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HK90mm viper mw44 conversion she Snaps!

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Old 11-08-2012, 10:37 AM
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eazyp
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Default HK90mm viper mw44 conversion she Snaps!

HI guys, took this little viper and put a little 44 in her, IM to the point where Ive had a barely sucessfull maiden flight. SHE SNAPS HARD ON ROTATION !! . Once in the air the rest of the flight was great. tried both flap settings and she had no pitch change what soever the landing was done with full flaps,holding a little power on final and then she just came in for a nice slow uneventful landing. MY question is is the problem possibly a AOA problem? not nose high enough, or possbly its to nose heavy with fuel,fuel is just foward of the cg ,two 16 oz. tanks one in front of the other,with the one in the rear closer to cg draining first. 4oz air trap near the nose balance full. after landing only the rear tank was empty after aprox 4 minutes of flight. I hope you guys can give me some insight on a possible fix. will upload a few pics dont be clowning me on the paint,if I can get it to fly right she will get a new paint job this winter,and a few other upgrade as she will be worth it,also if anybody is thinking mw44, she weight 10lbs full os fuel and the engine is detuned back to 180k. Paul
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Old 11-08-2012, 11:37 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Is it a stall caused by high wing loading due to the weight of the plane
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Old 11-08-2012, 01:15 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

I don't think it's going to be too heavy. (Assuming it's the 90mm version)


It sounds to me like you may be nose heavy, or that the main landing gear is too far back. (making rotation hard) Personally, I would double check the balance, and push it back a little if you can. Then... I would make the front leg a little longer, to bring the nose up.

Is your Wren the "Gold" version? If it is... 180K RPM's is only 7 Lbs of thrust. I would turn it up to 185K and see if it acceleration a little better. If it's the old version... I would run it up to full RPM. (195K)

Then finally... I would add a little reflex to the ailerons. Bring them up a couple degrees. And don't use flaps for take-off. (They may be pushing the nose down while in ground effect)
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Old 11-08-2012, 01:17 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Not enough take off speed?
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Old 11-08-2012, 02:50 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

guys I know it may be hard to come up with a solution,without a video,but i think airspeed was plenty for rotation,otherwise i dont think you could recover from such a bad snap.im talking stand the plane up on the right wing bad.
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Old 11-08-2012, 03:01 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Did you use takeoff flaps?
Scott
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Old 11-08-2012, 03:02 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

no no flaps on maiden flight.
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Old 11-08-2012, 07:12 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Hey p good thing YOU were so quick on the sticks and recovered from that snap on rotation.
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Old 11-08-2012, 07:29 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Not trying to be pessimistic , but I never saw a conventional small "complete" turbine jet flying as well as it looked. In fact, most of them where a handful.


IMO is better to keep this turbine for a glider, or something with a much lower wing loading.




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Old 11-08-2012, 08:16 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

well IM gonna try a few things,Im going to raise the nose gear,recheck the cg, check the cg,with fuel, use takeoff flap setting,gonna even turn the engine back up,depending what I find with the cg i might try adding less fuel. big Bob Kevin was on the sticks when the snap happend, the plane and this engine is really a great combonation and a pleasure to fly. I was aware that some of the edf guys had snapping trouble,but from all my reading all the planes were tail heavy. once the real cg was figure out no more problems until now.
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Old 11-08-2012, 09:23 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Use takeoff flaps and I bet it wont snap any more.
Scott
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Old 11-08-2012, 09:37 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

no this is not a wren, this is a simjet 700
i saw it for sale on rc jets
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Old 11-09-2012, 05:42 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Certainly less fuel and a slight amount of flap at takeoff will help. You can also get rid of the 4oz air trap using a felt clunk and trying using both aleirons slighty rised all the time.


anyway, good luck with it!

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Old 11-09-2012, 06:51 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Quote:
ORIGINAL: craigdyer

Not enough take off speed?

I can guarantee there was plenty of takeoff speed. Paul gave me the heads up before i flew it that guys were experiencing a snap on takeoff with their edf versions. i kept the plane on the ground for about 700 feet and slowly fed in elevator. the viper jumped off the ground and snapped hard to the right. the rest of the flight was fine ... nothing out of the ordinary for a maiden. Landing was easier than i anticipated after experiencing that takeoff, no bounce - settled right in. i think the nose being raised slightly and a recheck of the cg with fuel in the tanks will reveal the issue (or the mains are way too far back). even after the flight, the nose feels heavy while rotating the nose off the ground by hand.
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Old 11-09-2012, 07:34 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

p good thing you are flying vicariously through Kevin! lol
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Old 11-09-2012, 08:08 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

It is worth having a look at the videos of several EDF HK Viperjets crashing. This one has a 90 deg snap after takeoff.

