Online Community of Zenith Builders and Flyers
finished my 750 STOL and when I went to take off it flew left. I gave it full right rudder and when I touched down the plane flipped forward and left due to nose gear. Don't know why it fly's left. Checked wings and engine centerline thrust. Maybe gear is misaligned but don't know. My fix may be to install a trim tab on rudder. Any suggestions? Anyone else had this problem?
Thank you Robert for the comment. I have done what Jonathan suggested and put the power in slowly. That has solved the problem of pulling left and rolling (P factor). Apparently, the power plus the p-factor together is too much for the little plane. I have not yet been able to fly the pattern since I am having issues with the airspeed indicator not registering. I have had two put on and both work fine on the bench but when I fly the runway I don't get any movement? Can't figure that one out. I know I am faster than 30 MPH since I am flying down the runway. Pitot tube is clear also. When I blow on the instrument it moves. Just another glitch I need to fix. When I finally get the plane flying and I have some time under my belt I want to put some electronic tabs on so I can win the STOL competition. I know I have the power but gotta overcome the p-factor on take off with full power somehow.
Didn’t the big piston engined fighters like Mustangs have the same issue? They had to be careful not to add too much power until they had rudder authority or they went left.
Sounds like you have found the practical limit for CH750 horsepower- 130hp.
Geoff: Thanks for the information. I had not heard of the Mustangs having that problem. I think you are right though. Right now I am having trouble getting the air speed indicator working. I have had two not indicate and tomorrow I will check out the third one. If that doesn't show anything I am thinking the pitot tube is too small. I will try to fix that with a funnel type of tubing on the existing pitot tube. maybe the extra air will force the needle to move. Seems odd though that these are not working. The indicator is 0-150 and I have flown with "0" indicating? Just got the third one back today. Another thought is they may not be calibrated correctly. Don't know about that. What are the odd's. So when I finally get to fly my bird I plan to work slowly into full power take-off with control. I am sure I can beat Rogers award winning 110 feet take-off. I have some ideas I want to try. I will have to check out the Mustang history and see what they did to correct, but they were not trying for a short take-off. My 0-290 is rated at 135 HP at take-off and I have a Prince prop with high pitch and 58 inches. I have an AOA and will be putting some VG's on the wing just aft of the slat. I read where the vg's did not help with the slats, but after re-reading the article I think they were positioned incorrectly. there comes a point, as you know, where the plane will be hanging on the prop so it will take some practice and finesse to accomplish that task. Does anyone have any ideas about the air speed not showing?
The airspeed indicator is comparing ram air pressure to static air pressure. Are your static pressure instruments behaving normally?
I’ve been following the torque roll at liftoff discussion as well, but had to weigh in when a comparison to a 2,000 hp supercharged WWII fighter was made! If the theory is PFactor is causing this based on excessive power that exceeds the design limits of the flight control surfaces- based on 135hp, I am skeptical. You should not have to use a takeoff technique of anything less than full throttle, maybe the application should be gradual for testing, but as a permanent fix that makes no sense to me. Have you verified you get full rudder deflection at the maximum number of degrees as called out in the plans?
Jim: Thank you for the reply. The first three attempts at take-off with full power went immediately south. So as per suggested I put in gradual power until I had airflow over surfaces to get authority and control. As you suggest, I will check plans to confirm that I have proper rudder deflection. From observation though I don't see why it would be any different or less. As for the ASI, I attempted three times to correct the problem but all three had the ASI showing "0" even with the static line disconnected. I 'bench' checked both the instruments and both of them showed an increase in airspeed as I blew into the pitot tube. So today, I am going to install the second new ASI and see if it shows any movement. If it does not...I don't know where to go except to an official FAA repair station and ask why? When I get this problem fixed I will post it on the blog.
That rudder is crucial to keeping the nose straight after the nose wheel lifts off. The more deflection you have, the more directional authority you have to counteract the effects of torque and P Factor.
I also noticed you mentioned in an earlier post that you checked “centerline thrust” as part of your initial troubleshooting. Is your engine mount part of a proven commercial FWF kit or did you build the mount yourself? Is there an offset built into the mount?
The mount came with the engine which was hung on a piper cub. It appears to be a commercial mount and it fit perfectly on the Zenith with no alterations. Today I flew with the new ASI which did operate and indicate correctly. So now I think I have all the bugs worked out. Next step is to fly the pattern then check all is okay. Thanks Jim again for your input. Oh, BTW, today I did have plenty of rudder authority but only after I went past 35 MPH.