@LSLGuy, first of all thank you for once again taking the time to provide very thorough and informative information about aviation stuff. I never knew about the inherent nose-heavy flight dynamics and that, as you explained, this is for reasons of stability. Who woulda thunk it (or possibly, “who would of thunk it?”). Any sort of force required of the horizontal stabilizer, whether tail-up or tail-down, must necessarily contribute to drag, even if minimally, but it appears to be unavoidable.
I had a silly question but I think I figured it out. I wondered, if the horizontal stabilizer is such as to normally provide a downward force on the tail and nose-up on the airplane, why it is that the tail rises very soon into the takeoff roll on a tail dragger (I’m a pretty good DC-3 pilot, though sadly, only on Microsoft Flight Simulator! – the DC-3 was an excellent free third-party add-on.),
I guess the obvious answer now that I think about it is simply that the angle of attack of the horizontal stabilizer provides a great deal of (upward) lift when the tail is down and thus raises the tail to a position of aerodynamic equilibrium once the plane is moving fast enough, at which point it increasingly produces the nose-up tail-down force you describe as the plane accelerates and transitions from static balance to aerodynamic balance.
Do cargo aircraft have a big 'ol center-of-mass marker in the cargo compartment to make it really obvious where to start loading stuff? I know that modern craft have load cells in the landing gear to ensure the mass/CoG is within limits, and that in principle the pilot should be doing a full CoG calc each time anyway, but it does seem like there may be an advantage to having a bright painted line to help reduce human error (like using a sharpie to mark “amputate THIS leg” for a surgeon).
The answer is even more simple than that. The tail rises on a tail dragger because the pilot holds the stick (or yoke) forwards for the specific purpose of lifting the tail. With the tail up you can see better over the nose and there is less drag, it’s also a bit nicer on the tail wheel. I’m sure there are other reasons and exceptions that I’ve forgotten. You have to be careful with how you do it. You don’t want to dig the prop into the ground, and as the tail comes up, gyroscopic precession through the prop will cause the nose to swing to the right (on a typical clockwise rotating prop as viewed from behind).
If the tail is rising all by itself then that will be to do with how the elevator trim is set.
It doesn’t have to be this way. Back when I was regularly flying a Pitts Special I would sometimes hold the tail on the ground until take-off, which happened very quickly.
No, because the weight of the cargo doesn’t necessarily correspond to the bulk. The load distribution is calculated using software or paper trim sheets by the load controller. Load instructions are then given to the loaders and they use that to ensure the load is correct.
When I was flying BAe146 freighters the “cans” of freight were all numbered and the load sheet showed which can was in which position. The best the pilots can do is confirm the cans are in the proper order. Most of the time we couldn’t even do that because we weren’t able to watch the loading and most of the can numbers aren’t visible once they are loaded. As a gross error check, we would match the front can (which we could see) with the load sheet and trust everyone had done their job properly.
Cans going on:
Looking down the side of the front can to check the number:
While that makes perfect sense, surely the angle of attack is a major contributor. If you look at the pic below of a DC-3, the angle of attack of the horizontal stabilizer with the tail on the ground and the elevators in a neutral position is around 15 degrees, which should provide a major upward force on the tail all by itself as the airplane accelerates, no? (It’s also surprising, compared to modern airliners, how big the rudder and elevator surfaces are.)
Yes it is a contributor, I guess I’m just saying that regardless of the tail plane’s natural angle of attack on the ground, the pilot is in control of the elevators and can either hold the tail on the ground, lift it up early, or hold the stick approximately neutral and allow the tail to fly itself off the ground. Whatever happens is a conscious decision.
Here’s an article about flying the DC3 for the author’s first time where they talk about positively getting the tail up.
Once into wind, the Dak turned back into a pussycat. Power was applied by the pilot flying (me) and completed by the co-pilot to give 48in of boost at 2,700rpm, and I immediately applied a large amount of forward push to bring the tail well up and level the aircraft by about 50kt indicated airspeed. Suddenly the forward field of view was outstanding! Rotate was at 82kt and the aircraft lifted off effortlessly. Gear was brought up by the co-pilot immediately because this is not an aircraft in which to have an engine failure after take-off with the gear still down.
@Richard_Pearce of course nailed this in his various responses so I’ll add some details and anecdote.
Somebody (or some software) is doing a full CG calc on every freight and passenger flight. None of this stuff is WAGged. At least in airline use, any landing gear load cell measurements are used as gross error checks, not as the primary calculation.
