JoelP

My Feedback: (20)

Do rc jets respond the same as prop models when testing the cg after pulling to 45 and roĺling inverted?

why_fly_high

My Feedback: (19)

Depends on the jet. My experience is that most jets don't have symmetrical airfoils and I don't like to run the cg as far aft on my jets as I do my prop planes. it will get you close and is a valid check but you may not get it to be perfect like you might a prop plane.

rhklenke

My Feedback: (24)

I agree in general, but it will give you an indication if you're way off. For example, on my new TopRC Cougar, I put the CG just a tad in front of where the manufacturer said, to be safe. I did this test and I had to add "gobs" of forward stick to maintain the line. I wound up removing around 3-4 oz. of lead from the nose, moving the CG back by around 3/4" and it flys *much* better. Its still a bit nose heavy using that test, but as you said, I generally don't go as far back on jets - especially scale jets.

Bob

jsnipes

My Feedback: (3)

Springbok Flyer

+1, but flying a tail heavy scale jet is a bunny hop waiting to happen.....

Cheers,

JanR

Dieselman1220

My Feedback: (25)

It is one way to help for sure but keep certain things in mind.
The big one being expo on your elevator. About a year ago a buddy of mine test flew a new bobcat. After landing and conducting the "inverted test"
He claimed he needed to hold about 50% down so the airplane must be way too nose heavy and was about to start shifting batteries.
I then asked him how much elevator expo he's running. After looking in his radio he claimed 45%, and asked me what relevance that had to CG.
His CG was ok still required some tweaking but not to the extent he was about to do. Expo can mask some potential issues this being one of them.
I do use expo in conjunction with flight modes, but rarely run over 15% on a surface.

why_fly_high

My Feedback: (19)

One thing is worse than a nose heavy jet.

olnico

Yes. I systematically test all my prototypes of jet power hobby models, flying targets and UAVs with this method. It is part of our test flight program.

olnico

Then, you should try our Crusader...

Zeeb

My Feedback: (41)

Depending on the model and how quickly it responds to control inputs, I probably use more expo than most guys and this flying inverted thing always made me nervous as it seemed like it took a bunch of down elevator to keep it level. I told a friend about it one day while we were setting up a new model. I expressed my feelings about being inverted, he took my radio, some quick clicking and beeping noises and he handed it back to me with a "try that".

I was amazed at how much more comfortable it was to fly inverted. I looked at the settings when I landed and he'd increased the deflection rate and decreased the expo on the down elevator. That was all it took So when guys say don't overlook the small stuff, they know where of they speak....

Viper1GJ

My Feedback: (20)

I test all my new jets with this method. I learned it 20+ years ago when starting with aerobatic planes for IMAC.
My technique is slightly different as I establish the 45° upline and roll inverted and then let go hands off. Then I observe the flight path. Its subjective but if it pitches toward the canopy its probably too nose heavy. If it stays straight 45° up its probably a little tail heavy for me and may be a little pitchy and unstable on final. If it has a very gentle arc toward the canopy its probably close. Once I get the CG adjusted with the gentle arc, then further adjustments are after several approaches and landings to get performance I like.
Gary

Dieselman1220

My Feedback: (25)

Glad to hear, the other area where it bites people is on the base to final turn while slow (landing) with a crosswind pushing the model behind you.
The model begins to drift behind the flight line the pilot in an effort to get back on the center line pulls harder than usual on the elevator. Then bam
the model suddenly snaps over on its back (accelerated stall) and ends up in the dirt.
Then usually the pilot exclaims to not understand what happened, the model was not flying slow enough to stall.
1. Bank angle increases stall speed increases.
2. That 50% expo bit. The pilot pulled on the elevator but due to the crosswind and the model drifting pulled quicker and probably harder than usual
due to the high amount of expo the elevator, the elevator came in harder and more sudden thus causing the accelerated stall (also called tip stall or snap spin by some)
Unfortunately Ive seen in happen more than once.
Expo is a great tool, it would just behoove people to have a little better understanding of what exactly it does. Especially on the ever critical elevator.
It is much more forgiving on the other surfaces.

erbroens

IMHO Jets should optimize the cg setting for landings and skow speed handling, wich are the most critical and demanding parts of the flight, and not for acrobatics.

tp777fo

My Feedback: (28)

Right on Dieselman! See it myself.

RCFlyerDan

My Feedback: (54)

Besides the 45 degrees angle, which I figured out is only used to give a bad pilot with a nose heavy airplane time to react to the plane diving upside down, verses level inverted flight. I look at the elevators, if there is any detectable elevator out of center when sitting on the ground with the power "on", I make adjustments. Some scale planes/jets are hard to see any difference. If you have up elevator, nose is heavy, down elevator, tail heavy. I also look at the knife edge flight. Same thing; tucks to the gear, tail heavy, pulls to the canopy, nose heavy. Works for me. You can't properly trim a plane/jet until the Flight CG is established.

Zeeb

My Feedback: (41)

Yeah I garbage bagged a Shockwave doing that even knowing that it was heavy as not much fuel had been burned off. I always flew a pattern as I'm also a full scale guy and it gives me a pretty consistent sight picture for the approach at different fields.

However, after that rather nasty spending experience, I started watching the turbine guys more closely while they were landing. Almost without exception, they fly a 180 degree turn from downwind to final. That eliminates the corners of a pattern approach with a pretty much continuous load on the wings and, if you want to keep that puppy in tight, that's also possible with the appropriate technique be it a higher G, power on tight turn, or what we used to call a "Forrest Service Approach" in the full scale stuff. Due to the short, narrow fields in canyons where the Forrest Service has landing strips, the common approach is a 180 degree slip. Works great in a full scale and you can do it with a model. But watch the ones with swept wings, the more wing sweep, the less tolerant they are of doing slips....

jsnipes

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jsnipes

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JoelP

My Feedback: (20)

Thanks for the replies.