Greetings!

(I broke this discussion out into a new thread to help keep it together.)

Dave brought up CG placement in another message and this brought up another thought. In autoracing, there were teams reportedly who had a movable weight in their cars that would allow them to alter their CG during a race. It was ultimately outlawed, but I think you know where I'm going with this one.

I'm sure there are maneuvers that would benefit from a very rearward CG, and others that wouldn't. Thoughts? Bad idea?

Ben

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Ben Chiu
-OLR.PL v1.76-

• RE: CG manipulation,DHamblin, 18:19z, 05-20-02
  • RE: CG manipulation,Ben Chiu, 21:30z, 05-20-02
    • RE: CG manipulation,vgbaron, 23:45z, 05-20-02
      • RE: CG manipulation,Mike_Greenwood, 02:52z, 05-21-02
      • RE: CG manipulation,Ben Chiu, 06:12z, 05-21-02
        • RE: CG manipulation,vgbaron, 19:21z, 05-21-02
          • RE: CG manipulation,ChipB727, 23:54z, 05-21-02
• RE: CG manipulation,avbug, 23:02z, 05-26-02
  • RE: CG manipulation,Ben Chiu, 08:21z, 05-27-02

Dave Hamblin
(formerly DaveH on old site!)

Bad only if it became stuck at the rearward position. I guess the question is whether the benefits outweighed (literally) the weight and complexity to do this.

In my C150H once trimmed you can cause a pitch change by leaning forward or back (if passenger and pilot do so its kinda fun....)

I wonder if it would be disconcerting to the pilot having a changing CG (hence trim).

Greetings Dave:

> Bad only if it became stuck at the rearward position.

Yep, there'd need to be some sort of fail-safe system (a spring could return it to the forward position if the positioner gives out).

> I guess the question is whether the benefits outweighed (literally) the weight and
> complexity to do this.

That's really the issue. I think the weight shifting mechanism could be very simple, and the weight itself doesn't need to be very large (you just need to move it further).

> In my C150H once trimmed you can cause a pitch change by leaning forward
> or back (if passenger and pilot do so its kinda fun....)

Reminds me of the FAA inspectors trying to get a Piper Tomahawk out of a spin when they were researching them...

> I wonder if it would be disconcerting to the pilot having a changing CG
> (hence trim).

I'd imagine it'd only be used while in flight for the performance or certain maneuvers and we would be returned to "normal" for TO and landings.

Ben

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Ben Chiu
-OLR.PL v1.76-

>
> That's really the issue. I think the weight shifting mechanism could be
> very simple, and the weight itself doesn't need to be very large (you
> just need to move it further).
>
Probably small tanks filled with water or another liquid - pumped back and forth. Emergency blowout to empty the system in case of problem.

Vic

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Vic Baron
-OLR.PL v1.76-

Hi Vic,

>>Probably small tanks filled with water or another liquid - pumped back
and forth. Emergency blowout to empty the system in case of problem.<<

Excellent idea, and that's how the Concorde does it...except with fuel. Dunno if the emergency blowout is a great idea when you're using go-juice though <g>.

--Greenie
**6 miles SSE KSJC**

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Mike Greenwood
**** OLR.PL Build 1.73 ****

> Probably small tanks filled with water or another liquid - pumped back
> and forth. Emergency blowout to empty the system in case of problem.

Ah, the Concorde concept (except they use fuel). The only problem I see with that is if you're in a flat spin or some other unusual attitude, the water may not want to exit your drain. Nothing insurmountable, so I think that's an idea worth exploring.

Ben

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Ben Chiu
-OLR.PL v1.76-

> > Probably small tanks filled with water or another liquid - pumped
> back
> > and forth. Emergency blowout to empty the system in case of problem.
>
> Ah, the Concorde concept (except they use fuel). The only problem I see
> with that is if you're in a flat spin or some other unusual attitude,
> the water may not want to exit your drain. Nothing insurmountable, so I
> think that's an idea worth exploring.
>

Well you could have the system pressurized but, in a flat spin, I would believe the centrifugal force would empty the container no matter which end it were on.

Vic

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Vic Baron
**** OLR.PL Build 1.73 ****

>> > Probably small tanks filled with water or another liquid - pumped
>> back
>> > and forth. Emergency blowout to empty the system in case of problem.

I would have to agree with VG on the transfer of fluid. In the Extra it takes a lot of weight to make a change.

>>The only problem I see
>> with that is if you're in a flat spin or some other unusual attitude,
>> the water may not want to exit your drain. Nothing insurmountable, so I
>> think that's an idea worth exploring.

