View Full Version : RAF stab incidence
03-06-2004, 06:25 PM
After talking to several people on what would be a good initial incidence angle for my Parham stab...and observing the keel angle of RAF's in flight...I set mine to two degrees nose down with the keel. Most of the keel angles observed seemed to be just a couple of degrees down..so my thinking was that with two more degrees down...I should have some down force to keep the keel from pitching down with higher airspeeds.
This of course comes with the expense of a little more drag resulting in less airspeed.
Now that I am flying my RAF and getting the feel of it..I am now observing that there isnt any apparent nosing down with speeds up to 85. The stab must be doing its part. These initial observations were just seat of the pants ...and I am planning on measuring the keel down angle with my Warp drive pitch gauge. If these measurements show that I could try a little less incidence...or for that matter...show that I need a little bit more...I will adjust it down or up a degree at a time.
Before I make any changes..my next agenda is to start documenting GPS groundspeed readings at different rpms and trim settings. I will of course repeat each setting on a course 180 degrees the other way and then average out the GPS ground speeds.
If I do change my tail incidence,,,I will document before and after groundspeeds trying to do so at the same temperatures so as to keep the data as honest as possible.
Just some nit picky stuff to play with..but probably will be more fun than practical. But...should all this result in a net gain in groundspeed ...I will be confident it will be actual...and just not felt.
03-06-2004, 08:31 PM
Stan, here's how I do it. Set your keel level on the ground. Then place your prop protractor on the trim wheels or wheel and center the bubble. Take note of which way you have the protractor and the reading.
Go flying and when s/level place the protractor on the trim wheel. If the bubble is to the rear then the gyro is nose down. Adjust to center the bubble. The difference between the two readings is the nose up or nose down from level.
I would then, using a line drawn from the tip of the leading edge to the tip of the trailing edge as the stab line, situate the stab 3 degrees nose down from the level flight keel position.
Conduct some flying and take notice of speeds etc and the keel angle. Then add another degree and see what happens.
Even averaging ground speeds is of no use when conducting these exercises, only airspeed gives the results.
03-07-2004, 03:18 AM
Paul: Thanks for the tips. Your explanation of setting the protractor is what I had in mind..but I will be honest in that it was one of your posts months ago that gave me that idea.
I know the airspeed is what counts....but I can only trust my GPS to really give me my true groundspeed. My ASI seems to be fairly accurate...but I want it exact. So...I feel my GPS will do so at the extra pain of having to fly 180 degree courses and then averaging out the speeds to eliminate the wind error.
It will of course be simpler just to record my airspeed...but I still wont trust it to be as accurate at averaging out GPS readings which I have no doubt are dead on.
Your 3 degree figure is probably close to what I have right now if my keel is flying one degree nose down. If my keel is say three degrees nose down...then my stab which is already two degrees down would then be at five degreees.
You probably have tested more stabs and various thrust changes in the RAF tnat I know of ...... and I respect all your efforts Paul. Keep up the good work.
03-07-2004, 04:01 PM
I agree with Paul. Groundspeed is totally irrelevant. Even if your airspeed indicator is off, it will be "equally off" during all your tests. Therefore, you will note either an increase or decrease in airspeed with your different settings. Air temperature and altitude must be constant to give you good relative readings between the different settings.
03-07-2004, 07:21 PM
Gary..Paul... THanks for simplifying my tests.
I see your point....I am going to keep it simple then and next time I fly I am going to record what the keel is doing at different airspeeds.
03-08-2004, 03:43 PM
Paul: I put my warp drive protractor on the level keel...then decided to use my instrument panel as a calibration point.. The dash has exactly 19 degrees of forward tilt from vertical when the keel is level. The trim wheels were not a good spot for the protractor....they arent exactly flat where as the dash is....plus I can easily read it.
I was going to record some numbers today but the weather wasnt nice.
Any increase over 19 degrees that the dash is tilting is the angle that the keel is nose down. It will be interesting to see what the keel is actually doing at all speeds. I have had it to 85 now and it seemed to be staying the same. The protractor wont have any bias though.
03-09-2004, 02:17 PM
I went flying today to check out what degrees my keel was flying at. I had my dash indexed at 19 degrees from vertical when the keel is level.
I flew at 65...75....and 85 mph. All my readings were right at 21 degrees indicating the keel is running 2 degrees nosedown. I am not going to mess with the 2 degrees negative incidence I have my Parham stab at as I am pleased that the nose wasnt going down at higher speeds. My previous post stated I felt it was that way...but this proved it to me. So my stab is flying with a 4 degree nosedown attitude.
03-09-2004, 03:08 PM
Good test Stan. It is a fair indication that your stab is doing its job.
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