Optimizing Center of Gravity for Aerobatics

Did you ever wonder why two-seat aerobatic airplanes like the Extra 330LX, the Pitts S2 or the Decathlon are flown solo from the rear seat? It’s all about managing the center of gravity (CG) for optimum aerobatic performance. Since the passenger seats in the above examples are located almost directly on the aircraft CG the addition of a passenger has little or no effect on the overall CG of the airplane. The tandem two-seat RV-4 and RV-8 are flown solo from the front seat so adding a passenger to the rear seat will move the CG aft. In most cases the RV-4/8 can accommodate only very light passengers without exceeding the aft CG limit for aerobatic flight.

I have found that my ideal CG location for aerobatics is at about 25% of the Mean Aerodynamic Chord (MAC). At this position the stick pressure required for aerobatic flight is rather light and spin and snap roll entries are crisper and more controllable. The lighter elevator control is most noticeable and appreciated during inverted flight. To achieve the 25% MAX CG would require me to put 68 pounds of ballast or my nine year old granddaughter in the back seat. Of course an extra 68 pounds in the aircraft would seriously degrade aerobatic performance. I found that only 20 pounds of ballast in the tail of my RV would result in the same movement of the CG and a perfect place to put that weight is under the empennage fairing right between the front and rear spar of the horizontal stabilizer.

Now here’s the problem: While the twenty pounds of ballast would be great for aerobatics it would severely limit my ability to carry passengers and baggage without exceeding the normal aft CG limit. I would have to remove the ballast every time I loaded the aircraft with my wife, two Chihuahuas and 50 pounds of luggage. (Yes, that’s our typical cross country load!)

Competitive sailplanes carry water ballast to increase speed and move their CG aft to reduce induced drag. When no longer needed the water is vented overboard. Since there is no room for 2.4 gallons (20 pounds) of water in the tail of my RV-8 I am using 20 pounds of #9 lead shot which pours just like water! The aluminum container is pictured below. (My thanks to my good friend Gary DeHart who welded it up for me!) A filler neck sticks through the empennage fairing and a drain tube exits at the bottom of the fuselage. All made from PVC conduit I found in the aircraft aisle at Home Depot.



BALLAST BOX AFTER WELDING SEAMS







FILLER NECK INSTALLED WITH BOX MOUNTED





FAIRING INSTALLED





FILLING BALLAST BOX WITH #9 LEAD SHOT

Brilliant!

Carl

What about polar inertia? Any concerns on how this might affect spin stability and recovery? At a minimum, I'd expect that it would tend to flatten the spin out a bit.

ChiefPilot said:

What about polar inertia? Any concerns on how this might affect spin stability and recovery? At a minimum, I'd expect that it would tend to flatten the spin out a bit.

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You seem to imply that increasing polar moment of inertia is a bad thing. What I am doing is making the airplane less stable in pitch. To a certain point this is a good thing to have in an aerobatic aircraft. At 22% MAC my pitch stability was positive and stick forces were too heavy. At 25% MAC stick forces are much lighter but still positive and well within the designed GC range for aerobatic flight. I perceive no change in developed spin characteristics at 25% MAC but I have found that spin and snap roll entry is a bit crisper and more consistent. Recovery from spins and snaps is still very controllable.

ronschreck said:

You seem to imply that increasing polar moment of inertia is a bad thing. What I am doing is making the airplane less stable in pitch. To a certain point this is a good thing to have in an aerobatic aircraft. At 22% MAC my pitch stability was positive and stick forces were too heavy. At 25% MAC stick forces are much lighter but still positive and well within the designed GC range for aerobatic flight. I perceive no change in developed spin characteristics at 25% MAC but I have found that spin and snap roll entry is a bit crisper and more consistent. Recovery from spins and snaps is still very controllable.