Unitized cargo (“cans” or pallets) is carefully weighed before loading. For wide-body passenger aircraft, the ordinary checked passenger baggage is individually weighed then loaded into cans which are weighed again. In both the main deck of a freighter and the below decks area of a wide-body passenger or freighter, each position where units can sit is marked. So the load plan specifies that unit #123 goes in position A, unit #876 goes in position B, etc. They are loaded in order (more below) and locked into their labeled spots.
Loose last-minute passenger baggage is also weighed individually. On a widebody the “gate check” baggage is loaded loose into one designated separate below-decks compartment. On really big airplanes (777/A350 & up) that compartment is large enough to need to be logically divided into marked sub-sections and it’s tracked which specific bag goes into which sub-section so the balance calcs are complete. A heavy fabric or webbing barrier prevents the piled bags from sloshing between sections.
On narrow-bodies there are 2 or 3 physically separate compartments below decks. Which are each divided into 2 or occasionally 3 logical sub-sections. Again the individual bags are weighed and are piled in the appropriate sub-section and in many cases retained there by a webbing barrier.
For customer service reasons we can’t weigh the passengers. Nor is it practical to weigh their carry-ons. There are exceptions on very very small airplanes, like 9-20 passenger jobs. But not on the larger ones.
Normally we just assume X pounds per passenger (inclusive of assumed carry-ons) and the system knows which seats are occupied to do the balance calc. In the case of airlines with scramble seating, they have some statistical knowledge of how any given percentage of full load distributes itself.
On my airplane we can carry about 170 passengers. At about 200# each (I forget the exact number & it varies by season) that’s 34,000 lbs of movable load. If it all went forward or aft at once that’d be big enough to matter. Normally it’s spread out enough that minor variations e.g. a family re-seating themselves together doesn’t matter.
But …
In the COVID era with many very light loads a problem arises. First Class still wants to be full, whether of passengers paying the no-kidding FC fare or of lesser folks offered a free or low-cost upgrade, but there’s nobody sitting back in the tail to offset them. So we end up with an out of balance situation. Which means we either force some FC seats to remain empty, or we force some people to sit way back in row umpteen-lots when they’d rather be near the front of coach/economy/whatever term = rows 10-15 for quicker boarding / de-boarding. And once we force folks into those undesired seats, the FAs need to ensure folks stay in/near their assigned seats throughout the flight. Pax love the extra room, but they hate the seating restrictions. Oh well.
Cargo has their own version of loading follies. A parked airplane is a tricycle with ~10-15% of the weight on the nose gear. If you fill the ass end up first with heavy cargo, you get a tail sit. Oops. Funny to look at but the Boss will be PISSED.
So in addition to figuring out the end-state balance for flight, the load planning process also has to establish that all along the way during the loading itself, enough but not too much weight is placed on the nose gear.
Some airplanes, notably the DC-8, had/have an actual support post that was hooked to a connector under the tail to prevent a tail sit. In those airplanes all the main deck cargo comes in from the big side door just aft of the cockpit. So you’ve got to slide the growing load aftwards carefully. Just shoving each unit in turn all the way aft and locking it down would tail-sit every time. Unloading had the same concerns; you couldn’t just remove each unit in turn from the front. You need to slide the whole snake forwards so the empty space accumulates at the rear. The tail stand made that a less critical process.
The 727 had a similar issue, even the passenger versions. The tail stairs were reinforced and provided that same support post function. Decades after we stopped loading people through the stairs it was still policy on every arrival to lower the stairs to prevent a tail sit during either unloading or re-loading.
An apology to @Richard_Pearse for failing to spell his screen name correctly. It’s not like I don’t know it. Sheer carelessness on my part and I missed the edit window. I’m more like DumbGuy today (There really is an inactive user with that name too)
My late-stepfather’s father lived in Florida. Mom and Turkka went to visit, and then flew off to an island (IIRC) on a twin turboprop. Turk was a big guy. I think he was like six-three or six-four, and weighed 250 pounds. My mom was tiny; about five feet and 100 pounds. Mom got a seat in the back row, but the gate personnel apologised, saying that Turk was too big to be seated in the cabin and would have to take the right seat in the cockpit. Apologies were hardly necessary; Turk (and my mom, and my dad) was a pilot. For him, sitting in the cockpit was a treat.
