Strange behavior – dive, pull up, flattens out

[Peter Khor]

Subject more or less says it all, on an Electrajet that I'm trimming out, it won't loop, even from a steep dive. Climb performance seems comparable to my own Electrajet, will roll and invert fine, overall it’s stable. However, when I put it into a dive (shallow or steep), and pull up fully, the entire plane just kinda flairs out to a flattened flight profile immediately, and then flies on level with a nose-up profile:

\
_\
__\ flight profile from a dive
___\
____---------------

From level flight at speed, if you pull up, it will gradually try and get verticle until it stalls out.

Is this an indication of over-stable aircraft? (CG too far forward) or something else?

Thanks much!

[Mike Rolls]

Peter
From your description of what happens when you "pull up fully" from a dive it sounds as though the model is being stalled. An aircraft can be forced into a stall at any speed if the angle of attack is increased too far.
Have you actually tried the old tests for thrustline for downthrust and CG? From your posting it sounds as though perhaps you have not.
To check the CG climb to height, cut the power, push over into a vertical dive and neutralise the elevator. For a sports aerobatic model you want a straight dive for a 100 feet or so followed by a gentle pull-out. If it starts to pull out immediately the CG is too far forward. If it doesn't want to pull out, or even starts to tuck under, the CG is too far back.
To check thrust line for downthrust fly flat out in level flight and chop the throttle. If the nose drops the model has too little downthrust, if it rises it has too much. You are looking for it continuing in a level line for a second or two and then gently starting to lose height as speed decays.
Hope this helps
Mike

[vintage1]

Mike has it right...but my 2c worth is to try being more gentle on the 'up' elevator as well. If you try and foce the plane into too tight a loop, the wing will stall, you will get extra drag, and the thing will either mush along level, as yours is, or stall and roll out of the loop halfway round

There is a reason why 3D planes have very large wings, and are built very light...

Hing wing loading planes need to be flown round large diameter loops: They won't pull such high G as a lighter plane.

A rearward C of G should help a bit as Mike says.

Or you maybe have too little power, or the prop geared wrong..too fine a pitch and it can't get enough speed up in a dive to make it round the loop...

[Hammer]

Make sure your pusrods aren't "bending" when stressed.I once had a Cub that flew fine until you pulled on the elevator too hard.On the Cub it was a case of "wet noodle pushrod".

[Dorme]

Mike has it right...you are going into what is called" a deep stall condition". You are lucky to be able to fly level after trying to loop. I have experienced this on the Pico Jet and others like it when I have reefed on the elevator. It's fun if you're attempting to do it, but perplexing wonderment if you are not and it pancakes.

[Peter Khor]

Thanks much all for the load of info! learn sometime new everyday ...

Anyways, a quick run down of what I've done/observed so far:

* no bindings in any of the controls
* control throws are a little more than reccomended
* cut the throttle, the plane flies on straigth for about 2 secs, and then dips slightly - seems to be good to go in this department
* verticle down (need lots more than the max down trimm to keep it pointed down), once I pull up, it wants to level out immediately
* power - judging from audio, this particular EJet sounds "slower" than my EJet - then again, the Ejet in question has been trashed quite a bit

I ended up dialing in a travel adjust of about 70%, and then this time round, gave it a gentle pull-up on the stick - and it finally looped! Guess it has been a combination of a couple of factors (cg, control, power) that gave rise to my problem.

[Mike Rolls]

* verticle down (need lots more than the max down trimm to keep it pointed down), once I pull up, it wants to level out immediately <

Peter
As the "throttle chop" test seems OK that vertical dive test looks like a nose heavy aeroplane, but you seem to be trying to trim it into a dive. The idea is to drop the nose and then neutralise the controls. How does your balance point compare to the designer's recommendation?
Mike

[Hasina75]

Hi Peter! I use two different packs for my electrajet. The first pack is the 9.6 v 700 mah that comes with the kit; the second one is a home made pack, which is a 9.6 V 1600 mah. The EJ can' t loop with the stock battery. I can perform a nice loop with my 1600 mah pack that gives me a 13 mn flight time. It is a radio shack flat battery, but I modified it into a block to fit the EJ fuselage.

Good luck!!

hasina75

[Peter Khor]

Balance point is just a little ahead of the recommended (1/8" to 1/4" ahead, so it is nose heavy).

I can't get it to dive w/o giving alot of down stick, and even then when I release the stick to neutral position, the plane immediately rights itself. Then again, this particular EJet has had it share of crashes, and the wings (esp. TE area) has significant warps (in a bad enough crash, the motor took off an entire chunk of the rear section - not due to prop damage, but actually ripping out the foam ;-) ).

Hasina75, is that the ~$27 NiMh pack that RatShack sell?

Either way, for a parkflyer (it's no zagi foamie for sure), I'm having a blast with it, enough to get another one in a couple of weeks. I'm pretty impressed that the stock kit and battery will:

* loop for sure, make that 2 or more in a row
* loop FROM INVERTED (albeit in the first 2 min of flt time)
* vericle and roll for about 25' (est)

That is, once I got the right throws, power, and balance.

[Hasina75]

Yes, the pack is the one that you find at radio shack. I modified it into a block to fit the ej fuselage. It gives you around 12 to 13 mn flight. However, the pack is heavier than the stock one; so, you should be careful when you land the plane. It takes one hour to charge the pack at 1.6 A rate. I usually bring 6 packs when I go to the field; therefore , I don't have to recharge them after each flight. It's boring to recharge a pack at the field because you are eager to fly all the time.

Hasina75