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You're making it less stable in pitch, but you're also making it less stable in yaw as well. I was just curious if you had actually tested spins with the ballast in place or not. I had considered fixed ballast last year for similar reasons but after consulting with an aerospace engineer I decided not to based on concerns he raised. Glad it works for you.

weight

I'm just thinking out loud and I don't know much so I'm not saying it a wrong set up for you.For me I would have wanted a set up that did not put anymore stress in the tail were it attached to the airframe .I get real nervous about anything around my elevator controls. For safety reasons I would have installed a mounting plate in the bottom inside the fuse with anchor nuts and bolted a streamlined lead bar on the outside that could be taken off or installed easy and if for some it became lose it would not be a factor.I always thought there should be two more bars on a bulkhead holding the front of the horizontal on just like the back.Anyways not trying to offended anyone .
Bob

reminder?

Ron, that's a perfect solution to shifting the C.G. for aerobatics. I'd have to add that to my check list and something in the cockpit to remind me that I had lead in my tail such that I didn't inadvertently take up a passenger with the lead in. Can't tell you how many times I started aerobatic practice and forgot to switch tanks to the flop tube one. The engine sputtering is a good reminder. Wishing you the best of luck this year in competition.
Bill McLean
RV-4 slider
lower AL

ChiefPilot said:

You're making it less stable in pitch, but you're also making it less stable in yaw as well. I was just curious if you had actually tested spins with the ballast in place or not. I had considered fixed ballast last year for similar reasons but after consulting with an aerospace engineer I decided not to based on concerns he raised. Glad it works for you.

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You are correct; it is less stable in yaw. And, again that is a good thing when flying aerobatics. I tested spins above 6000 feet AGL with the ballast at 10 pounds and again at 20 pounds. As far as pitch and yaw stability is concerned the addition of 20 pounds at the tail is exactly the same as having a 68 pound passenger aboard.

For those of you concerned about security of the ballast box mount: It is secured by four AN-4 bolts into plate-nuts that are riveted to aluminum doublers. The box is NOT attached to the horizontal stabilizer, rather to the plate between the front and rear spars of the stabilizer.

Guys, I'm not trying to rewrite the laws of physics or volunteering to be a crash dummy. I recognize that a very large majority of RV pilots are hesitant to deviate from the basic design of the RV and are content to fly from point A to point B without venturing anywhere close to the edge of the design's flight envelope. Some of us enjoy exploring the outer edges of the envelope and we do so with the utmost respect for the limitations suggested by Dick VanGrunsven. This is not a death wish. On the contrary, becoming familiar and comfortable with the aircraft at every point within the envelope helps us become safer pilots.

The T6 Harvard and Texan were/are flown PIC from the front seat. They use removable "suit case" weights in the tail. However, every now and then.. well actually only once per student, somebody would forget and leave the extra weights in the tail and get into a spin with a passenger in the back seat. Recovery was somewhere between difficult to impossible. It did however work well for solo flight from the front seat.

F1R said:

The T6 Harvard and Texan were/are flown PIC from the front seat. They use removable "suit case" weights in the tail. However, every now and then.. well actually only once per student, somebody would forget and leave the extra weights in the tail and get into a spin with a passenger in the back seat. Recovery was somewhere between difficult to impossible. It did however work well for solo flight from the front seat.

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Perhaps a visible reminder of when the weight is inside the tank. Perhaps use a RED plug when shot is in the tank and use the SILVER plug when it's drained. Store them with the shot when it's not in use. Kind of like a Remove before flight" flag..

control said:

What is the consequence of you being in the wrong configuration? Acceptable or managed by extra controls?

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Not good. You could exceed the maximum recommended aft CG limit for normal flight.

And Jerry, I appreciate your suggestion about a red plug. I'll do that.

ronschreck said:

You seem to imply that increasing polar moment of inertia is a bad thing.

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Without knowing the effect, it very well could be.

