Oh Boy, this plane is going to be a lot of fun......

I got to the field late today and had absolutely no distractions, the temperature was high 60's with no wind. An absolutely perfect evening for a maiden. I did my check list and everything was A OK. I have a new Jet Cat P120, and she runs awesome, so here we go down the run way on low expo steering and this bird stays on the center yellow line, rock solid, and then there she goes a very flat take off with little or no rotation, just floating off the run way. It was very pretty, but that was the end of the easy flying!!!!!!!

Upon trying to make the first bank, it seemed that the elevator had little or no effect, as the bird is flying further and further away, I switch to high rate elevator and still she is slow to respond, my banks are about an 1/8 mile in radius and it feels like there is absolutely no up elevator in the turns. I finally manage to bring it in for a landing after 2 go arounds and she has alot of speed on final so I pray and go full throttle for a go around. After thanking God that the plane actually came around again, I decided to kill the engine and glide it in. She floated a distance and landed only slightly nose high.

OK what the heck is going on here???????????

Here are some ideas
1. Instructions say CG is 180 mm behind furthest point on wing. Is this wrong?? is my bird way way nose heavy with CG at this point??

2. Am I losing elevator deflection while in flight?? I am using JR 8611 servo, 4-40 wire and dubro heavy duty ball links??

3. I had forgotten to attach the bottom air brake cover (left it at home) could this negate elevator throws???

For those of you who have maidened there Hawk, please respond with your experience, and please measure the CG from the wing Tip leading edge.

I was lucky today, This Hawk looks great in the air and I am sure this problem is a set up problem and can be corrected. I am looking forward to bringing it to Tucson Jet Rally.


Thanks


Kevin Marks

seems that you had a hard time on it..... congrats in taken it back in one piece!

Enrique.

Hi Kenvin,

Congrats. Sorry for not having a good first flight. Just a couple of questions:

1. Did you employ any flaps during total flight?
2. Did you pack the wheels away?
3. Are the elevator belcrank and servo horns perpendicular and at futhest point from centre with maximum ATV on radio? If too much travel reduce the D/R not ATR.
4. How did you intall your fuel system?

Hope we can solve this problem real soon. CG sounds good.

I have pleny of time on this bird and i think it is just a small setup problem.

Regards

Here are some photos of the front gear door, I added layer of carbon fiber for strength, and a photo of engine set up.

Thanks

Kevin

Hi Morne, this may be a duplicate, the first one I sent may not have gone through, here are some pics of the set up.

I did not use flaps on the entire flight.
I did retract the gear.
I did forget to put the rear cover on the speed break.
I placed the ball link on the 2nd hole from the end as not to touch the outer wall of the exhaust pipe. I am using a JR8611, and upon inspection after landing the elevater seems to be rigid and strong??

So take a look at my set up, and tell me if you see anything. Also, the ailerons worked perfact.

Thanks

Kevin

Hi Kevin,

Congrats on the maiden! I wish I could have seen it.......my friend. I'm going to be in town at least until next Tuesday. We should get together. Also, I plan on going to the Tuscon Jet Rally. Maybe we need to start a rool call thread.

John

We had a similar problem on two hawks that we flew in the uK about 4-5 weeks ago. We tracked the problem to two Futaba servos and incorrect linkage set up.
When We tested out stab at full deflection you could feel a relatively strong servo Resistance. But from Neutral if you put any pressure on the stab and tried to pull up on the elevator stick it took very very little pressure to stop the stab from moving. See if this is your problem.
The only other thing it could be is the missing air brake cover. It is quite a big hole and that far back could result in it doing strange things.
we need one of those aerodynamic experts to pitch in here and solve that one.

OK Kevin.. I need to know a couple things..

1) length of servo arm from the center of the output shaft to where your pushrod is attached and arm type (nylon, aluminum, etc.)
2) length of control horn on the elevator
3) radio programming info for the elevator (atv's, dual rate setup, etc)
4) control surface travel

Hi Kevin,

Nothing i can see on the photos to suggest such problems! Doug is on the right track and i bet is an incorrect linkage/radio setup. Had a problem like this on my canards of my Gripen.

Regards

Kev, congrats on the maiden. Did you have good elev authority on the landing with enough to flair and land at slow speed?? call me today.
V..

Kevin, If the servo arm and the control arm are in a strait line, try using a heavy duty clevice insted of a ball link. Some times ball links tend to flex a little during high loads. I read this on my ES instruction manual! Ball links works better using dual contorl horns and to put ball link in centre. Hope this help to solve your settings.

Hey Kevin:

I´m Jose, from Spain. Sorry for my bad English. Last saturday i went to my field with the idea of testing all in my Hawk ran well. But when i start my JetCat P120 and was sure all was ok, cannot resist to try to fly her. All that happened was exactly your case. She took off and when i try to turn right had no response of the elevator. It was very difficult to me to pilot the plane where i wanted she goes, and with many problems i could land her. Thanks to god!. I´ve been thinking about what could be the cause of this behavior, and i think the problem was that she was very nose heavy, because probable the c.g. isn´t 180mm back to the front of the wing. Also i think the setup is incorrect and the elevator must have more deflection. More than 40-50mm. I have a Hitec Servo with 9.6 Kgs. of power and i´m going to change for one with more power.

If you have another consideration i want to hear it.

Thanks and congratulations for your first flight.

