Adding nose weight for penetration

[deh6]

After a windy/gusty day a flier at the field was telling me about adding nose weight on his plane for windy/gusty days. How effective might this be?

My thinking is that the weight increases the wing loading two ways. 1) it adds weight, thus increasing the total weight, 2) requires more download from the horizontal stab to offset the more-forward CG, thus adding to the wing load. The increased wing loading then moves the polar so that there is less sink at high speeds, i.e. penetration, but increased drag from the tail load would be detrimental.

Moving the CG forward would also help the stability. One problem we were having on that day was that gusts were such that the planes would pitch up violently (adding to the difficulty of getting back upwind to the field).

This guy was also thinking about moving a lead weight inside a tube in the fuse with a servo, thus giving some adjustment to the CG.

(Obviously the nose weight can't be so large that the horizontal stab loses pitch control.)

Am I on-track with this reasoning?

[nuevo]

I don't agree.

On windy days, you want to increase the wing loading, not alter the CG. Yes, you're thinking is right about the polars, and penetration. If you have a rearward CG already, then you might want to ballast so that the CG moves forward maybe 1/8-1/4", but that's all.

If you add all the weight at the nose, you will need a lot more uptrim on the elevator. Why not just add all the weight right at the CG and no trim changes required?

On a 3-meter plane, I'm talking adding anywhere from 8-24 oz of lead, depending on wind speed. I don't think you want that much weight in the nose.

[Quacker]

You are close...adding weight improves penetration, but the best place for it is on the CG. You can put it just a bit forward to improve stability if you tend to normally trim toward tail heavy. If you go nose heavy, the plane will be more susceptible to zooming from gusts as the sudden increase in airspeed will make the required up-elevator more effective, raising the nose. Many model gliders have built-in ballast chambers--all on or near the CG.

Best,
Q

[deh6]

Thanks for the thoughts.

JonStone,

Added pitch stability would be the reason for putting the weight forward, rather than all (i.e. more since there is less tail download) at the CG.

Quacker,

I'll have to ponder the notion that in a gust a nose-heavy, i.e. a high static margin, plane will "zoom" more. The textbook shows that the short-term oscillation of a nose-heavy plane is stiffer in pitch and more quickly responds to changes in trim. However, I'm not sure that these gusts do the same thing as change in elevator setting/trim.

The nature of these gusts is not clear. If they are horizontal "swirls," then they would cause a pitch up, or down as the case maybe, as the plane passes through them. I had been thinking in terms of active pitch stabilization, such as a control loop with a gyro or angle-of-attack sensor, then this chap at the field sprung the idea on me of using weight in the nose. Actually, he was thinking in terms of putting a lead weight in a tube and sliding it fore/aft using a servo.

Maybe the answer is to wait for calmer days!

[Quacker]

I think that in gusty conditions that most pilots would want:
1. increased penetration
2. neutral as possible, response to gusts.

I believe that adding ballast at the CG is the best way to achieve this. My comment about placing the ballast just forward of the CG was directed toward those that like to trim tail-heavy. Tail-heavy craft perform well but are easily upset because of inherent marginal stability. Correcting tail-heavy trim during gusty conditions might be a good idea.

Best,
Q

[raptor22]

It would be. However, airfoil polars stay the same with weight. It is an aerodynamic function only.

The plane still gets the same max lift/drag at the same angle of attack, but it will glide faster at the same speed with lead.

--Alex

[deh6]

My understanding is that the L/D is not affected by changes weight only if the CG remains the unchanged, i.e. the weight added at the CG. The polar remains the same shape, but shifts so that the max L/D occurs at a high sink rate, and hight speed (the ratio being the constant L/D).

However, if the change in weight changes the CG the drag of the horizontal stab/elevator will change, thus affecting the L/D. Thus adding weight to the nose would lower the best L/D since the more downward lift is required, but it would offer some increase in pitch stability.

[Quacker]

If you shift the CG forward and compensate with increased elevator, your pitch stability with changes in airspeed will suffer. As airspeed increases, the increasing force generated by the elevator will raise the nose. The best pitch stability should be obtained at a point where changes in airspeed do not effect pitch. This is where DLG gliders are trimmed.

[deh6]

I think we are thinking about somewhat different situations as well as use of the term "stability."

The textbook clearly shows that moving the CG forward reduces the time for the plane to stabilize from one elevator/speed setting to another, and that the pitch becomes less sensitive to elevator deflection, however it is talking about the time it takes a plane to adjust small changes when in relatively horizontal flight.

The DLG is quite different since the plane travels nearly vertical after release. As a result, during the near-vertical ascent a forward CG produces little, or no, forward pitching moment (with respect to the neutral point of course). The elevator/horizontal stab therefore must limit the downforce to offset only the forward pitching moment of the wing airfoil (i.e. effect of Cm). However, when the CG is forward and the plane returns to level flight, a large up-elevator is suddenly required to prevent a dive/zoom, and in some cases the leveling out may take place at a speed too slow for the tail to counterbalance the forward CG. It makes sense that in this situation a forward CG makes the plane less controllable.

The issue then seems to come down to the nature of these gusts. Maybe tomorrow will be calm.

[nuevo]

The textbook clearly shows that moving the CG forward reduces the time for the plane to stabilize from one elevator/speed setting to another

What textbook?

Forward from where? Neutral Point?

[raptor22]

I don't know what he is using, but moving the cg forward from any point makes it more stable, with stability being defined as time it takes to neutralize from a disturbance.

--Alex

[deh6]

Textbook: Dynamics of Flight: Stability and Control, by B. Etkin, L. Reid, |Wiley (1996). Very good book but requires some math background beyond high school algebra. They present some root-locus and period time versus static margin graphs that shows the effect of moving the CG forward.

Forward CG is from the Neutral Point, though I think modelers tend to consider a "forward CG" as one that is forward of the recommended CG for the particular plane. Static margin is the difference between the CG and the neutral point, usually normalized so that, for example, .05 (5%) would be a good safe CG location regardless of size.

[surfimp]

To increase penetration, add ballast at the CG. Without trying to sound like too much of a jerk, that's the end of the story.

Adding sufficient ballast to the nose (assuming you could even find the room to do so in a DLG) in order to increase penetration in any meaningful way will make your plane so insanely nose heavy that it will be unflyable. Take if from me, I'm a sloper. I always fly in windy conditions.

Steve