I believe the centrifical dump should work in a flat spin depending on the placement of the outlet.
Have to get a bottle of water and go try it in mid July.....

Weight shift for trim and control is one of the most origional and basic forms of aircraft control. In a hang glider, for example, body english-weight shift is everything.

I've flown a variety of aircraft that involved vast changes in payload and consequently CG, throughout the course of the flight. When performing ag, the weight loss and CH change might occur over fifteen minutes as the chemical was sprayed out over the crops. In this case it wasn't a shift so much as a loss, though the CG certainly did change; it required considerable retrimming.

Flying skydivers in Cessna 182's and 206's involved having everyone lean forward during the takeoff. After a thousand feet, everyone would sit back to a more comfortable position, with a significant change in the feel of the airplane, and the trim required. The CG would move aft noticably. The same when people got out of the airplane, or climbed out on the strut or the wing.

When dropping retardant on fires, we would lose 18,000 to 27,000 lbs in varying increments, from 1/8 at a time, to all of it in under two seconds. This created very noticable rapid chagnes in center of gravity. Typically we would drop one aft tank, then a forward tank, then an aft, etc. If this change were done at one second intervals, the result would be a significant porpoising tendency that had to be anticipated to be controlled. Even in large airplanes such as these, someone getting out of their seat and walking to the back of the airplane could be felt in the controls.

In small aircraft such as the Learjet, I've noticed a definite control feel change as someone gets out of the rear bench seat and walks forward to the cockpit. The pitch change can be felt as the weight and CG shift. A degree of attitude change, or more, may be required to compensate for the change. This doesn't sound like much, but it can be felt.

During flight test operations to explore the CG envelope and attendant aircraft performance, very often a sliding weight is used to vary the CG. This weight is usually run on a cable and rod arrangement or with a chain and sprocket, in order to explore a variety of CG applications during one flight. The CG is precalculated, and can be set as required at intervals to conduct the test.

Some aircraft trim systems have used weight shift arrangements using a weight in the floor or in a track somewhere in the aircraft.

Some helicopters, such as the Rotorway Executive (an experimental) must use weights on the skids to compensate for passengers in or out of the aircraft. Some aircraft use a battery case that is mounted on a track, and the battery moves to adjust the CG in flight.

Some major aircraft accidents have occured over the years due to shifting cargo in which the cargo moved aft (or forward) and prevented recovery. Somewhere I used to have photos of a Skyvan that stalled and rolled over into a spin when too many jumpers gathered on the tailgate in preparation for an exit.

Remember that when having a discussion of spins, the aft CG isn't what causes a spin. A spin results following a stall; the aircraft must be stalled first. Many other factors are involved in spin recovery, including aircraft design, power application, control input, etc.

Greetings avbug:

First off, thanks so much for the very detailed insights about carb ice and CG manipulation. I'm sure the others feel the same as I do--well done!

> Weight shift for trim and control is one of the most origional and
> basic forms of aircraft control. In a hang glider, for example, body
> english-weight shift is everything.

That's right. Thanks for the reminder. I tend to use body english when I sit in the back seat of an airplane watching another pilot making a landing.
I doubt my weight shifting/body english actually does anything, but that doesn't stop me from subconsciously doing the same when I go bowling either!

> During flight test operations to explore the CG envelope and attendant
> aircraft performance, very often a sliding weight is used to vary the
> CG. This weight is usually run on a cable and rod arrangement or with a
> chain and sprocket, in order to explore a variety of CG applications
> during one flight. The CG is precalculated, and can be set as required
> at intervals to conduct the test.

This is the kind of mechanism I was thinking of at first, but it may be too elaborate (complexity and weight-wise) for aerobatic application we envision.

> Some aircraft trim systems have used weight shift arrangements using a
> weight in the floor or in a track somewhere in the aircraft.

I never knew that. Can you give us an example of an aircraft that does/did use this?

> Remember that when having a discussion of spins, the aft CG isn't what
> causes a spin. A spin results following a stall; the aircraft must be
> stalled first. Many other factors are involved in spin recovery,
> including aircraft design, power application, control input, etc.

Rgr. I should have explained that my example of a flat spin and rearward CG worst case scenario was to emphasize that in that situation a rearward CG would reduce elevator effectiveness (due to the shorter arm between the CG and elevator) and may make recovery from that maneuver impossible--hense requiring a foolproof weight ejection system in that situation.

Thanks again!

Ben

-= VPC OffLine Reader Version 1.1.0.0 =-
Registered to: Ben Chiu
-OLR.PL v1.76-