I am sure it is not a CG issue as we had the same problem with 2 hawks in the Uk and to fix it all we did was alter the linkage set up not the C of G. Deflection is Ok also. I use maximum throw on my hawk which is only 50 mm so I can not see 5 to 100 mm making too much difference.
Try the test where you hold the stab from neutral to see how much pressure it takes to hold it still. This will be a quick test to see how much pressure it takes to stall/ blow back the servo.

Guys, one thing I'm confused about, I have done load testing on most the best that servo manufacturers have to offer, including JR 8411's and 8611's. I loaded an 8611 up with 15 anda half pounds of water in a pitcher one inch off the center of the spline and it jerked that pitcher around like the servo was hardly working.

I can't imagine a linkage being so out of wack that you'd be able to hold an 8611 down or move it back with your finger. I would try another 8611 or stick an 8411 in there and see if there is an immediate change with your finger pressure on the control surface.

Or can someone explain to me in words I can understand how the geometry is working so in favor of the control surface as to stall one of the most powerful servos on the market.

Thanks Guys for the input, I think we are onto something here,,,,,

I held the elevator at the nuetral position and gave full up, the pressure on the elevator surprisingly was not a lot. I will replace the heavy duty PLASTIC control horn on the jr 8611 with a hanger 9 metel arm, and remove the heavy duty ball link on the elevator pivit arm with something metel???? I will also move the control rod to the furthest most hole on the elevator pivit arm, even though it slightly hits the outer wall of exhaust. This should give me the greatest leverage on that arm.

Thanks to All, for excellent recommendations

Kevin

I would like to see a pic of the stab linkage.
V..

Ali wrote:
<<all we did was alter the linkage set up not the C of G.>>

Ali, what exactly did you do to correct the linkage??

V..

Hey Kevin,

You need to hook up with Ali. From what I have seen in the videos, he has the Hawk dialed-in. Do exactly what he did....my friend.

John

Kevin
My friend;glad you got it down without any damage .
Semper Fi
Joe

First thing we did was make sure the servo was center in relation to the elevator joiner arm. This meant that the servo arm was not 90 degrees to the servo. It was angled toward the back of the model by about 1 or two servo splines. Now we had a servo arm that was level to the elevator arm if you look at them from the side. No wno need for sub trim and no need for any ATV one way . All 100%-100%
Now I used the outer hole on the stab end and one of the middle holes on the servo end. Not as first set up with the push rod on the outer hole of the servo arm and middle hole of the stab arm and 130% atv one way and 70% the other. After that things were much improved.
I have just come in from the work shop where I have set up a 8511 on the stab of a hawk. I tried the finger test (Put one finger on each stab trailing edge outboard tip and get somebody to move the elevator stick) Set up right the pressure at the back was almost lifting the front end off the bench.
Hope this helps. If not let me know.
Regards Al

Ali,
Thankyou for the detailed explaination. I spoke to my friend Kevin today and he will make those adjustments and try flying the Hawk again this weekend. I am surprised that this info was not made available to other Hawk owners out there considering you have seen this on two other jets. Kevin is VERY lucky to still have this jet and also not cause any property damage with it.
V..

I was just on the point of installing the elevator system in my Hawk on final gearing up.

The servo is a JR 8511, (same as a 8611.) On my set up I am usinga heavy duty JR servo arm withan X-cell helicopter ball link screwed into the arm with a ball link connecting to the push rod witha 4-40 sullivan link on the elev. arm.
I tried pushing down on the stabs. at full deflection and the servo arm seriously deformed (and these arms are STRONG) and the ball link popped off, there is clearly a LOT of mechanical advantage for the stab in the setup.

Back the installation, this time with a metal output arm and a clevis. I also note that there is too much slop in the stab., the holes in the horn are drilled too large for a Sullivan clevis.

Regards,

David Gladwin.

David, I think the perfect solution would be to use a metal housed ball link system, tap the metal elevator horn for the threaded 4-40 bolt, use a 4-40 rod, and connect it to the JR. 8611 with a metal (hanger 9) control arm. I am going to install the elevator linkage to the furthest out hole, and conect it to the servo horn closer in toward the hub. This should give incredible stregth and rigdity.

Any idea who might sell a metal housed ball link clevis. Dubro sells some nice carbon fiber ones but I have never seen metal ones sold. I did get a set with my AD F16, so I know they exist,????

Kevin Marks

OK, a re-rig seems to have improved things greatly :
The X-cell ball was fitted to the outer hole of the stab. horn after tapping it as required. The servo end of the pushrod is now a Sullivan 4/40 clevis attached to the JR heavy duty horn. I tried the outer hole of the horn and the servo could be stalled by holding the stab. Moving the clevis to the next hole in greatly improved things , much more force was needed on the stab. to stall the servo. The required travel was reached using an increase in ATV (120%) I would not have believed what I have just seen, the difference is enormous, on just one clevis hole.

Some time ago I made myself very unpopular at BVM when I sugggested the BobCat elevator problems (some models had just flown into the ground with "soft" elevator response) could be fixed by a move to a stronger servo which would probably counter any slight misrigging on the small 3301 servos, by an upgrade to 9411 servos. (I was, and remain, conviced that there was nothing wrong at all with the BobCat airframe.) After playing around with the Hawk I am even more conviced that control surface blowback was the problem on the BobCat and probably has been on some Hawks.

Servo torque was also causing some flexing of the fuselage in the region of the servo mounting plate as the servo approached the stalled position so may I suggest that the servo mount is made very strong and stiff.

Regards,

David Gladwin.

Sorry Kevin your post came up whilst I was typing but you are proabaly right. Incidentalyy almost all of the slop in the stabs has now gone, that remaining is gear lash.
Reagrds,
David Gladwin