's out of CG range, then I'd plan on doing the calculation to see how much & how far to move or change something.

Dan

 

[erikpmort]

Thanks guys, so the speed is ok then ? I will double check the cg numbers

 

[PCHunt]

I've never heard of a CG, even one out of range, being the cause of not being able to slow down.

 

[rvbuilder2002]

Hi guys investigating a 6a, flew in it yesterday- the weight and balance shows my suspicion confirmed. It's nose heavy. 180hp lycoming w cs prop- not surprising considering setup. 1110 EW

Check it out- 2 inches forward of forward limit. 68.7- 76.8 normal limits, his is 66.4

How much does this hurt ? The place I noticed it the most was landing. It kept hauling butt down the runway and wouldn't slow down- finally it stalled and settled the last couple feet and he was good at jerking back on the stick to compensate. Infact the only damage history on the plane is a bent nose fork from a hard landing- I guess is also due to the cg. He also said stuff like the pitch trim is sensitive and it's hard to slow down- something I don't hear from owners with cs props.

How much could this effect cruise and top speed ? We went to 10,500 turned on AP went full power and saw 2680 rpm 21" mp and 161 ktas. I thought that was slow considering the setup. Am I right or wrong ? I know nothing about cs props and don't know what mp to expect. Any analysis welcome.

Couple of thoughts....

An empty C.G. fwd of the limit isn't automatically a bad thing (and pretty common with RV-6's with a 180/C.S.)
Before doing anything to the airplane, do an accurate weight and balance with the tanks fully drained so you know for sure what it is.

A fwd C.G. would make it less pitch sensitive (and trim less sensitive). If it really is, it is because of any number of other causes.

C.G. will have very little effect on at what rate the airplane bleeds off speed near the runway. Most likely is the way it was being flown, not how it fly's.

161 Kts.... is that indicated, true, ground speed....?
If it was indicated at 10,500, it was actually quite fast. More info needed...
21 " MP is about right for the max you can get at 10,500.

 

[BillFear]

RV6a

Our 7a with 180hp 0360 and Hartzell BA CS prop averages 185 mph true at 8-10k. According to my old flight computer that is about 161 knots. Can't remember what the manifold pressure is at that altitude.

 

[erikpmort]

That is helpful advice and I thank you. Perhaps I am looking for stuff to jump on because I'm a nervous buyer about to drop a lot of money . Thanks

One thing I just realized was his final speed was 75 kias that must be too fast, and the real contributing factor. What do you guys fly final at ?

 

[Neal@F14]

Remember that in an RV-6 as you burn the fuel down, the CG shifts further aft too. Also I've read that 1 lb at the tail equals about 15 lbs on the spinner in the RV-6 series. Maybe just relocating the battery to the baggage area could help out a lot. I know of an RV-6A with a 200hp angle valve IO-360 and CS prop that has its battery back there and he doesn't seem to have any unmanageable CG issues.

The "hard to slow down" is just simply part of flying RVs period... if you're coming from a Cessna or Piper spamcan. Floating down the runway then dropping thru the flare is one of the skills you've overcome thru repeated practice at landing an RV. It took me quite a while to get that part down, having come from almost a thousand hours in a Cherokee and all the bad habits I had from that. The flare-to-touchdown-on-the-mains process finishes much more quickly and airspeed control down final and thru the flare is just simply a lot more sensitive. The airspeed you need to enter the flare is also a lot more sensitive to how much weight and density altitude you have too, than in a common GA airplane. THe numbers change whether you're solo, or carrying a passenger, or a passenger plus baggage at full gross.

In my humble little O-320/wood prop RV-6, I've actually got aft CG issues when fuel is low and I've got baggage in the back while solo.
I typically come down final at 80-85 MPH , and 75-80 over the fence, the lower numbers when solo and light. Choosing when to flare is an acquired skill. I still have a tendency to flare too high, then slow down too much and fall thru the flare and bounce once on the mains . All my more experienced RV friends still tell me I'm landing too fast.

 

[gasman]

In my humble little O-320/wood prop RV-6, I've actually got aft CG issues when fuel is low and I've got baggage in the back while solo.
I typically come down final at 80-85 MPH , and 75-80 over the fence, the lower numbers when solo and light. Choosing when to flare is an acquired skill. I still have a tendency to flare too high, then slow down too much and fall thru the flare and bounce once on the mains .
All my more experienced RV friends still tell me I'm landing too fast.

I agree. Try 70 mph over the fence. You might just like it.

 

[Larry Ocker]

CG

I would weight the aircraft again and actually measure from the datum to the nose and main wheel axles. My measurements were different than Van's distance. Come across the fence at 65 - 70 Knots.

