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Blue on Top

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  1. Blue on Top
    So, back to having fun and dreaming ... I HOPE that someday there will be energy densities greater than 100LL; the real-world technology is no where near at the moment. In the meantime, though, we should be designing new airplanes that are electrically powered (hybrid until the power becomes available). Electric motors have vast advantages over current IC engines in many, many ways. In addition, the regulations will need to be "updated" for these new propulsion devices (motors), distribution systems and storage systems. Failure modes are completely different than today. PS. Electric airplanes are no more quiet or efficient than current airplanes. Aerodynamics are, well, aerodynamics. As our "father" once said, "They all fly through the same air."
  2. Blue on Top
    … and that was the original, mis-informed plan from day one for the M10J. We can do better … much, much better. -Ron
  3. Blue on Top
    Yes, and a lot less dihedral. You can also change the fuselage shape (make it smaller earlier … before the wing trailing edge). Which, btw, causes drag on an M20
  4. Blue on Top
    @PT20J … and now you're seeing the complexity and tradeoffs of design. Another huge one is part commonality. There are parts from the Bonanzas that are used on the 1900s … including most of the certification basis. It's a very complex world. There are pros and cons with a stabilator design, most of which is aero-elastic considerations and pilot control force considerations (which drive internal control system loads and weights). Bottom Line: It's a big tradeoff with typically little differences. Thanks, Ron PS. A high wing is actually faster than a lower wing, but landing gear are very long and don't fit in a high wing well. Hence the ugly Cessna landing gear.
  5. Blue on Top
    Way freakin' cool video! Yes, Flight Test is fun! I spent 30+ years of my career in it … so far. Because they had close-up video of this, they had to know that it was there and that it was not instantly divergent. We typically shake the wing and tail, symmetrically and anti-symmetrically, from 0-50 Hz. to excite any coupled modes. Natural frequencies are typically a little under or a little over 10 Hz. Lower Hz for larger more compliant parts and higher for smaller, stiffer parts. Again, way cool video @takair!
  6. Blue on Top
    @PT20J Skip: There is definitely something to it. The vertical stabilizer is much smaller, too, due to the forward sweep. It's less drag and less effective at high speeds (which both are good) and it's more effective at low speeds (which is also good).
  7. Blue on Top
    I feel badly for all the Mooney employees that have poured their hearts and souls into Mooney Aircraft (the actual name is not relevant). They deserve all the best for their efforts.
  8. Blue on Top
    1) Agree (not something to worry about) 2) The bungees are there to "center" or "fix" the elevator. They also add off trim force (good to a point as it will hold trim condition much better … more stable). The C172 is not real applicable as its horizontal is fixed and the elevator is positioned with the trim tab. The early C180/185 on the other hand is similar to the M20 with a trimmable stabilizer. 3) The tail volume equation is a rough guess and doesn't include things like dihedral, sweep, downwash and thrust. The M10 POC looked good on paper but not so good in the real world. We didn't do powered tunnel testing . If you look at spin characteristics, I can tell you from personal experience that the NASA guidelines are worthless. With that said, you would be surprised at what little items make or break spin testing. Wind tunnels are great, but are limited to static conditions, which don't include dynamics and inertial effects. CFD is getting closer, but again, it gets better when put into full simulation with inertial and dynamic effects. Now, with that said, $$$ get really high for these simulators (close to or more than the cost of a POC airplane). For $$MMM, Boeing "flies" their airplanes 2 years before the real prototype flies … It's a risk reduction process that pays off (if they trust the simulation … which is only as good as the CFD). Bottom Line: We still need flight test.
  9. Blue on Top
    1) @takair CG should not affect elevator trail position (stabilizer position yes). You have done awesome work! A little up or down is probably correct ... and with little to no affect. 2) Right on! 3) 100% in agreement with all @takair has said! The higher one flies the longer you need to stabilize, too. In the jets at FL410, we would wait 10 minutes before starting any data … and then another 10-15 minutes to stabilize. Boeing also does potential energy changes up high to determine if they should take data at all (of course they weigh a bit more . You're right on.
  10. Blue on Top
    1) Thanks for the symmetrical affirmation. 12% sounds really thick for a stabilizer; half of that would sound better. I actually doubt it is an airfoil per say. My guess is that it is more of a flat plate with a rounded (or elliptical) leading edge and a tapered elevator (to simulate the wood tail). The beading/forming on the elevator also causes drag, but it is easier to assemble when the skins can be rivetted together without ribs. 2) Yes. All the springs, bungees, bob weights, bent trailing edges, control system weights, etc. that cause any elevator hinge moment. 3) It's all about total down force. Yes, it is relieving some of the downforce, but it is also acting like a reflexed airfoil (remember it is up side down) which is lower drag in cruise. 4) Symmetrical tail surfaces are easier to build and can be flipped right to left (less part count). But, on that note, many tail surfaces are not symmetrical, especially horizontals that are more typically cambered … up side down. Verticals are more likely to be symmetrical, but typically they are either offset (to offset the engine torque and slipstream) or they are cambered, too.
