737 hori-stab load

[cjam]

Hi folks,

I am interested in finding out how much force or "apparent weight" is produced by the 737-300 and the -400 tail planes at different cruise c of g's and weights . I've spent quite a few hours scouring the net and tried to work it out from my Kermode but I'm not having much luck. Can any of you suggest a basic formula (I'm a non-engineer) , or even give me a basic ange off the top of your head?
Thanks, CJAM

[gr8shandini]

Sorry, I wish I could help you out, but there's nothing simple about that question. To get an exact answer would require a lot of proprietary information that I don't believe you'll find floating around the web anywhere.

As far as rough estimates go, I seem to remember that trim drag is usually on the order of 3% of induced drag, which would mean that the tail load is roughly equal to 3% of the GW.

[Genghis the Engineer]

For non manoeuvring flight, if you take moments about the CG datum (or any other convenient longitudinal station), consider the weight at wherever the CG is, and lift to be acting at quarter chord, then the tailplane load to be acting at quarter chord of the tailplane - a bit of quick algebra will drop an answer out.

It won't be exact, because of the pitching moment effects of the mainplane and the lift on the fuselage, but the answer should be within 10% or so of the truth, which will give you a fair idea.

For working data, you just need a 3-view of the aeroplane, and a W&CG report. If you haven't got a suitable W&CG report, rumour has it that the example aeroplane used for the CAA/JAR ATPL exams is a B734 so you can create a reasonable approximation from that.

G

[gr8shandini]

That'll come close. But the lift acts at ~25% MAC not just 1/4 chord (I know that's what you meant, G, but it could be confusing for some). If you have a W&B sheet that lists both MAC and FS, that'll be pretty easy to come up with. If not, you're going to need a scale drawing of the aircraft and some good geometry skills.

Just curious, what are you using this data for anyway?

[Mad (Flt) Scientist]

You may be able to come up with an estimate by an entirely different means, if you have the right info to hand.

IF - and it's the big IF - Boeing have provided any kind of sensitivity info in your cruise info as to the effect of cg on range/fuel burn, you can use that info to derive the apparent weight due to trim.

Let's say they've told you that every 5% of aft cg movement gains you 1% in range, or fuel burn, or whatever.

Simply work out how much change in mass would give you the same change in range/fuel burn, and bob is at least a close relative - that is pretty much the tailplane trim load.

(I'm ignoring direct tail drag changes, for example, but those are definitely second order for any reasonable tail load).

Apologies if you're doing this to derive the cruise info, obviously it won't be available as a starting point in that case ....

[cjam]

Thanks a lot for your replies , I am just interested in fuel efficency and became interested in tail loadings. I can't find any data from boeing on what sort of fuel savings come with each percent MAC so thought I might try and look at it practicallly and esimate it myself. The loadsheets I have don't have FS so I am not sure of the distances involved for the moments.
Pretty much stuck really. The info I'm after at the end of the day is really the advantages gained between an a/c the way we load now and the same a/c loaded with an aft c g.
Anyway, thanks again, I'll keep plodding along! Cheers.

[Mad (Flt) Scientist]

It's not 737 data of course, but if you're looking for some order of magnitude numbers, on the Global Express (this is taken stright from the Flight Planning Manual) the effect of a 5% cg shift is:

Climb Perf : 3% on time, distance and fuel
Cruise : 1.5% on fuel burn (and thus range ...)
Holding : 1% on fuel burn

[cjam]

Thanks MScientist,
That is helpful, cheers.