 

[Neal@F14]

I agree. Try 70 mph over the fence. You might just like it.

My rate of descent spikes abruptly when I slow to 70 mph and the ground sure comes up at me alarmingly quick. A little too quickly for my comfort at this stage.

That's why I've been hesitant to let it get slower than 75 mph over the fence.

Maybe after I get a few hundred more landings under my belt I'll get more comfortable landing slower, but for now I'm not trying to win any spot landing contests with it.

 

[Sam Buchanan]

My rate of descent spikes abruptly when I slow to 70 mph and the ground sure comes up at me alarmingly quick. A little too quickly for my comfort at this stage.

That's why I've been hesitant to let it get slower than 75 mph over the fence.

Maybe after I get a few hundred more landings under my belt I'll get more comfortable landing slower, but for now I'm not trying to win any spot landing contests with it.

Adding a little throttle will arrest the sink without increasing airspeed. Use elevator to set the speed you want and adjust power to set sink rate. It takes practice to get comfortable with using throttle to control sink but it will add immensely to your skill set and expand your RV's performance envelope.

 

[Neal@F14]

Adding a little throttle will arrest the sink without increasing airspeed. Use elevator to set the speed you want and adjust power to set sink rate. It takes practice to get comfortable with using throttle to control sink but it will add immensely to your skill set and expand your RV's performance envelope.

I used to do that all the time in my old Cherokee and got really good at spot-landing it that way, but this RV-6 is quite a different beast and I find myself increasing the airspeed much more than arresting sink rate while doing that and then end up floating down the runway... it's just gonna take practice to get more accustomed to this airplane. Constantly having tons of turbulence on short final to the primary runway at my home airport with a downward sloping runway doesn't help either.

I didn't have such issues landing the RV-8, it was much easier to finesse a landing in. Probably due to the CS prop and the thicker, much less bouncier main landing gear.

 

[Sam Buchanan]

I used to do that all the time in my old Cherokee and got really good at spot-landing it that way, but this RV-6 is quite a different beast and I find myself increasing the airspeed much more than arresting sink rate while doing that and then end up floating down the runway... it's just gonna take practice to get more accustomed to this airplane. Constantly having tons of turbulence on short final to the primary runway at my home airport with a downward sloping runway doesn't help either.

I didn't have such issues landing the RV-8, it was much easier to finesse a landing in. Probably due to the CS prop and the thicker, much less bouncier main landing gear.

You are probably using too big a correction with the throttle. I fly a fixed-pitch prop on my RV-6 and am talking about a 20-30 rpm change--very, very slight. But it will be enough to make the descent shallower...if...you haven't already "fallen into a hole" and let things get too slow. 70mph is about right as you come over the numbers and speed should decay to ~60mph as you touch down. I don't pay much attention to airspeed numbers, watching the AOA instead, but these numbers should work pretty well for you if you aren't heavy. But "flaring too high" will guarantee some exciting arrivals!

Yep....lots of practice to develop the sense of timing needed for smooth landings. I've been flying my -6 for 13 years and it is still training me......

 

[L.Adamson]

You are probably using too big a correction with the throttle. I fly a fixed-pitch prop on my RV-6 and am talking about a 20-30 rpm change--very, very slight. But it will be enough to make the descent shallower...if...you haven't already "fallen into a hole" and let things get too slow. 70mph is about right as you come over the numbers and speed should decay to ~60mph as you touch down. I don't pay much attention to airspeed numbers, watching the AOA instead, but these numbers should work pretty well for you if you aren't heavy. But "flaring too high" will guarantee some exciting arrivals!

Yep....lots of practice to develop the sense of timing needed for smooth landings. I've been flying my -6 for 13 years and it is still training me......

With my constant speed, I preferred 70 knots over the fence, and then making sure I was very close to the runway at 60 knots. The Hartzell constant speed slows the plane very quickly from 60 kias.....if it's a power off approach. I found it quite different from a fixed pitch RV6A that I've flown.

 

[Sam Buchanan]

With my constant speed, I preferred 70 knots over the fence, and then making sure I was very close to the runway at 60 knots. The Hartzell constant speed slows the plane very quickly from 60 kias.....if it's a power off approach. I found it quite different from a fixed pitch RV6A that I've flown.

The airspeed window is smaller (and lower) with an RV-6. Fly the RV-6A onto the ground a little fast and it will stick, but try that with an RV-6 and you will most likely start a series of bounces as the tail goes down and the wing returns to positive AOA. Hence the famous "RV Skip".

But each plane/pilot is a little different, timing comes with practice. I've found that when my landings start getting a little sketchy, I've allowed airspeed to creep up a bit. Getting airspeed back down into the sweet spot will restore good landings.