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Thread: Cadet Dutch Roll

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  1. Cadet Dutch Roll 
    #1
    Join Date
    03-01-2002
    Location
    Houston, TX
    Posts
    132
    A couple of us have built Cadets from the Earl Stahl plan for a rubber-powered free flight version (downloaded from the Plan Page). I scaled mine up to a 32" wingspan. Power is by GWS 4.8V LPS with a 4-cell NiCad battery. RTF weight is a little under 5.5 oz. The plan shows a scale outline for the vertical tail and an enlarged outline for the FF veresion. We both used the enlarged outline and both used the dihedral angle from the plan (about 5.5 degrees). Both planes exhibit pretty bad Dutch Roll characteristics. I realize that Dutch Roll is generally due to too little vertical tail area and/or too much dihedral, but don't know how much they should be adjusted. Can anybody give me any guidelines for adjusting the tail area and/or dihedral angle to fix the Dutch Roll without causing the plane to become spirally unstable?
     

  2.  
    #2
    Join Date
    10-05-2001
    Location
    Farmville, VA
    Posts
    2,291
    Hello Don,
    When you changed the model to R/C you more than likely lowered the CG with the R/C gear. This has the effect of increasing the dihedral. Of course, all of this is assuming that there was no Dutch Roll in the original model. Free flight modelers often preferred a little Dutch Roll to Spiral instability. Personally, I would add area to the existing tail by taping light balsa or foam in place, and experiment with finding the best combo. Then, you could replace the tail with a more traditional structure.
    Regards, Dave
    Dave Robelen
     

  3.  
    #3
    Join Date
    03-01-2002
    Location
    Houston, TX
    Posts
    132
    Dave, Thanks for the info.
    It looks like I might have been mis-interpreting the Cadet's flight characteristics. I have a program that computes the center of area for about any shape. So I used it to compute the CLA of the Cadet and then used the method in Andy Lennon's "R/C Model Aircraft Design" to analyze spiral stability. According to it, the Cadet, using the larger vertical tail on the plans, has a bad case of spiral instability. Using the stock (smaller) tail size shown on the plan would give it slightly positive spiral stability. So it looks like what I thought was Dutch Roll might have really been spiral instability coupled with an inept pilot's overcontrolling. Have you ever tried Lennon's method for analyzing spiral stability?
     

  4.  
    #4
    Join Date
    10-05-2001
    Location
    Farmville, VA
    Posts
    2,291
    Hello Don,
    No, I have not tried the Lennon concept. I do have a great deal of respect for his writings and concepts, so it is well worth paying attention to. True Dutch Roll is cyclic in nature, and a decent test is to establish level flight and ease the stick back while reducing power. If you have Dutch Roll, the model will begin rocking, probably with a fairly modest amplitude.
    Cheers, Dave
    Dave Robelen
     

  5.  
    #5
    Join Date
    03-01-2002
    Location
    Houston, TX
    Posts
    132
    Dave, thanks for the reply.

    Out of curiousity as much as anything, I have decided to resize the vertical tail per Lennon's method and see what happens.
     

  6. Update on my Cadet 
    #6
    Join Date
    03-01-2002
    Location
    Houston, TX
    Posts
    132
    Dave,

    It appears that my problem with the Cadet was not Dutch Roll, but spiral instability coupled with pilot-induced-oscillations when i tries to recover. It also appears that Lennon's method is a useful tool for estimating spiral stability.

    Yesterday I flew a revised version of my Cadet with the smaller 'scale' vertical tail and one more degree of dihedral on each side. According to Lennon's method these changes should give it slightly positive spiral stability. It was too gusty to really wring out the spiral stability, but the tests i could conduct indicated that the estimates were at least in the right
    ball park. It was certainly a lot easier to fly than the first version.
     

  7.  
    #7
    Join Date
    10-05-2001
    Location
    Farmville, VA
    Posts
    2,291
    Hi Don,
    I am glad you got on top of it. Some of these issues can be tricky to judge. It certainly sounds as if Andy Lennon can be a fine resource.
    Regards, Dave
    Dave Robelen
     

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