It should be high on the list of concerns for anyone doing modifications that add weight to outer positions on the airplane (such as tip tanks, outboard aux fuel tanks, etc.).
Once flight testing (which I know you are doing, because the resultant handling is the primary purpose you are doing it) has proven that there are no negative influences, then the concern can be set aside.

My concern is that a thread like this might cause people to take a casual attitude regarding polar moment of inertia influences. The are very real and are fully capable of transforming an airplane with good spin recovery in to one where there is no spin recovery.

CG vs Polar Moment of Inertia

ronschreck said:

... As far as pitch and yaw stability is concerned the addition of 20 pounds at the tail is exactly the same as having a 68 pound passenger aboard.

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Just be very aware that while 20 pounds in the tail results in the same CG as 68 pounds in the passenger seat, the polar moments of inertia is significantly greater for 20 lbs in tail (approx. 2200 lb-ft^2) vs 68 lbs in the rear seat (approx 650 lb-ft^2).

Test thoroughly!

Skylor

skylor said:

Just be very aware that while 20 pounds in the tail results in the same CG as 68 pounds in the passenger seat, the polar moments of inertia is significantly greater for 20 lbs in tail (approx. 2200 lb-ft^2) vs 68 lbs in the rear seat (approx 650 lb-ft^2).

Test thoroughly!

Skylor

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Thanks for your concern. Yes, the moment of inertia is greater with the tail ballast and after testing at gradually greater weights at altitude I have determined that the 20 pounds of ballast changes control response in yaw and pitch and I am pleased with the result. Next project is to DECREASE the roll moment of inertia.

Respectfully

ronschreck said:

Thanks for your concern. Yes, the moment of inertia is greater with the tail ballast and after testing at gradually greater weights at altitude I have determined that the 20 pounds of ballast changes control response in yaw and pitch and I am pleased with the result. Next project is to DECREASE the roll moment of inertia.

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What Skylor and Cheif pilot are respectfully trying to shed light on, is that if you entered an accelerated spin, the 2200 ft-bs vs 650 ft-bs might flatten the spin to a non recoverable point. The extra inertia will pull the tail down/raise the nose if the spin rate was increased either accidentally, or on purpose with ailerons.

Please wear your parachute, while testing thoroughly from a very safe altitude.

Best of luck!

Just an idea

I'm thinking maybe there could be a way to jettison your extra weight if you were to forget. Maybe a electric solenoid or cable operated valve on the drain tube. Just a slot cut in the tube with an aluminum blade, a small return spring and a light cable or something.

MeAndMyToys said:

I'm thinking maybe there could be a way to jettison your extra weight if you were to forget. Maybe a electric solenoid or cable operated valve on the drain tube. Just a slot cut in the tube with an aluminum blade, a small return spring and a light cable or something.

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Brenden, have you priced lead shot lately! I paid $60 for 25 pounds!

I took the plane up this morning and tried everything I could think of to put it into a flat spin. (By the way, an accelerated spin is not a nose high spin. And it is one of the easiest spins to recover.) I started at 11,500 feet and recovered at 5000 feet. Tried it three times, both directions, with and without aileron input, power on and off. Bottom line... it flies great! And, yes I always wear a chute when doing aerobatics. Is Bill McLean the only one who agrees that this is a good thing? Are there no more experimental pilots out there? Jeeze!

(OK, rant over. Thank you for your patience.) Out.

ronschreck said:

Is Bill McLean the only one who agrees that this is a good thing? Are there no more experimental pilots out there? Jeeze!

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I don't think asking questions about your design process and thought process is bad and I haven't seen any comments in this discussion that would imply such a thing. I'd submit that asking these kinds of questions are in fact the kinds of things experimental test pilots actually do as bolting a modification onto the airframe and launching into the wild blue yonder went out of vogue quite some time ago.

You have clearly done the flight testing to validate that you still have sufficient positive stability to meet your needs. Considering that many RVs go through their entire phase one in essentially "point A to point B" flight as you described it earlier, I think you're ahead of the game. Thanks for sharing your idea and engaging in the conversation.

