SinCityJets

I apologize if this is a stupid question, but I am curious.

If the answer is; you want the plane's CG correct for landing, then gear down and a full UAT are usually not the only landing configuration.

What I mean is, I have had planes where a "hopper" tank was set forward of the CG, and I would typically land with 1/2 to 3/4 of the hopper full. With that being the case, would it be better to CG your plane in it's true landing configuration (ie 1/2 hopper full, or whatever fuel you normally land with in the tanks)?

I appreciate the responses.

Chad

LGM Graphix

My Feedback: (22)

I somewhat agree Chad, but I would prefer to be landing a little nose heavy than tail heavy in most cases. I've had situations where due to some circumstance (waiting in the pattern, forgot to set the timer, to busy trying to trim out a plane etc) where I've ran out of fuel, if I had balanced with the center tank half full, I'd be tail heavy in a deadstick configuration, that would almost certainly spell crash for many airplanes, no power and tail heavy. As with many things in life, it's probably safer to err on the conservative side and know where you're at, and trim things out for a slightly nose heavy landing than risk running out of fuel for whatever reason and being tail heavy.

With a few exceptions, most of my jets don't change a huge amount on the front tank half full or empty in the CG, the Super Bandit is one of the exceptions, but even that one doesn't change a significant amount. My CAI Razor was really bad for this, and my Raptor is a little noticable in regards to this.

I think maybe the smarter solution is plumb our tanks so that the front tank drains first, then the saddle tanks into the UAT, is there any reason we don't do this? I have been thinking about that in my Bandit and Raptor, then you can balance it more critically and whether you have fuel or don't, what is left is on the CG.
Anyway, that's my thoughts behind it.

Gordon Mc

My Feedback: (11)

Chad - it depends I guess on the size of that hopper tank.

If you set the CG with the hopper 1/2 to 3/4 full to match your normal landing, what happens when someone farting around on the runway when you want to land, or 3 guys stacked up to land ahead of you, causes you to have to stay airborne longer ? Now you drain your hopper tank (ahead of the CG) draining more than normal, and your CG moving backwards.

A significantly tail heavy aircraft isn't fun, so you try to avoid that wherever possible. So set up for what is close to your worst-case (empty) scenario (4oz in a UAT doesn't make THAT much difference compared to 1/2 or 3/4 of a 20oz hopper that's futher forward.

Gordon

jetfreak

My Feedback: (9)

Chad my 02,

You balance the cg based on how you expect to land. Many gear sweep forward causing a slight change in CG towards the nose. nose heavy is ok. When the gear come down it will move the cg slightly back. So you always measure with gear down if they sweep forward on retraction. That way there is no chance it will be tail heavy when the gear come down. I believe the change is very minimal but you know in this hobby it is always better to follower the "safer" way.

The UAT in theory should always be full. If you land and you got into your UAT, you are lucky to be landing with a running motor (turbine speaking). Thought behind this is that with the UAT always full you want to balance with that weight accounted for no matter if it is forward or aft of CG.

Hope that helps...

sean

Gordon Mc

My Feedback: (11)

Jeremy - I know some peole HAVE replumbed Bandits etc to drain the forward tank first, and are happy with the results.

I don't know if BV's rationale for the Bandit plumbing matches mine, but when I set up the Jag with my custom tanks etc, I thought long & hard about how to run all the plumbing.

In the end, I adopted the same approach as BV has for the bandit, with the dual mains (in-wing in my case) T'd into a (more forward) central tank and then to the UAT, rather than having the central tank feed into the mains which then go into the UAT. My reasoning was that I expected to get uneven fuel flow in some flight conditions from the 2 mains, such that one could feed fuel and the other air. Based purely on volume, it seems safer for that air to go into the larger central tank instead of directly into the UAT. In other words, I saw that central tank as a kind of pre-UAT UAT.

Dunno if that makes sense to anyone other than myself.

mr_matt

My Feedback: (10)

If you have so much fuel in front of the CG that landing in a near empty fuel state results in a dangerously aft CG, then that remaining fuel is just dead weight.

Move things around so that you can balance with less fuel on board, at least the plane will be lighter overall.

mr_matt

My Feedback: (10)

I agree Gordon. Having 2 tanks, especially flat ones like BobCat wing tanks feeding into a very small air trap (like a UAT) is not an ideal situation.

