Help please! DC10 question about cruising

[UKJim 502]

So I am cruising at 5000 feet at 250knots - the autopilot is switched on and flaps are at 0 - but when I look at the dial in the VC is looks like it is constantly at 5 degrees incline and on the outside it looks wrong too. I have taken a look at the manual and could not find anything different.

This is what I got ...

Can someone please say if this is correct? Or should I have something like flaps set to a different level for cruising at 5000 feet?

Cheers,

Jim

[allardjd 1,853]

It all comes down to weight and CG. For an AC to maintain level flight, the wing and elevator must create a net amount of lift to exactly equal AC weight. Wings make more or less lift by increasing or decreasing angle of attack. If you fly slower, you'll have to pitch up a little. If you fly faster, the nose must come down. If the weight is less, less AoA is needed than at full load. Flying at a higher altitude requires a higher AoA, all else being equal, than the same flight parameters at a lower altitude, because the air is "thinner" and the wing needs a bigger bite to produce the same lift at the same IAS.

CG complicates things. In most AC, most of the time, the ideal longitudinal CG location is near the center of lift, which is often expressed as a % of MAC (mean aerodynamic chord). As the CG is displaced longitudinally from the center of lift, the elevator must contribute more and more pitching moment, either up or down (usually down in most AC) to maintain the wing's AoA at the required value to offset weight. If the elevator is producing a large down force, the wing must make even more lift so that the net lift (positive from the wing, minus the negative lift from the elevator) equals the AC weight.

Swept wings further complicate it.

In your case, it may be entirely normal for that AC at that weight, speed, density altitude and CG, or it may be that the CG is not optimal.

A final possibility is that the designer of your FS version of the AC screwed the pooch and just got it wrong.

By the way, you should NOT need flaps for level flight at 249 knots at 5,000 feet.

John

[UKJim 502]

Yeah I thought not - I think its the autopilot that is not smart enough on this model doing this but who knows its all good - I just wanted to know if I was flying her right

Thanks John

[TedG 0]

Something you might try to counter the issue is climb 500-1000' higher than planned altitude and then descend to whatever is planned for cruise (although 5000' for a big ole jet like that is pretty inefficient) prior to cutting to cruise power. Depending on the aircraft you may need to add more throttle to keep a low AoA until the fuel load burns off.

As John mentiones its related to load, CG, and AoA required to maintain lift.....what I am describing is called "getting on the step" which is similar to trimming and getting a boat cruising efficiently. I discovered this myself when working on a recent B377 review thanks to a nice article in the POH from A2A sims. I tried the procedure on other aircraft when heavily loaded and it works quite well.

In my better payware aircraft there are definitely warnings going off if the flaps are deployed above a certain speed....and you would probably rip the wings right off too!

**edit**Most POH's have a flap deployment schedule related directly to airspeed and in some cases clearly printed within the cockpit itself. Similar numbers appear in relation to landing gear (if retractable).

[allardjd 1,853]

Ted's tip is right on - I'd forgotten that - an intentional overshoot in the climb and then descending back to your intended cruise altitude does help sometimes (usually when heavily loaded) in some AC. It's worth a try. As little as 200' is enough in a GA single but for an airliner it's going to take a bit more. I wouldn't blame it on the autopilot.

John