I think Larry Flesner hit it right on the nose. Hope your HZ is adjustable On Mon, Mar 30, 2020 at 5:56 PM Flesner via KRnet <krnet@list.krnet.org> wrote:
> On 3/30/2020 9:35 AM, Dr. Feng Hsu via KRnet wrote: > > The plane wants to go nose down from the first flight so we installed a > > fixed trimtab on the elevator. Problem solved. I think we made the > elevator > > a bit to heavy, so the weight of the ellevator is forcing the nose > > down.....I think. > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > Stef, > > I don't think it is the weight of the elevator causing the nose down > pitching, it is the angle of incidence of the horizontal stabilizer, > which you can't change at this point. > > Consider this. The early Cessna 170's (rag wing) did not have a > balanced elevator. Setting on the ground ,with engine shut down, if you > pulled back on the yoke to put the elevator at neutral and then let go > of the yoke the elevator would drop to the down limit with a noticeable > bang. Air loads in flight tend to push the elevator to a more neutral > position. > > A more relevant example is my KR. It has a balanced elevator and on the > ground the elevator tends to stay near the neutral position. I can > place my adjustable trim tab in the neutral position, take off and climb > to altitude, push the nose over to cruise (150 mph), and if I let go of > the stick, I will get a violent nose down pitch. It is enough to throw > things up off the seat A bit of nose up trim removes the stick forces. > In flight photos show the elevator still in the near neutral position so > a very small change makes a big difference at higher speeds. Once > established in cruise I seldom touch the trim until I'm in the landing > pattern or sometimes on a long letdown from altitude. That same setting > is sufficient for takeoff and climb out and then adjust again for > cruise. One or one and one half degree more nose down angle of > incidence on my horizontal stabilizer would make it more efficient in > cruise. > > Bottom line. If you're lucky you will set the horizontal stabilizer > incidence to the correct angle for your normal cruise speed with an > ideal C.G. location and the elevator at neutral. That would create the > least amount of drag and give you the best speed. Unfortunately life is > not that simple. Every other speed, power setting, and C.G. location > would require a different angle of incidence on the horizontal > stabilizer. A number of aircraft designers did exactly that by making > the horizontal stabilizer adjustable. All the early Pipers through the > TriPacer and the early C182's come to mind. So, you have two options. > One is a fixed tab to get zero stick forces at one given speed and C.G. > location or two, some type of an adjustable tab or other device to > relieve stick forces at different speeds. Your KR is no different than > any other airplane flying that requires pitch trim, adjustable or fixed, > to relieve stick forces. > > One important thing to remember is if you use an adjustable trim tab > make sure there is no slop in the system. Any free play where the trail > edge can move up and down can induce flutter and you don't even want a > hint of that. That's the reason I removed my steel cable operated tab > setup and went with electric servo in the elevator. With a coupling of > only a few inches between the servo an the tab I was able to remove all > free play in the system. I recall the P51 that crashed at Reno > developed a problem in the trim tab that caused flutter and that didn't > end well. > > Second bottom line. Don't worry about needing some pitch trim. All > aircraft have that need. If you can make C.G. changes to eliminate as > much stick forces in cruise as possible, great. That would be ideal. > If not, go with what you have as long as it is safe. > > Larry Flesner > > > > > _______________________________________________ > Search the KRnet Archives at > https://www.mail-archive.com/krnet@list.krnet.org/. > Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options. > To UNsubscribe from KRnet, send a message to krnet-le...@list.krnet.org > _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/krnet@list.krnet.org/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to krnet-le...@list.krnet.org