  11. Blue on Top
    So, this sounds like a fun test, but very difficult to get good results (too many variables … including the pilot). So get a good, trimmed (hands off) cruise speed. Since your J is flying with the elevator a little TE down, you will need to pull back on the yoke slightly to align the elevator to the stabilizer. The airplane will start to climb. You'll need to trim nose down slightly. Then readjust your force to align the elevator again. Repeat this iteration until you're within the elevator flight control hysteresis band. After the airplane is again stabilized, see if the airspeed has changed. Good luck! Remember that when you are done with the test, the airplane will be slightly out of trim.
  12. Blue on Top
    @PT20J Skip: I'll try to do this by paragraph, but I am a little confused by the second part of the first paragraph. First, I agree with the 1990 Mooney engineer and LoPresti … fixing it would not be worth a drag reduction (if there would even be one). Where I am confused is "changing the fixed incidence of the stabilizer". The pilot does that every time he/she moves the pitch trim. The float (stick free) position of the elevator is determined by all the forces/hinge moments acting around the elevator hinge line, summed to zero: aerodynamics, bob weights, elevator control system, springs, bent trailing edge (or dinged TE due to hangar rash), etc. The position won't change with (minor) stabilizer (trim) position changes. It will change with major trim changes because the stabilizer position changes the amount of force the down spring applies. My understanding is that the horizontal stabilizer does not have any camber (it replicates the flat wood tail). With that said, a little camber is more efficient than a flat plate at a higher AOA. The Ralph Harmon metal-metal gap seals are awesome! So being not in trail is not like airplanes without a trimmable stabilizer. The fixed stabilizer on those airplanes are set at a minimum drag for a typical cruise flight. The amount of down force required of the tail is fixed by the pitching moment of the wing. How efficiently the tail produces that moment determines drag. Now for the more fun one
  13. Blue on Top
    The fixed tab is only aerodynamic and not airspeed dependent. IOW, if nothing else was attached to the elevator (control tubes, springs, bungees, etc.), moving a fixed tab will move the elevator to a set position … independent of airspeed. So there you have the rest of the story. All are there for different reasons (and all are required, legally). They all are effective during different conditions. Some are fixed forces (which may change with geometry/position) and some vary their force with airspeed. Clear as mud. Let the learning begin .. for ALL of us. Thanks, Ron PS. It's going to be great to meet all y'all in person at Sun-N-Fun, Oshkosh, Mooney Summit, etc. I am honored to be a part of this family.
  14. Blue on Top
    Centering springs on the other hand, try to keep the surface in the trimmed position (or, in the case of Mooneys, faired with the stabilizer). The advantage is that the control forces can be tailored to be higher when flying off trimmed condition. In addition (and if strong enough) can enhance the stick-free (hands off the controls) stability of the airplane. The tradeoff/compromise here is part count, weight and complexity (and failure modes).
  15. Blue on Top
    Springs and bungees (often used interchangeably) are not changed by airspeed; they are (sometimes) changed by geometry/positioning. A good example of this is the elevator down spring on the K and later airplanes. This spring puts a constant force on the elevator control to push the nose down (by pulling the trailing edge of the elevator down). Ingeniously, the force it applies to the elevator changes with empennage (trim, horizontal stabilizer) position to be more force when the trim is positioned more nose up (slower flight). Now, for that reason and the fact that it's force is fixed with position, aerodynamics are much stronger at higher airspeeds so it does not do as much when the airplane is flying fast.
  16. Blue on Top
    We'll start with balance weights since it should be close to complete with the discussion in Rudder-Aileron Interconnect. Depending on highest speed, shape of the airfoil, shape of the elevator, etc. designers will balance these surfaces from not balanced at all (most slow, early airplanes) to over balanced in transonic and supersonic airplanes, and everywhere in between. Clear as mud. The compromises are between aeroelastic effects (flutter), overall weight and aerodynamic balance, including control forces. There are pros and cons to each of those. Well, okay, there are no pros to flutter, which can be both bending and torsion.
  17. Blue on Top
    For reference, this thread originated within (dated 1-5-20 posts) the Aileron-Rudder Interconnect thread. We got a little off topic . I'm an engineer; I talk too much .
  18. Blue on Top
    @N201MKTurbo @ArtVandelay @PT20J and I know @carusoam will find it without me saying so …. I'll @blueontop start another thread called "Elevators - balances, springs and bungees" to get answers those questions that have been posted here … and get the discussion out of "Rudder-Aileron Interconnect".
