The other half of the weight equation.

Last week, I wrote a blog post about helicopter weight. It was my response to a blog post by Tim McAdams on the AOPA helicopter blog titled “Gross weight.” The comments to that post indicated to me that some of the commenters were confusing weight with CG — center of gravity — issues. My blog post concentrated on weight, putting CG aside. But CG is the other part of the weight equation. And for most helicopters, CG is vitally important to calculate as part of preflight planning.

CG Defined

Center of gravity is pretty much what the phase indicates: a calculation of the center of gravity on an aircraft. It’s the aircraft’s balance and it’s calculated as part of the “weights and balance” computations.

For helicopters, CG is extremely easy to envision. After all, a helicopter with a single main rotor system (as most have) is supported at one point in flight: the main rotor system or mast. If you held up the helicopter by its rotor system, the distribution of its weight — not just what’s inside it but its engine, battery, tail rotor, etc. — would determine how level it hung.

Take, for example, my R44. Its full passenger load is in front of the mast. Its fuel, engine, and heavy components are slightly aft of the mast. For this reason, if I’m flying solo (just one person up front) with low fuel, the helicopter would be a bit tail heavy. As I set down from a hover to the ground, the back of the skids would touch the ground before the front. In fact, the right back would touch first. That’s the lowest point closest to the center of gravity for that load.

The same applies to R22 helicopters. In fact, it’s often terrifying for student pilots to pick up into a hover after their flight instructor steps out when its time for that first solo flight. (Sure scared the hell out of me.) The helicopter feels as if it’s going to flip over backwards!

But stick a pair of fatties up front in my R44 and the CG will shift forward. In fact, if you have enough of a load up front, the fronts of the skids will touch first on landing. (I remember the first time I flew with my brother-in-law on board. I thought I was landing on a slope!)

Watch any helicopter take off or touch down and you’ll probably be able to tell where its center of gravity is.

Why CG Is Important

CG limitations are important for aircraft operation. For example, if you’re loaded with too much weight up front, the helicopter will tilt forward more in flight. When slowing down, stopping, or hovering, you might not have enough aft cyclic to counteract this forward tilt. Ditto for lateral or aft CG.

Remember, every aircraft control has a “stop.” That’s the limit to the control’s movement. You pull the cyclic back to slow down in flight. If you’re heavy up front, you may have to pull it back to simply hover in place. If you’re so front heavy that you can’t pull the cyclic back enough to stop the helicopter from moving forward, you have a serious problem — a problem with your CG or balance.

Want to see how far your controls will move? You should be checking their movement before starting up by simply moving each of the controls as far as it will go. The idea is to make sure none of them are stuck on anything or binding in any way. You don’t want to learn about a control problem when the engine is running, rotors are spinning, and you’re picking up into a hover.

You Can Be Under Max Gross Weight and Still Out of CG

What was bothering me about the comments on that AOPA blog post was that a few of the early commenters kept referring to CG and “weight and balance.” But the blog post was about gross weight. CG wasn’t discussed at all.

I didn’t address CG in my post because that wasn’t being discussed. In fact, it’s quite possible to load an aircraft out of CG and still be within max gross weight.

This aircraft is out of CG but still within gross weight limitations.

Want an example? Here are the plotted points for a W&B calculation for an R44 helicopter with 4 good-sized people on board (190 and 250 up front; 190 and 145 in back). It’s a short flight, so only 16 gallons or about an hour’s worth of fuel is loaded on board. The total weight of the aircraft is 2408 — that’s nearly 100 pounds below max gross weight. But as the graph points show, this aircraft is out of CG — too much weight up front.

If you’re having trouble reading this, envision the pink line as the mast. Everything to the left is the front of the helicopter. Everything to the right is the back end of the helicopter. The blue box is the CG envelope for the aircraft, as determined by the manufacturer. In this example, the aircraft with and without fuel is loaded forward of the CG envelope. That’s a no-no.

The same passengers as in the previous example, but with heaviest and lightest passengers switched. This load is within CG.

This situation is extremely easy to fix. Simply rearrange the passengers. While you can’t move the 190-lb pilot, you can move the fatty beside him. In this example, the 145-lb passenger in back has switched places with the 250-lb passenger up front.

Adding fuel might help, since fuel is loaded aft of the mast. But you couldn’t add much — you’re already pretty darn close to max gross weight.

Of course, both of these examples show only longitudinal CG — forward to aft. Lateral, or side to side, CG also needs to be calculated. As many of the commenters pointed out, they’ve created spreadsheets to perform these calculations for them. So have I — where do you think these two diagrams came from?

Take the balanced load of the second example and top off the fuel tanks and you’ll get a situation like this.

Can you be over max gross weight and within CG? Technically, no. As these two diagrams indicate, if you’re loaded too heavy, the plot points will be outside the CG envelope. That means you’re out of CG.

But are you really out of balance to the point where you’ll run into control issues? I wouldn’t want to find out for myself.

Which is More Important: Weight or Balance?

I’ll always argue that you need to consider weight before you worry about balance. If you’re too heavy to fly within manufacturers max gross weight allowances, you’re too heavy to fly legally. Who cares about balance at that point?

So if you’re too heavy, it’s time to reduce weight. Leave behind some gear. Take less fuel. Leave behind the passenger who really doesn’t need to come along. Can the mission be flown with the load changes? If so, go the next step with the new load and do the complete CG calculations. Are you in CG? If so, you’re almost ready to go.

Almost? What else is there?

Performance, of course. I’ll discuss that in a future post.

Gross Weight ≠ Center of Gravity.