The other alternative is to run everything is series. You have a bit of lateral trim to deal with but int he set ups I have seen (fuse tank not wing tanks) it looked manageable in flight

highhorse

My Feedback: (2)

I set mine (jet and otherwise) based on the worst case scenario as far as fuel and gear are concerned, without specific regard to landing configuration. To set it in "landing configuration" for it's own sake because "that's how it lands" is, IMO, arbitrary and potentially dangerous to your pocketbook. It implies that other flight conditions are somehow not important, or even critical.

Worst case scenario = the condition or combination of conditions that will move the CG farthest aft.

I don't EVER want a case of an aft/unstable CG, so I make initial calculations based on the UAT empty (because it's usually located forward, and if I one day screw up and run dry I don't need the added complication of an unstable airplane due to "missing" nose weight) and the LG in the most AFT configuration (up if, for instance the NG retracts aft, and down if it retracts forward).

Every other condition, then, is more forward and safer. If you have too many variables moving the CG too far forward, of course, you will have problems there too. But fwd CG issues are almost always fairly benign, where too far aft can cause a total loss very quickly.

PS: That was certainly NOT a stupid question!!

mr_matt

My Feedback: (10)

duplicate sorry

SinCityJets

I appreciate the responses. I knew people much more intelligent that I would have the answers.

Chad

DelGatoGrande

My Feedback: (23)

set your cg with landing gear down..cose this is usually how planes land.



as for fuel with this way mention every where you will be always nose heavy i guess(safe side) .. as much fuel as you burn

yeahbaby

My Feedback: (21)

Good points above Gents

my technique is to balance all fuel tanks empty (almost teeter tottering in the middle of the recommended CG range) and try to maintain as neutral a CG balance as possible. most fuel installations have the UAT forward of the CG, so even if you get pinned in the stack of airplanes waiting to land and flame out....you can be assured your CG will still keep you safe even with zero fuel. as Gordon mentioned, the 4 oz left in the UAT isn't going to make a huge difference (unless it 10' in front of the CG highly unlikely scenario) except for the B1 perhaps

draining fuel from a forward tank first can be risky, even if the tanks are on the CG. I think you'd find yourself adding unnecessary lead in this configuration. i could be wrong though.

ultimately, assuming a forward or "its nose heavy so that must be safe mentality" isn't necessary the way to go either. a nose heavy plane can create other problems.

I experienced this with my SM F15 years ago. balanced in the recommended range it would tuck the nose in the turn (just like Buffallo Bill in Silence of the Lambs) . the tucking tendency was removed when i slowly removed lead strips and moved batt packs aft bringing the CG range into a more appropriate range. eventually i could easily do a wheelie landing on rollout if i kept the speed up just a bit.

each airplane is different...if the gear retract "aft" might want to consider CG'ing with them in this configuration....especially if they are big ol' heavy tires like Yellow F18/ BV Rafale etc etc

Bryce Watson

My Feedback: (6)

What they said. For no question is stupid, unless you haven't asked it.

Now what I've heard. Is that some people intentionally balance their jets just a little bit tail heavy for their worst case scenario. (empty tanks and UAT) not enough to be out of control, but enough so that the model would touch down with the slowest possible ground speed. Then again, it depends where one would plan on touching down in that scenario and whether the gear would be up or down (sometimes more often than not flying over grass, the gear is up) not forgetting to take into account which direction the nose gear retracts and how much of an impact being clean vs dirty may have on that worst-case scenario dead-stick CG.

So the best answer I could come up with to your question based off of what I know is to balance any model where the manufacturer recommends; which is usually gear down (flaps too if you want to get technical) and UAT full.

Unless you want to look cool after touchdown by holding the nose wheel off for 3 or 4 seconds like a full-scale, then balance your model completely level with your hopper tank and UAT full (assuming you will fly a bit into that hopper, which will make you just a tad tail-heavy) and remember that you're on your own if you end up in a worst-case flameout situation at the end of your awesome flight due to fuel starvation, but at least one thing will be certain. . ., you will have had all eyes on you during the entire flight, and even more on you for the impending landing that you are about to execute. . . whether it be on the runway or on the field, clean or dirty. Good Luck!


* now always remember, the larger the model you are flying, the more pronounced all of the above will be. Can't stand when those 747's go rolling past us down the runway after landing in SFO sometimes with the nosewheel still in the air at 100 KTS or less! [X(] Then again, they've been flying a lot longer than us I guess. (by a few hours) (and the terminal is right there next to the inboard runway on the left, and our passengers onboard, rolling next to them on the right.) It's all image I tell ya!