Rocket (W&B)

[lshapnek]

Looking at a Rocket and making up a sample W&B calculator. I'd heard about the forward CG, but my first looks and calculations show a nearly impossible loading situation. Anyone have a good sample loading worksheet that I can use to see if my "engineering" is wrong? Or where is my error(s) on this calculator (attached), or is the path "just fly it"?  

 

Edited April 24, 2021 by lshapnek
too much info "out there"

[carusoam]

Welcome aboard Ishapnek!

What was the impossible loading situation?
 

Do you have two big people sitting up front and nothing in the back?

 

Many Mooneys have added some heavy updates forward of the firewall....

From extra cylinders to heavy props...

 

Like you are doing... working with the WnB is the way to find the solution...

 

Just fly it?  No... not ever...  Not just this once... this is never going to be a good idea.  
 

Being out of the WnB envelope doesn’t immediately destroy the plane....   but, it is a good way to cause an accident within a minute of leaving the ground...  departure stalls are terribly unfriendly.  
 

So... ‘Just fly it’ doesn’t usually get a yes vote...
 

Kind of like skipping T/O distance calculations on a high DA day... or short runway...

 

PP thoughts only, not a CFI...

 

Best regards,

-a-

[carusoam]

See if this helps the conversation...

What is causing the odd shapes to your envelope?

Best regards,

-a-

 

[aviatoreb]

Just now, carusoam said:

See if this helps the conversation...

What is causing the odd shapes to your envelope?

Best regards,

-a-

 

Math answer: The lack of convexity.

[lshapnek]

All, the front seat weights are just samples. Two (x 195 lb) in front would be a more normal crew. Then I'd like to take some fuel along just 'cause.

The envelope is defined from the 252 limits plus the Rocket limits (on top).

I'm looking (hoping) for errors in my sample calculator. The first line is actual empty AC data from a Rocket.

Sample Rocket WB Calculator (4-24-21).xlsx

[carusoam]

WnB is a two pronged solution...

When you run out of envelope...  you can adjust the weight... and where it goes.

The Rockets have a pair of Batteries in the tail...

They don’t need a pair of batteries... but they are more useful than Charlie weights...

If the Batteries aren’t heavy enough... consider getting the taller/heavier battery...

After that... adding a Charlie weight back by the batteries can be the solution...

This is a typical job for a Mooney mechanic to do... not something a new owner would want to take on by himself...

For your calculation experiment...   See what Concorde has for battery weights that fit the same footprint... X-C stands for extra capacity... aka heavier weight...

you can use that station for experimenting with how much of a Charlie weight you may want to add to make your imaginary  WnB experiments work out...

Long Body Mooneys have a specific Charlie weight location near the batteries...shorter Mooneys have a hidden location closer to the end of the tail...

Rocket Engineering has great support for their STCs... they may be a great source of an answer.  They can be a challenge to make contact...

There is always a technical answer to most problems in aviation... MS is a great resource for finding where that answer is kept...

PP thoughts only, not a mechanic...

Best regards,

-a-

[lshapnek]

Erik, Do you have W&B data for your Rocket that you would share? Maybe I can spot an error in my engineering. Larry

[RJBrown]

I found that with 2 people in the front seats the plane was near the forward limit. All additional items loaded into the plane move the balance rearward.

with 120# in the baggage area (the weight limit) as long as you are not overweight you cannot load it too tailheavy. 
If your math disagrees with this statement find your math error.

Assuming a 1000 pound useful.

340 in front seats 360 in fuel tank 120 in baggage area and 180 in rear seat. Balance is fine.

360 fuel is your minimum fuel load at 3200 gross. This assumes 10 gallons of useable fuel (30 minutes at cruise rate of 20 gph) at maximum landing weight of 2900.

That leaves 640 pounds to distribute in the airplane. I suppose if all that weight was placed in the front seats you could get out of balance forward.

I never tried the math with more than 340 in front. 
‘Airplanes are more stable nose heavy. Elevator authority for landing is the forward limit.

Airplanes become less stable as the COG moves rearward. The rear limit keeps the airplane controllable. An airplane is fastest at the rear COG limit. Less downforce on the horizontal stabilizer combined with less wing lift need to compensate for that downforce creates the least amount of drag.

Bonanza has a problem where the COG moves rearward as fuel in burned. It is easy to get a Bonanza out of balance to the rear.