I’m reminded of a story I heard decades ago. I do not vouch for its veracity. A couple of hunters were in Alaska and wanted to be flown out with the moose (or bear) they’d killed. The pilot had (IIRC – it’s been decades) a Cessna 185. He expressed some doubt, but the hunters told him their last pilot told them they’d be OK. The plane gets loaded, takes off in ground effect, and then crashes with no injuries. The pilot apologised but the hunters said, ‘Oh, that’s OK. The last pilot didn’t even make it that far!’
I’ve told the story before, but I spent one season flying Navajos, specifically the Chieftain model, in the Grand Canyon.
We carried 9 passengers with no baggage beyond women’s purses. They were usually Chinese folks on package tours, so generally small with a few exceptions.
We weighed each person and always had to put the biggest one (often an obese male) in the co-pilot seat. Never the delicate cutie. Damn.
I love that joke. Even seeing it coming a mile away I get a laugh. Once in awhile it’s Cajuns huntin’ 'gators in the bayou. Still Comedy Gold.
When I lived in L.A. and coming up to Bellingham, I’d catch the last Horizon flight from SEA to BLI. The DeHavilland Dash 8s didn’t have a lot of cargo capacity, and more than once my bag did not fly with me and I’d have to wait until the next day to get it. After the first time, I learned to carry on necessities in a rucksack.
I don’t know about a 185, but I know you can easily gross out a 172 before you space out.
Of course, exceeding gross weight is a separate issue from getting CG out of bounds. I had a scheduled flight with a pilot and one other friend in a 172 a few years ago. When I invited a fourth along, the pilot canceled the 172 reservation and reserved a 182 instead, saying that a 172 couldn’t fly with four adults aboard.
This of course led to a discussion of why they ever built a 4-seat airplane that couldn’t fly with 4 people aboard. The pilot’s idea was that these planes were built in the 1960’s and 1970’s, at a time when the manufacturers (Cessna in particular) were pushing the idea of “family airplanes”, suitable for flying Papa Bear and Mama Bear in the front and the Baby Bears in the back.
Thanks both for the wealth of information. That is especially interesting about the new behavior seen with COVID (though personally, I always sit very nearly at the back when I have a choice, though ideally with a few rows between me and the restrooms).
That said, what I had in mind with my comment wasn’t necessarily modern craft and attitudes towards preflight checks. I was thinking more of someone still flying a stripped-out DC-3 or the like out of BFE and being a little approximate on the details. Or, perhaps, a military situation with too much urgency for the full suite of CoG calculations. CoG accidents do still happen, so at least a few people out there aren’t being as diligent as they should be. And in any case, I support the idea of “defense in depth”, so a simple sanity check (is this heavy as shit item roughly at the CoG?) seems like not a terrible idea…
IIRC, the Skyhawk could carry 740 pounds with full fuel. That’s 185 pounds per seat. At the time, the ‘standard weight’ for an adult American male was 170 pounds.
Even the rear loading 146 had a tail-stand (yellow support in the photo). The cans lock in to position so you could load it just with cans towards the back and cause to sit on it’s tail.
P.S. Quite important to ensure the tail-stand is removed prior to take-off! The 146 freighters had a tail-stand light in the galley area. The problem was that it said nothing about whether the stand was connected to the tail, it just indicated that a stand (not necessarily the one being used by the loading crew) was in its stowage in the lower rear cargo hold.
At least in big airplanes, the CG accidents I’ve seen / heard about are driven by cargo shifting. e.g. National 102. Which is still a failure on the part of the loading crew.
As you suggest, it’s a darn good bet the scruffier parts of aviation handle a lot of this stuff by eyeball. The guys on Ice Pilots NWT probably WAGged more than they calculated, at least on their smaller aircraft. Certainly even smaller bush planes worldwide adopt the “heavy stuff mostly in the middle and not too much total” approach to weight & balance.
A gotcha is that big airplanes can tolerate wide CG ranges. But they have the capacity to hold a lot and a lever arm long enough to get not only outside CG range, but WAAAY outside CG range.
For a Beaver or a Caravan the pilot can see the entire load, may well have loaded it him/herself and has some hope of eyeballing it to the “safe enough” range. Fat chance doing the same on DC-4 or larger aircraft. A DC-3 full of chickens & people & luggage is potentially eyeballable with enough practice. But throw a few crates of metal machinery in the mix and now eye-balling it is playing with fire.
Looking at a 172M POH, the highest useful load is 913 pounds. With 42 gals of fuel (the one I fly has 52 gallon tanks but we don’t fill all the way) that uses up 252 pounds. Which leaves 661, or 165.25 per person.
