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As we have been discussing in another thread, I have been struggling with CG issues in my design calculations & ballast may be needed to solve the problem. While doing research into the situation, I ran across this report on nose ballast in the F15, I haven't got all the way thru it yet, but some of you may be interested in reading it.
I guess this shows there are no new problems. :)
Navions with larger engines all have a 9 to 12lb lead weight in the very far end of the tail cone. Some other people have done stuff like moving the battery (already behind the rear seat) further into the tail cone as well.
Talking about ballast, I've got a 20 pound chunk of lead under my work bench that has a Beechcraft part number on it. It looks like a pound cake cut in half and made to fit on the last fuselage bulkhead of a BE-19.
I am certainly not the engineer in the room. I'm just asking. What are the drawbacks of concentrating mass at the extreme ends of the airframe? Such as engine at one end and ballast at the other? Or a Cessna 310 with all the fuel weight out at the wing tips?
I did find a pusher that concentrates the mass near the CG. Its a Molt Taylor design called the Mini Imp. This is a link:http: //www.mini-imp.com/inboard_profile.htm
I was chatting with a retired USAF Aerodynamicist about something else and in passing he mentioned nose ballast in a fighter. I guess they all need some. He said that the dwgs specified titanium attaching bolts TO SAVE WEIGHT!
Bob
If the fuselage is designed to distribute the loads like a truss, it isn't that big of a deal as the loads should be tension or compression with no bending moments. That is partly why I am using a steel tube fuselage, as I have done those type of load calculations many times. An aluminum or composite monocoque structure can still be designed to behave like a truss, even if it doesn't look like one, but the calculations are more difficult. In the case of tip tanks, while airborne the force is distributed evenly along the bottom of the wing, lifting up, so it counteracts the weight of the fuel. I would imagine there are some flexing loads while landing. Repetitive loading & unloading of a member is what will eventually cause it to fail if it is not designed properly. Here again with steel it is easy, as long as you don't exceed the yield strength of the material it won't ever fail. Aluminum is trickier, as there really is no yield strength, because of the crystal structure it does bend at a microscopic level with every loading/unloading cycle, which is why you see some airframe parts with a life limit. You can calculate how heavy it has to be for a certain number of cycles, the longer you want it to last the heavier it is.Quote:
Originally Posted by Bob Dingley
That is an interesting design. Not real practical for carrying baggage, but it looks fun. I briefly toyed with the idea of sticking the engine between the baggage compartment & the seats, but the prop drive would get overly complicated & heavy.Quote:
Originally Posted by Bob Dingley
Now that is funny. Must have been an MIT grad that specified lightweight bolts to hold ballast in. ;)Quote:
Originally Posted by Bob Dingley
Where the weight is located in your airframe is very important for maneuvability and spin dynamics. Weight at the extremities means more inertia which restricts maneuverability as noted in the USAF report on the F-15 ballast. Spin recoveries do not like weight at the extremities of the aircraft. In most Piper twins you get one rotation to stop the spin (from speaking with the former chief designer at Piper about twin engine airplane spin dynamics). In a C-310 I wonder if you get that.
I can report from my hours flying a C-310 that full tip tanks (50 gals or 300lbs on each tip) are a mass that in turbulence has to be controlled with firm and immediate aileron input.
Best of luck,
Wes
N78PS
Now that is something I would not have thought of. Obviously my previous post was strictly from a structural standpoint.Quote:
Originally Posted by WLIU
I sure hope this thing does like it is supposed to & wont stall or spin.
I am trying to come up with an accurate engine weight & cost effective ways to reduce weight - I think I can bring the engine down to 368lb (which is slightly lighter than a O-470) but I don't know what, if any accessories that includes.
Then, of course, there is the prop & prop drive to consider.
Yes, the pros of putting the weight out at the end of the arm is you get a larger CG shift for the same amount of weight.
As long as the mass is centered and you're flying coordinated, there really isn't a disadvantage (provided the structure can support it). As pointed out, as soon as you start flying much uncoordinated or turning on some random axis (like the spin) then you may have untoward handling changes.
My Navion had a 20lb weight in the tail to offset a heavy GO-480. After I changed engines to an IO-550 I flew with it for a while. Then I did some CG calcs and realized I could remove it. I did it primarily to get the 20lbs in useful load but the pitch handling change was a dramatic improvement and pleasant surprise. Slinging that bob weight around with every pitch change was more effect than I imagined.Quote:
Originally Posted by FlyingRon
Thanks for the pirep on the C-310 Wes. It confirms my guess. I'm more familiar with the Baron with its more inboard fuel. Sweet handling. Like dancing with Angelina Jolie. Bigdog's Navion experience points out that while lead ballast may be a necessary evil, it feels so good to get rid of it. Many just toss a sand bag or two in the baggage compartment.
Remotely locating the battery also has drawbacks. First, they continue to improve battery tech by making them smaller. Just add another in a few years? Another issue is the weight per foot of starter cable. I SWAG it as 0.4 lbs per foot for AWG 4. Many go to more flexible welding cable (AWG 0) at about 0.5 lbs per foot. Then there is the price per foot factor. Thinking outside the box, a remote battery and an air start system like a Russian radial could be an overall weight saver.
Bob
Everything in an airplane is a compromise. I have friends who wrestle with their Russion air start system. They carry a scuba bottle to recharge it after it leaks down when they are parked for a while. I will guess that the scuba bottle is not figured into the official empty weight of the airplane but they never leave home without it. Electricity is much better if you are flying somewhere south of Siberia....
Best of luck,
Wes
N78PS