Mooney does not have this problem. They are hard to get out of balance to the rear at max gross weight. As fuel burn in your Mooney the COG moves forward.

 

 

[A64Pilot]

Sometimes the fwd CG limit is due to elevator effectiveness, which often results in a higher stall speed, some aircraft the fwd CG limit equates to 61 kts stall speed due to the elevator, but just as often it’s a nose gear structural limit.

I’m sure it would fly just fine forward of the CG limit within reason, likely be a better instrument platform, but I’d be careful especially if the limit is a structural one.

I woud not put heavier batteries in, not unless the mount has been structurally analyzed for the heavier weight, be a bummer to have batteries flying forward in a crash.

[Hank]

As we say in model aviation:  nose heavy flies poorly, tail heavy flies once.

[MIm20c]

Like all things aviation, no problem that money can’t fix. Slap a 4 blade on the nose of that rocket...

[lshapnek]

So true Mlm20c. It takes an AMU for each knott faster that you want to go, takes another AMU to shed each lb. 

[lshapnek]

Hank, Are thoise the States you have landed in ? I only have two more to go myself ! One I'll have to rent a plane when I go there.

 

[carusoam]

lshap,

If you like...

You can get your own map...

It gets mentioned how in here...

-a-

 

[Hank]

51 minutes ago, lshapnek said:

Hank, Are thoise the States you have landed in ? I only have two more to go myself ! One I'll have to rent a plane when I go there.

 

Yes. So far I've visited 47. N. Dakota, Arkansas and Idaho . . . . Only a few in my plane. I rented in Alaska, a great experience!

[louisut]

I've puzzled over the Rocket's peculiar CG for a long time now. I've asked others for comparison, but the numbers are inconsistent from plane to plane. We decided to put the plane on actual scales recently, and measured an empty weight of 2390 lbs and 42.3" CG. Others I've asked are 100-150 lbs lighter but I haven't figured out why.

With these numbers, if it's only me in front (140 lbs), then I can carry 60 gals fuel if I also carry 120 lbs in the baggage. With two people in the plane and one person in the back, I can add another 10 gallons. With two up front, I'm limited to something ridiculous like 20 gallons before hitting the forward limit, even with full ballast. Part of this is due to the forward CG of the Rocket, and for me, it's exacerbated by the high empty weight. I can't load much before hitting the top "slope" of the limits. In short, it's a fussy experience.

Your setup looks similar to mine. A few additional notes:

• If you have Monroy tanks, the station is at 71" (about the same as the aft seats). It begins filling once there are about 20 gallons in the main. This is specified in the flight manual supplement. Mine didn't include one, but apparently later ones did. See the pics attached.
• Remember that the front seats full forward and full aft stations are different--34" and 39", respectively.
• If you need ballast, a case of oil is about 20 lbs. If you need something more compact, check out the lead weights for wakesurfing boats. I use two 50 lb bags and a case of oil to get 120 lbs.

On fixing the issue, someone already mentioned the composite prop. Performance improvements aside, it also takes ~36 lbs off the very front, but last I checked it's about $20k. It's an expensive proposition if you aren't looking for a new prop.

The service manual describes how to add up to 19 lbs of counterweight (aka charlie weight) in the tailcone at station 197.5". I'm trying to figure out how to go about adding more. By my estimates, 60-70 lbs there would get the CG into the "meatiest" part of the limits and solve my issue completely. Supposedly there's someone who did just that (which was reported secondhand to MS), but I'm haven't had any luck getting more details.

Can you ignore it? As already mentioned, in the Rocket it's primarily an elevator authority issue. If you're landing and the elevator stalls in the roundout, your nose is going to slam down. With the already low prop clearance, you're probably looking at a prop strike. I wouldn't "just fly it". Some may suggest it obliquely, and I suspect many do anyway. Remember that once you're outside limits, you're a test pilot #1.

edited for grammar

Edited April 25, 2021 by louisut

[A64Pilot]

I’m not abdicating doing anything against regs, but if you know for sure it’s elevator, meaning 61 kts stall is the limit, just add 5 kts to an approach speed and learn when to lower the nose on touchdown. fly it like it was a lancair or something

That may be better than overgrossing the airplane or exceeding structural limits from excess lead in the tail or even lead weights in the baggage as they may be tough to secure.

‘Or the lightweight prop is the “best” answer but it’s $20K.

‘Sometimes you have to pick the lesser of the evils.

 

Edited April 25, 2021 by A64Pilot