Why does a Comp Arf Hawk behave this way
 

My friend built a Comp Arf Bae Hawk as per the plans and instructons. Powered by a Jercat P 120 and with quailty radio and equipment. He has double and triple checked the CG and has it set at 7 7/8". I have the priveledge of being the guy who does his intitial flights and I assist him it getting the throws and rates and trims setup.

Here is what I have noted about the way this airplane flies that is to me unusual and somewhat troublesome. This airplane is EXTREMELY pitch sensitive to changes in power and airspeed. For example : I trimmed the airplane to flight straight and level hands off at half or 2/3 throttle, then, if the throttle is advanced to full power the airplane will climb almost instantly and I dont mean a gradual climb either. I mean it will pitch up to a near straight up vertical climb. Conversely, if the throttle is cut to idle the airplane will drop the nose to a very nearly straight down attitude. Naturally any airplane will climb or descend to some extent when power is added or cut and no pitch trim is or elevator input is given. But I have never seen one like this. My initial thoughts were that we had a major thrust angle problem but he has checked and tweaked the engine angle and it is now correctly lined up as far as being parallel to the tail pipe but still the pitchy issues persist to more or less the same degree. Now this is not to say that the elevator ( or in this case flying stab) lacks the authority to overcome these issues because it absolulely does. The airplane files quite smoothly and well except for this. I have learned to overcome the pitches up and down with elevator inputs and or trim inputs but I have seldom if ever seen a plane that requires such massive trim changes with speed and power changes in order to fly level hands off.

I tend to think that it is nose heavy to some extent but I cant imagine that moving the CG back wll solve this or make it reasonably tolerable even.

This airplane has a farily small wing and therfore a pretty high wing loading but its not an X-15 or F 104 either. He had also read somewhere that some guys were flying these with both ailerons raised ( to what extent he couldnt recall).

I want to add more tail weight at this point, he wants to set up a mix with throttle being the master and elev the slave. Add more power and get more down elev automatically and vice versa. I am not sure this is a good idea or the proper thing to try at this time.

I am interested in hearing any and all suggestions, especially from other pilots who have owned and or flown this airplane.

RE: Why does a Comp Arf Hawk behave this way
 

Mine is not like that at all.  I don't know what the problem is but it certainly isn't a design one - its a great flying airplane.
I have my CG and throws set per the manual and that is fairly nose heavy IMO.  I needed a lot of nose weight to get to where they suggested.  I plan to move mine back next season (flying is done here for the next few months.)

The only negative thing I have noticed with mine is that it's tricky in a cross wind landing due to the large size area.  On slow landings you need quite a lot of elevator to flare.

I have a Rhino in mine - I adjusted the position of the engine to stop it climbing under acceleration.  On a P120 it should not climb under power.

My ailerons are not set level with the flaps - they are set so the ailerons are level with the wing tips.  That means the flaps are slighly lower (about 2mm.)

Any chance you have some binding on the elevator mechanism?  It almost sonds like something is moving inside the airplane for things to be as drastic as you described.  My airplane is ROCK SOLID in the air, not pitch sensitive at all, no bad habbits at all and very easy to fly.  There is definately a problem somewhere.

RE: Why does a Comp Arf Hawk behave this way
 

Raised Ailerons are required for the early Sky master and Fei Bao T-45 / Hawks to try to minimize the chance of tip stall during low speeds i.e landings, this should not have the effect that you are describing.

I would stay away from a mix as you described, I would look for the source of the problem.;)

RE: Why does a Comp Arf Hawk behave this way
 

Brian, you mentioned slaving the elevators to the throttle. Check one thing before you do anything else and that is retract the landing gear and then check to make sure that there is no slave mix that is active only when the gear is retracted. It is highly unlikely that this is the case but it only takes a minute to rule that possibility out.

One other thing to try is rolling inverted and check to see if it still pitches skyward under power.
If it does then as you said check the cg. If not then go back again to triple check the decalage and pushrod mechanics.

Good luck,
Ross

RE: Why does a Comp Arf Hawk behave this way
 

If the P120 is sat on packers as the original manual then it does climb slightly under full power, When we raised the elevator servo mounting plate after the first 6 kits, the turbine can go on the rails, or even mounted above the rails (the JC mount on the top side of the rails model on its wheels, or under rails looking through wing opening)
But- even with the spacers it did not "pitch" up and climb, it would just climb slowly at full power. I only ever used full power for loops etc so it never bothered me and I did not notice it.

Something odd for sure

Dw

RE: Why does a Comp Arf Hawk behave this way
 

The engine may be aligned to the pipe, but is the pipe in proper alignment?

RE: Why does a Comp Arf Hawk behave this way
 

I have a Wren 160 in mine and it sets on top (when the plane is right side up) of the engine mount and my cg is aft of the recommended one. My plane does not exhibit pitching at all. Elevator is not overly sensitive either, but I use about 60% expo. I don't use dual rates.

First thing to check is your mixes for any elevator input to throttle.

Jeff

RE: Why does a Comp Arf Hawk behave this way
 

The pipe alignment is more or less set by where the engine rails are and where the opening in aft end of the fuselage is. Having said that, the more I think about this the more I still think this is some sort of thrust angle issue.

If I recall correctly the front end of the tailpipe ( engine end) is lower than the aft end relative to the wing chord line. To me that would equate to "up" thrust, I think ??

RE: Why does a Comp Arf Hawk behave this way
 

ahhhhhhhhh. we may be on to something here. I think he has the engine mounted to the bottom of the rails and not the top.

RE: Why does a Comp Arf Hawk behave this way
 

It would take a boat load of thrust offset to produce the symptoms you mentioned...

RE: Why does a Comp Arf Hawk behave this way
 

Probably right, I would check the mixes and also the wing incidence.

RE: Why does a Comp Arf Hawk behave this way
 

Is your engine mounted to the top or bottom of the mounting rails ?

RE: Why does a Comp Arf Hawk behave this way
 

The wing incidence is ths same as everybody elses Comp Arf Hawk and they fly correctly. The Stab is a full flying stab so there is nothing to check there.

RE: Why does a Comp Arf Hawk behave this way
 

I will have him check for any mixes but I really dont think that is the problem.