What do you think?

I'm wondering how you will turn the aircraft on its side to shake that shot out the little hole

Raymo said:

I'm wondering how you will turn the aircraft on its side to shake that shot out the little hole

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There is a drain tube on the bottom of the box.

Weight

The Extra 230 was derived from the Stephens Acro and the many variants that followed. Walter Extra copied the Stephens fuselage dimensions almost exactly but then added a constant speed prop. This put the CG well forward. His fix was to put the 25# battery at the lower leading edge of the vertical fin, inside the removeable turtledeck.
I always believed the weight that far aft in the fuselage would be very noticeable. Not so. The 230 flies better than any of the homebuilt Stephens derivatives by a huge margin. The battery in the tail has no noticeable effect on the way the airplane flies including all spins.
Going in the other direction, when I replaced the standard Maule tailwheel on my Pitts S1S with a lightweight Aviation Products tailwheel the snap rolls improved noticeably but nothing else changed significantly.

ronschreck said:

Brenden, have you priced lead shot lately! I paid $60 for 25 pounds!

I took the plane up this morning and tried everything I could think of to put it into a flat spin. (By the way, an accelerated spin is not a nose high spin. And it is one of the easiest spins to recover.) I started at 11,500 feet and recovered at 5000 feet. Tried it three times, both directions, with and without aileron input, power on and off. Bottom line... it flies great! And, yes I always wear a chute when doing aerobatics. Is Bill McLean the only one who agrees that this is a good thing? Are there no more experimental pilots out there? Jeeze!

(OK, rant over. Thank you for your patience.) Out.

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I didn't mean to bring on a rant about the handling characteristics, I was referring to if there was an inadvertent issue of forgetting to empty it and having a passenger aboard. Mostly for the benefit of any others contemplating following your idea.

I think yours is a great idea, although I am in no way qualified to pass judgement on it. As I have had no aerobatic training at all, YET!!

Looking forward to completing my project so I can remedy that.

Haigh tail wheel weight

I remember years ago several guys would put 5 lb weights on the tail wheel
rod attachment(Pitts S1S). They said it would improve presentation on the snap. I never tried it because I thought the spin might be affected. I'm sorry but I'm just not a believer in messing with aft CG and Acro, too many variables.
John

CG Wanna Be

I'm fascinated by this topic. I love Ron's innovative approach to the stability / maneuverability problem. The airplane I want to fly is the one that is inherently UNSTABLE in every axis and fantastically maneuverable. Unfortunately, that's called a Joint Strike Fighter, F-18, or a Russian something-or-another and i can't afford any of those. So, like Ron, when engaging in aerobatics, I'll always be searching for the best compromise between stability and maneuverability.

On the practical side, and with only six months of experience in my -4, I'll say that I can definitely tell the difference up and away when somebody is in the back. I LOVE that increased pitch authority! So, like Ron, I'll be looking for a way to shift the CG back when nobody's IN the back. I don't want to pay a weight penalty either though so I'm thinking some sort of tailwheel strut weighting would be the best approach. Great, another research project!

Don't you just love experimental aviation!

RV8iator said:

Perhaps a visible reminder of when the weight is inside the tank. Perhaps use a RED plug when shot is in the tank and use the SILVER plug when it's drained. Store them with the shot when it's not in use. Kind of like a Remove before flight" flag..

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Jerry,

Appreciate your comments. I went to the hangar and painted a plug DAYGLOW ORANGE. I had some paint left over so I wrote a note to myself. I think you would approve!


Preflight

ronschreck said:

Jerry,

Appreciate your comments. I went to the hangar and painted a plug DAYGLOW ORANGE. I had some paint left over so I wrote a note to myself. I think you would approve!

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Do you havea any extra paint? That should be written and heeded by everyone.

Better than a Remove Before flight banner!