Why This Group is Moribund

"The Natural Philosopher" wrote

I'm not sure which hairs you are trying to split here, or what definitions you could be using differently.

Perhaps the fact that the center of lift is ahead of the CG, and the tail is providing downward pressure to keep the plane stable, as is true in most conventional light aircraft and models?

On Mon, 27 Aug 2007 23:18:44 +0100, The Natural Philosopher wrote in :

Yes--but it can be achieved by a little down elevator instead of by the native lift of the surfaces.

You are, of course, correct.

When we use the term "center of gravity", we are concerned only with one axis instead of three--fore and aft balance. We ignore the lateral and vertical location of that center for all practical purposes.

"The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight. Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance; that is, the location of the CG along the longitudinal or lengthwise axis. ...

"The basic aircraft design assumes that lateral symmetry exists. For each item of weight added to the left of the centerline of the aircraft (also known as buttock line zero, or BL-0), there is generally an equal weight at a corresponding location on the right.

"The lateral balance can be upset by uneven fuel loading or burnoff. The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition. [Figure 1-4] This is corrected by using the aileron trim tab until enough fuel has been used from the tank on the heavy side to balance the airplane. The deflected trim tab deflects the aileron to produce additional lift on the heavy side, but it also produces additional drag, and the airplane flies inefficiently. Figure 1-4. Lateral imbalance causes wing heaviness, which may be corrected by deflecting the aileron. The additional lift causes additional drag and the airplane flies inefficiently. Helicopters are affected by lateral imbalance more than airplanes. If a helicopter is loaded with heavy occupants and fuel on the same side, it could be out of balance enough to make it unsafe to fly. It is also possible that if external loads are carried in such a position to require large lateral displacement of the cyclic control to maintain level flight, the fore-and-aft cyclic control effectiveness will be limited."

Marty

| Sometimes rambling is a good thing. I'd hate to be part of a world where | everything is linear. How boring it would be!

Perhaps I didn't make myself clear.

I'm not saying it's a problem. I'm saying it's a remarkable accomplishment, and wanted to point it out lest somebody miss it.

Actually I'd meant to add one more line to my post, but sent it off before I did. I'll include it here --

Good job! (to Marty)

On 29 Aug 2007 15:10:19 GMT, "Doug McLaren" wrote in :

Thanks!

Back to the original topic: The group isn't moribund. It's just playing dead in the great tradition of the Possum Lodge. ;o)

Marty

wrote in :

I just refuse to play either possum, or, moribund.

Unless.... my wife has just found a job that needs doing >:-)

In order not to be moribund I went off flying yesterday (tuesday)

I wanted to try my recently built and (75% own design) glow plane as it's first flights indicated it was going to be a "good one". If I had thrown away the original fuselage design I could have claimed

100% own design.... but I had kept it... penny pinching I guess.

The forecast was for 2 mph winds so, more in hope than expectation, I also took along a recently built A2 glider, originally designed back in the 50's and complete with 66 inch undercambered wing.

First off the side of the slope was my little glow model, all 36 inches wingspan, and it does indeed fly a treat. Several flights confirmed that it was just the thing for throwing in the back of the car and nipping off for a quick flight when the occasion is right.

Having had fun with the small glow plane it was time to give the A2 glider a go, this was originally for free flight but I had fitted radio for rudder and elevators.

There was virtually no wind and I was doubtfull that I could get the glider airborne. First throw wasn't good, a brief glide and back to earth as there was no lift.

O.K one last go... I threw it out again and ended up scratching along the hillside trying to maintain height. A few minutes later I found a little bit of lift and gained about 50 feet. Another few minutes and I had gained enough height to get some confidence. That was it ! Thermal after thermal came along and it was about an hour later that I brought the glider back in again. My kneck was aching and so were my legs and I needed to quench my thirst ! The A2 was the only glider I have that could have got away in the extremely light breeze, but in these conditions it comes into it's own.

A really excellent flying session, and just goes to prove that we DO actually fly what we build !

I must have been suffering from euphoria, today I set to and did some chores !!!

Reg

Backwards. The center of pressure is BEHIND the CG for stability. With the pressure behind the weight, the tail's downforce balances it. Airplanes with the CP ahead of the CG are illegal in the real world and require a lifting tail. The last airplanes to do this were built during WWI. If they stalled they were unrecoverable. Canards, of course, are different, but the legal requirement is that the nose falls when the power is reduced so that airspeed is maintained without unusual pilot skills.

Wrong again. That is a most unstable configuration. All aircraft have a CG range that must have the CP behind the aftmost CG, for the above reasons. Many airplanes have gone out of control and crashed when they were loaded too far aft.

See

Regarding "moment": the airplane is longitudinally balanced around its CG. An airplane that turns out nose-heavy, let's say, will need weight added to the tail to balance it. But it doesn't end there: this airplane will now have some less-than-desirable characteristics. If it enters a spin, the tail's weight and the nose's weight are rotating at different levels around a common pivot axis, and they want to come into line with each other: nose up, tail down. Because of this, a balanced airplane that has increased weight at both ends will have a stronger tendency to go flat in the spin and may become unrecoverable.

Dan

Dan

On Wed, 29 Aug 2007 12:26:55 -0700, Dan_Thomas snipped-for-privacy@yahoo.com wrote in :

AH--that's it. I was thinking yesterday that I had gotten it backwards ... I had the right picture of the needed trim (down elevator) and got the reasoning why all wrong.

Great stuff. I don't see the need to use down pressure in recovering from the spiral dives I or my students have gotten into, but it sounds like potentially life-saving information for a full-scale pilot.

Marty

As speed increases, so does the nose-up tendency. It's the counterpart to the nose's tendency to drop as speed decreases. In a spiral dive, the airspeed can increase to well above Vne and when the wings are rolled level the nose will come up by itself. If the nose rises too quickly, the G forces get too high. The pilot may have to push ahead a little to control it. Many airplanes, especially slippery ones like the Beech Bonanza or Cessna 210, have entered spiral dives when their pilots flew into cloud and their inner ears got confused. Often happens to well-heeled people who can afford expensive toys but don't have the time to take instrument training and who sometimes think they know it all anyway. They get into trouble in the cloud, think they're in a turn when they're not, bank the airplane without back pressure (thinking they're levelling it), get into the spiral dive, pop out of the cloud at high speed and either hit the gound, or see the ground coming up fast, pull back hard to try to recover, and the high stabilizer forces caused by both the turn and the pullup usually pulls the tail (stabilizer/elevator) off the airplane first. Because CG is ahead of the CP, the thing now instantly flops over forward onto its back and the wings fail downward. The airplane is found in many pieces spread over some distance. See

Dan

On Wed, 29 Aug 2007 15:36:42 -0700, Dan_Thomas snipped-for-privacy@yahoo.com wrote in :

OK.

With the planes I've gotten into (and out of) a spiral dive, leveling the wings and gently pulling up elevator has been sufficient.

I can see where full-scale planes would be fighting much more severe forces of every kind (lift, drag, G).

Sobering stuff.

I read "178 seconds to live" years ago. I know the details about the study are a little bit off, but it's also clear that moral of the story is true: it's easy to get into the graveyard spiral and not so easy to recover from it.

Marty