  19. Blue on Top
    So, … weight is … weight, and weight is bad (takes away from useful load). Weight away from the CG is doubly bad. Now let's look at those elevator balance weights in particular (my last post explains a little of the aeroelastic (flutter) issue and why we balance the flight surfaces. Remember that EVERYTHING in design is a compromise or tradeoff. We pounce on those items that are not … an engine that has more HP per lb. of weight AND has a 4000 hrs. TBO AND costs 50% of current. I wish. So on to the elevator balance weights. We analyze the aeroelastic (flutter) modes to give us good characteristics with some static margin. This is similar to how the aircraft aft CG is set for the entire airplane. Note also that the balance weight is 2-4 times the weight of the actual elevator (its a geometric thing of balancing the moments around the hinge line (which is what your knife edges are in the real world). So, if your elevator weighs 2 lbs., the balance weight might be 6 lbs. to balance that 2 lbs. (it has a shorter moment arm). Now your 2 lbs. elevator weighs 8 lbs! Another tradeoff is where to put that weight. The balance area at the tip is ahead of the hinge line … trying to destabilize (flutter) the elevator. This area also lightens the control force. The easiest way for me to think about this is an arrow (of a bow and arrow). The CG of the arrow must be ahead of the aerodynamic center, too. If those 2 points are at the same location, the arrow will fly in any attitude (and very draggy). As weight is added to the pointy end, the arrow will fly more stable (less wandering). If the weight continues to be increased, the arrow will become more and more stable, but will travel less and less as the total weight increases.
  20. Blue on Top
    @ArtVandelay Tom: Balance weights and springs are installed for two very different reasons. Balance weights on the elevator are for aeroelastic bending (flutter) and springs are for flight stability. Lead weights installed on the forward portions of the flight controls (elevators, rudder and ailerons) are to (more closely) statically balance the flight control surface. Not all surfaces are 100% balanced (like the elevators in the manual above. Thanks, @PT20J). Let's look at the elevator (it's the easiest). In turbulence (vertical in this case) the airplane will move up and down (both pitching and vertically), which causes the stabilizer to move up and down with the airframe. The elevator on the other hand is free to move, controlled by the pilot. When the stabilizer goes up, (if not balanced) the elevator in relationship will go down, causing the first excitation of the cycle. The opposite is true on the down movement of the stabilizer. And now we have the beginnings of a flutter cycle. The springs on the other hand (I believe there are 2 sets for stability reasons on Mooneys). The first is the lateral-directional springs, know as the rudder-aileron interconnect, which loosely ties rudder and aileron control position together. The other is the down spring (newer airplanes), which pulls the elevator down in slower flight to give the airplane more static (speed) stability.
  21. Blue on Top
    @Andy95W I agree with you 100%! I should have seen that the springs were not attached. Also yes, Mooney control travels are small, which equates to higher loads in the systems. I love your idea to look at the system working on the ground, normally and cross-controlled. You'll need to either elevate the nose off the ground or put it on a grease plate to get rid of its effects. Way cool, and thanks!
  22. Blue on Top
    @PT20J Skip: Nail on the head, and why I started this thread originally. We (ASTM, which also includes FAA, EASA, TC, etc.) are looking at this regulation (23.177) and wondering if it really meets the original intent of why the regulation was put there in the first place. Ironically, I know Russ well (he was the Director of Flight Test my last couple years at Cessna). Part 25 airplanes have this same regulation. So … Part 25 airplanes typically have less required stability (and they typically go through a much broader altitude range … which makes meeting stability requirements more difficult). Part 25 airplanes also must get home after any failure plus any probable failure (those failures that occur more than 10^-5). They also have to deal with a jammed flight control surface. Part 23 (our Mooneys) don't have to. As a not quite "equivalent", Part 23 airplanes must be more stabile. For example, we have to fly home with any one bolt not installed in the flight control system. IOW, disable the rudder and fly home on ailerons alone (btw, even with the rudder aileron interconnect, the rudder would not move). Disable the aileron(s); fly home on rudder. Disable the elevator, fly home on trim (if you haven't tried this you should … at altitude first). Disable the pitch trim, fly home on elevator only. Hope this helps! -Ron
  23. Blue on Top
    @Andy95W Awesome photo of the rudder-aileron interconnect! A picture is worth a thousand words (or more). Also @PT20J with the IPC information! It appears that there is no intentional preload on the springs. On a more detailed observation, I'm curious where the unloaded spring goes (it must buckle in compression) when the opposing one is tensioned? And now everyone knows how a T-strip centers the surface (you'll see them on ailerons, too), and what a rudder-aileron interconnect does. BTW, The Wrights used them on all their flying airplanes, too
  24. Blue on Top
    OMG. All y'all are hilarious! The thread on the pencil holder, allen wrench holder, tool holder, flag mount, etc. is just funny But now I know a little more about how pilots think 1) First and foremost (and maybe only), please don't remove ANYTHING from the airplane! The airplane was certificated that way and is illegal if modified (and may not be insured either). I will assure you 100% that if you remove a stall strip (or both) you will not like the answer. Don't try this at home … or in the air. 2) Someone in the other thread mentioned that the rudder wagged a little when they removed the flag holder. This is why it was originally installed. It is a centering strip. Ironically, it is a "lower drag" centering strip (because it is at such an angle to the local flow), but in this case a little higher drag works better (but could be made shorter to get similar drag). A Gurney tab is a little different in that it is specifically a 90 degree angle and adds force in only one direction. Dan was a race car driver and patented his device. He died 2 years ago this month.
  25. Blue on Top
    @carusoam Very interesting. @KSMooniac mentioned it was round during lunch yesterday. I wonder if Mooney had extra material laying around, though one would think that T-strip would be laying around, too, and easier to attach. This is one of those odd cases where separation off the trailing edge is good (though it adds drag, it's a little less draggy than round).
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