[youtube]http://www.youtube.com/watch?v=Z7Lu4E21alQ[/youtube]

Maybe the plane is just not a very nice flyer, but looks great. The size is a bit marginal for a MW44 especially if the rib profiles are not correctly chosen.

John
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Old 11-09-2012, 09:21 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

If you have to pull up on the elevator to force the jet in the air you do not have sufficient Angle of Attack. The nose needs to be raised. I had a F-16 that had the nose slightly down and required a yank on the elevator to get her airborne. Because there was not sufficient speed she would yaw from side to side. The F-16 is a forgiving airframe so it rocked for a moment until it obtained suffiecient airspeed. Looks like from the attached email the bungee also did not supply sufficient airspeed for take off. Once the yaw starts to occur the airflow over the flight surfaces is disrupted and proper airspeed is not achieved for stable flight.

Another thing to check is to make sure that the horizontal stab and the elevator are at the proper allignment. Down angle will cause the plane to not fly off the ground smoothly.
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Old 11-09-2012, 09:47 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Bob I think I. Made the right choice, I'm not the guy with to much pride to make the right choice on a maiden flight. So what's the big deal, I also can admit he's a better pilot then me, I still have the plane nuff said.
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Old 11-09-2012, 10:26 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

use takeoff flaps and it should be fine. I have a 90mm Hawk and it did same thing. no takeoff flaps it will want to snap, with takeoff flaps its perfect.
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Old 11-09-2012, 10:34 AM
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Default RE: HK90mm viper mw44 conversion she Snaps!

thanks jetpolit I actually came here to get help with solveing a problem and I appreciate the guys who are trying to actually help. Im out for now will report back after I or my budy re fly it. thanks to all that have contributed something I can use. Paul
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Old 11-09-2012, 02:21 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Ive copied my reply from the other forum incase you missed it. I have flown this model many many times and it is perfect when the correct CG is used.

With all your fuel ahead of the CG.. I would balance the plane upside down with the tanks 1/3 full. With gear up or down depending on which way the nose gear folds(i cant remember)

98mm is good. 100mm is perfect. Balance for 98 and if you go less than the 1/3 in your tank your CG will move back to the 100mm mark(hopefully not more)

Nice install BTW


In regards to the Video posted in post #15.. Clearly it was tail heavy.

Here's how my70mm(yellow one) flys with the correct CG(its the same plane as in the video above) My 90mm Version flys the same way(Copper one)
http://www.youtube.com/watch?feature...&v=lv8gZBhPx5I
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Old 11-09-2012, 02:45 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Also make sure your not using to much throws in your surfaces. My 90mm was set to 10mm up/down for the Elevator and 8mm up and 6mm down for the ailerons. I dont recommend flaps or crow. The plane doesnt need it. It has a huge wing and will lift all on its own if the correct CG is used. Dont use your fingers!! Use a proper balancer.

Get video this time
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Old 11-09-2012, 03:01 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Never owned one but from the one's I've seen fly (and crash) the Viper jet appears to me
to be one of those planes that, if not set up exactly right, with conservative CG & control
throws, is a nasty piece of work waiting to bite you.

Of course, the bigger they are the more forgiving they are like most models.

But when you get a small one, heavily loaded, built with all the precision & care you would expect
from Hobby King ...........

John.
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Old 11-09-2012, 03:19 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!


Quote:
ORIGINAL: Boomerang1

Never owned one but from the one's I've seen fly (and crash) the Viper jet appears to me
to be one of those planes that, if not set up exactly right, with conservative CG & control
throws, is a nasty piece of work waiting to bite you.

Of course, the bigger they are the more forgiving they are like most models.

But when you get a small one, heavily loaded, built with all the precision & care you would expect
from Hobby King ...........

John.
Well I have owned one and I can tell you that you coudnt be more wrong in your opinion.

The reason there are crash videos..Is because ProDesign(the manufacturer, NOT HK) posted the wrong CG. I was the first one to post video of the plane flying with the correct CG and it flys just like a really fast Cessna.

As for your comments about setting is up correctly with the right CG and throws... Does that not apply to EVERY! model out there?

Clearly you are a HK Hater so we will just dismiss your comments as it does nothing to help the OP.
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Old 11-09-2012, 03:46 PM
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Default RE: HK90mm viper mw44 conversion she Snaps!

Quote:
As for your comments about setting is up correctly with the right CG and throws... Does that not apply to EVERY! model out there?

Clearly you are a HK Hater so we will just dismiss your comments as it does nothing to help the OP.
I'd agree, it does apply to EVERY model out there, but you would have to agree that some will tolerate incorrect CG & control throws
with few issues & others will bite hard.

No, not a ''HK hater", more a regular customer who's very careful with what he selects from the website.

At least yours lasted more than one flight. - John.
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