CG with retracts

[foofighter]

What is the preferred method, CG exact with gear up or gear down??

The model in question would have the CG move forward slightly with the gear up. I had thought that having the model slightly nose heavy for normal flight would be better than being tail heavy for the low speeds of landing and T/O?

[Brandano]

If the CG moves back with the gear down then I'd get the CG exact with the gear down. Essentially, it's safer to have the CG too far forward than too far back.

[skylane42]

I`m pretty sute the practice is to balance with gear retracted since this is the configuration you will be flying in.

[JetPlaneFlyer]

It's true that the model will spend more time flying with gear up, however it does have to fly with the gear down for take-off and landing which is more than enough time for a too far aft CG to get you into serious difficulty. Also landing approach, assuming it's an IC model, will be made with little fuel in the tank which on most models moves the CG back anyway.
Personally I'd make sure that for the first flight the CG never went further back than the recommended location in any flight configuration. Once you have flown the model you may well find that there is scope to ease the CG back a little but for initial flights safe is better than sorry.

Steve

[foofighter]

Its electric so the CG moving with the fuel load isnt an issue.

I'll go with my instinct of not being tail heavy with gear down. Only because I had the impression that at lower speeds any rear CG was more noticeable.

[skylane42]

Ok, but it will be more tailheavy during gear up flight... good luck. Just remember if you pull the gear up and things go strange, drop the gear.

[Work in Progress]

Full size practice is that there's no "exact" single CG point anyway, but a fore / aft band within which the CG may be permitted. On types where the gear retraction moves the CG significantly, the relevant CG shift is part of the pre-flight loading calculations along with the moments of fuel, oil, people, baggage etc.
We may as well accept that models with retracts or other moving masses are going to be the same - think in terms of a suitable CG range, with fore and aft limits, instead of a point. If it makes enough of a difference that you're unhappy with the CG range that your model operates in, consider lighter wheels.

[JetPlaneFlyer]

Ok, but it will be more tailheavy during gear up flight... good luck. Just remember if you pull the gear up and things go strange, drop the gear.
If you read Foofighters original post you will see that the opposite is true for this model... The CG moves aft when the gear is down and forward with gear up. The last thing you would therefore want to do if a too far aft CG is suspected is drop the gear. There again you would have little choice but drop the gear given the unavoidability of take off and landing, hence my advice to make sure the CG did not go aft of the recommended postion in any flight configuration.

Work in progress is of course quite correct in that there is an acceptable CG range for any aircraft. However unlike full size practice most model aircraft plans only show a single CG location. I for one would take this indication as the furthest aft CG position untill actual flight testing proved otherwise...

Golden rule: CG too far forward can be adjusted after the first flight... CG too far aft there wont be a second flight :rolleyes:

Steve

[Gary Warner]

Just curious... How much is the CG moving between gear up and down?

[foofighter]

Havent done an exact measurement yet but I imagine it will be a few percent of MAC. Already lightened the U/C as much as possible but with forward retracting double bogies of 145gm each side you cant eradicate the problem completely.

[Jurgen Heilig]

What is the preferred method, CG exact with gear up or gear down??

The model in question would have the CG move forward slightly with the gear up. I had thought that having the model slightly nose heavy for normal flight would be better than being tail heavy for the low speeds of landing and T/O?

I would not worry too much about the whole thing. Most IC powered models are ballanced with the fuel tank empty, and are nose heavy when the tank is full.

:) Jürgen

[Brandano]

if it really becomes an issue you could try and put the batteries on a sledge, and link it to the gear servos or to a servo on the gear channel so that it keeps the CG steady as the gear is extracted. Overkill, I think, but an interesting stunt :)

[macboffin]

Bear in mind that the center of drag will move downwards gear down ; the plane will tend to climb more as the gear comes up.