I was recently pointed to a blog post by Tim McAdams, AOPA’s helicopter blogger. [A side note here: It’s nice to see that AOPA has finally realized that the first “A” in its name, which stands for aircraft (not airplane), does indeed include helicopters. I’ve been waiting more than 10 years for this. But I digress.] The post, titled simply “Gross weight,” is a brief discussion of helicopter gross weight and an accident caused by a pilot attempting to fly over max gross weight.

Commenters Miss the Point

Many of the comments following this post mentioned Center of Gravity (CG) or Weight and Balance (W&B). At least two commenters offered links to their Excel worksheets to perform weight and balance calculations for the aircraft they fly. (I also have my own Excel solution for my R44.)

It seemed to me, however, that they were missing the point. The article wasn’t about CG or W&B. It was about weight. You can do the calculations in your head for that:

+ Helicopter Empty Weight (including oil and unusable fuel)
+ Weight of Fuel On Board
+ Weight of pilot and Passengers
—————————————————
= Gross Weight

Take my helicopter, for example. Using the above formula for a flight with half tanks of fuel, me, and two people weighing 150 and 200 pounds on board, the formula would be something like this:

+ 1500
+ 150 (that’s 6 lbs x 25 gallons)
+ 500
————————–
= 2150

Because the max gross weight of my aircraft is 2500 pounds and this total is less, I’d be flying within max gross weight limitations. This does not mean I’m within CG — that’s a different calculation.

This is not higher math. A pilot should be able to make this calculation in a matter of seconds without a piece of paper — and certainly without a computer.

In my opinion, this should be the first calculation a pilot makes when planning a flight. If you’re over max gross weight, don’t even bother doing a W&B calculation. You’ll need to offload some of that weight — fuel, a passenger, cargo — before you can even consider whether it’s loaded within CG limitations.

Max Gross Weight

Although the blog post was titled simply “Gross Weight,” what McAdams was really talking about was max gross weight — the maximum allowable aircraft total weight on takeoff. The two stories he related in his post dealt with aircraft that were likely over max gross weight on takeoff. One flight was uneventful; the other ended in a crash.

There are two aspects to a helicopter’s max gross weight:

• 

  IGE Hover Chart for a Robinson R44 Raven II. According to this, a helicopter may be able to hover in ground effect at max gross weight at 6,000 feet when it’s 95°F out.

  There’s the max gross weight for the aircraft, which is something you should never — in a perfect world, anyway — exceed. The manufacturer has created this limitation for a variety reasons and the ability to lift off or hover may not be one of them. For example, it could be structural. Would you want to fly a helicopter over max gross weight if you knew that every time you did, you could be putting additional stress on the airframe/blades/etc. that could lead to a failure in a future flight? I wouldn’t.

• There’s a practical max gross weight as it applies to high density altitude. Look at the performance charts. Can you maintain an IGE hover at max gross weight when the LZ is at 6,000 feet and it’s 95°F out? I do a lot of high density altitude flying in Arizona and I’ve experienced a wide range of weight and density altitude situations. I can assure you, max gross weight might be a limiting factor at sea level, but it’s an optimistic number at high DA.

I flew at the Grand Canyon one season and got to know the operation very well. Some helicopters simply weren’t as powerful as others — even within the same make and model. For example, on a hot day, if I were flying a certain ship, I wouldn’t take a load that put me within 200 lbs of max gross weight. To the company’s credit, they knew which ships were the “dogs” and tended to load them lighter. I distinctly remember being asked over the radio while waiting on the pad if I could take 4000 pounds in the ship I was flying that day — one of the dogs. It was over 90°F and the ship had been flying sluggishly since noon. When I said no, they shuffled the passengers around to lighten my load.

It’s Up to the Pilot

McAdams’ first story explains how one pilot refused to take a heavy load while another stepped right up and took over. He quotes a non-pilot bystander as saying “Now there goes a real pilot.”

This is the attitude that’ll shorten a pilot’s career — and possibly his life. The attitude that says, “I’m a better pilot because I can handle the heavy load that this wimpy pilot won’t try.” The attitude that ignores aircraft limitations and flying conditions just to prove you can do something that a wiser pilot won’t attempt.

You know about old pilots and bold pilots, right?

[Another side note: When I flew at the Canyon, late in the season, the last fight of the day for one of the routes had the pilot facing right into the low-lying sun at a critical point in the flight: just as we were attempting to climb up the side of a canyon wall in a relatively confined area to fly over the North Rim. My eyes are extremely light sensitive — which is why I just about always wear sunglasses outside — and I simply could not see in front of me when I flew this part of the route at this time of day. So I asked the lead pilot to not put me on that route for the last flight of the day. Although he was obviously pissed off, he complied –probably because there were at least eight other pilots on duty who simply didn’t care and would do the flight without question. My point: don’t fly beyond your comfort level.]

Real Weights

McAdams’ second story, about the crashed heavy helicopter, relates how the pilot estimated weights: 150 lbs per passenger. Ah, if only our passengers were that light!

The FAA’s estimate is closer to 190 per person average.

In a perfect world, we’d be able to weigh every single passenger, fully dressed and holding their carry-on things, moments before the flight. Those weights would be used in our calculation. This is exactly what they did where I worked at the Grand Canyon, so they never loaded a ship over max gross weight and always had perfectly accurate numbers for CG calculations.

Nowadays, most of my flights are booked by phone and I have to trust passengers to give me accurate weights when they book. But I don’t. I add 10 pounds to each person. If the caller is estimating the weight of another passenger, I add 20 pounds. It’s better to overestimate the weight than underestimate it.

Center of Gravity

One more thing: being out of CG and being over max gross weight are two separate things. While you can be in both conditions, you can also be in either condition. This means that you can be within max gross weight limitations but still be out of CG.

But I’ll save a discussion of that for another post.