Weight and balance Hippert's plane

Jeff Brooks · 10 October 2009, 07:31 AM

edit ... edit

jumpinjan · 10 October 2009, 07:57 AM

Quote:

Originally Posted by buck toenges

What else do I need?
Buck

Buck,
You need to collect accurate dimensions on YOUR AIRPLANE (and not using data from other aircraft or someone's plans). So we can calculate where your CP is located. We need the "actual" wing stagger, gap, wing area and leading edges location, cords, and so on with respect to your datum. Didn't you say your A&P measured all this, or maybe I misread it?
Jan

baldeagle · 10 October 2009, 09:43 AM

I think that Fokker and Co. knew that a tail heavy airplane is more maneuverable, and designed them that way, then compensated aerodymanically with the positive incidence on the tail. Not tail heavy enough to not recover from a spin though, but the question is how far back is still safe for a modern replica?

Jeff, you may have already posted this, but have you figured out the Mean Aerodynamic Chord for the D.VII referenced to the upper or lower wing leading edge?

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Jeff Brooks · 10 October 2009, 10:35 AM

Baldeagle,
I have posted this in a few places, just not here on this thread.

I did not use the shadow method, where you combine the length of the leading edge of the top wing with the overlap of the trailing edge of the bottom wing.

I used a formula for biplanes with different size wing area. I found this in an aerodynamics book.

Here it is for 25% of the MAC ...

And here it is for the 23% MAC ...

I have to go back and redo this formula for the ranges given in the Roussau construction notes. I want to see if the method I used jives with the figures the construction notes gave.

I also believe that Fokker did not make the tailplane heavy just to make them manouverable ... without the thrust avialable from the Merc IIIa engine. Being able to hang on the prop makes all the diffiernce!

buck toenges · 10 October 2009, 12:06 PM

I just took a quick measurement of the plane and found out if I use 6" from leading edge of lower wing then my total length from prop hub to 6.8" aft of leading edge of bottom wing is 66". Right now we were playing with a cg of 62-63" aft of prop hub as cg. If cg is more aft and I am considering this from the info I have on those 2 aircraft then I would assume I would need about 120lbs up front to get to the cg at 66" from front of prop.

I want to go back and remeasure the plane using a point on the fuselage frame close to the leading edge of the lower wing and recalculate and try to get all the pertinent details IE detailed chart of dimensions & weights of the total airplane (including the wing stagger, gap, wing area and leading edges location, cords, and so on).

The point I was trying to make on the two planes I mentioned was about where cg "should" be on a DVII. I know different engines, plans, and material can be variables to where cg is but what I would like to know is where should cg be. Should cg be calculated by modern ideas and norms or should cg be more aft and is that where Fokker intended it to be?
If we need it to be by the "norm" of today's standards I have to add at least 200lbs. If cg should be more aft then I need about 120lbs.
I hope it is more aft so I don't have to add some much dead weight.

Joe Perkel · 10 October 2009, 01:19 PM

Quote:

I think that Fokker and Co. knew that a tail heavy airplane is more maneuverable, and designed them that way, then compensated aerodymanically with the positive incidence on the tail.

Interesting,... an advantage that you would likely want in a wartime environment in order to perhaps more easily bring your guns to bear on a target. Defeat one target, and quickly snap around to the other while at altitude and at full power, this was after all, an aerial gladiatorial contest to the death.

Quote:

Should cg be calculated by modern ideas and norms or should cg be more aft and is that where Fokker intended it to be?

I think that I would answer this question by how the aircraft is aerodynamically best served in a traffic pattern environment as opposed to above. A comprehensive flight test program should determine "where that demon lives", and what action is required.

jumpinjan · 10 October 2009, 05:39 PM

Buck,
Look up in the index of ANY engineering aerodynamics book the words "Stability, Longitudinal" and you will read that the aircraft can tolerate some slight amount of aft CG (countered by trim), but its condition is unstable.
Jan

Jeff Brooks · 10 October 2009, 06:26 PM

Jan,
I wish we had gotten some time to get together an review your data at Dayton ... I was tied up in the evenings with my son.

mustang · 10 October 2009, 11:47 PM

Quote:

Originally Posted by jumpinjan

Buck,
Look up in the index of ANY engineering aerodynamics book the words "Stability, Longitudinal" and you will read that the aircraft can tolerate some slight amount of aft CG (countered by trim), but its condition is unstable.
Jan

It certainly is. As anyone who has flown a P40 or P51 with a full fuselage tank will confirm.
Mustang

Nickh · 11 October 2009, 08:30 AM

Sorry to intrude,

We've experienced similar C of G problems with our Sopwith Pup restoration, which I thought might be of interest.

Basically the airframe is designed tail heavy, but is sorted out with its' armament and full ammo box. As we are building the beast to its' civilianized guise (unarmed) we have a tail heavy aircraft at the moment, but we may have solved this with a nose extension on the engine.

Spotting the diagram above I noticed that the C of G is calculated as 2.5" in front of the leading edge of the lower wing. Will the weight of the machine guns and their full ammo boxes bring that aft substantially . . within the designed limits ? Just wondering.

All the best