# P-51C at ~70% scale as ultralight?

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#### J.L. Frusha

##### Well-Known Member
3 times the MAC between the wing at 25% chord and the HT MAC at 25% chord, is considered (on average) to provide the optimum sized Tail feathers (HT and VT)
3(4.39+1.24) 16.89

MTOW in Lbs.=[1+0.67(St/Sw)]
500 = 1+0.67(21/101) = 1.393 I truly do not understand this one.

I've been able to resize the pics of the docs. Will upload them ASAP. Battery about dead on the laptop, at the moment.

#### mcrae0104

##### Well-Known Member
Log Member
Please don't design your plane by a potpourri of rules of thumb. You will get nowhere without ever having to understand why it does or does not work. Get the book recommended earlier and set things in context. I'm on my way out the door or I would elaborate a little more, but aircraft design just can't be be discussed meaningfully (at this entry level) in disparate fragments. This is not intended to sound condescending--I'm saying it because I want to see you successfully design and build your plane.

#### Lendo

##### Well-Known Member
J.L. the answer is 1.393 times the 500 lbs or 696.5 lbs is what the wing has to lift OR an extra 196.5 lbs of down force by the HT. That's a fair bit about 3 times more than my Light Sport design - why because your lever arm is so short, a longer lever arm = smaller HT = smaller down force.
Also with a smaller down force you get away with a smaller wing as it doesn't have to have carry that bigger download.
You will notice that everything is reliant on everything else.
Someone earlier suggested you can take off the wing weight, that's because the wing carries it's own weight. I'm unsure about fuel load within the wing, perhaps some!

I concur with MC 104, you can pretty much nail dawn a preliminary design with, with the right tools and the formula are not that hard to manage.
I would also check that HT area with the formula I gave you , if you haven't already.

BTW have a good look at that Light Sport Carbon Cub, ( from Europe) which has the impressions of Rivets all over it, you will get a good visual representation of the 70% P51 and how the tail looks compared to the wing and Lever arm - it's an exact copy reduced to 70%.
George

#### Lendo

##### Well-Known Member
J.L.
Oops should always check someone else's work, 21/ 101 = 0.207920792 x .67 = 0.103930693 +1 x 500 = 569.6534653- only 69 extra lbs - not too bad!! I think you could get that down a bit with some extra lever Arm, depending upon how that looks visually - for you.
Your getting there just watch those calculations.
Sorry for not checking that first.
George

#### J.L. Frusha

##### Well-Known Member
~65%scale w/HS and VS increased to 110%
21 ft 9 in LOA
24 ft Span (not including wingtips)

#### pictsidhe

##### Well-Known Member
You should design your tail after determining the loads it will need to generate as well as for static stability. A tail stall is a Very Bad Thing. Fowler flaps will need a bigger HS than plain flaps.

#### BJC

##### Well-Known Member
One advantage of slotted flaps over plain flaps, for some airplanes, is that the nose will be lower at stall with the slotted flaps. That allows for a more nose-down approach to landing, providing better forward visibility.

BJC

#### mcrae0104

##### Well-Known Member
Log Member
21 ft 9 in LOA
24 ft Span (not including wingtips)
The length of your airplane isn't terribly important. What is more important with regard to longitudinal stability is the distance between the wing and the tail (25% MAC to 25% MAC). This is what Lendo was getting at in post 275.

There are many other things that will play into longitudinal stability such as wing incidence and decalage, pitching moments (including flaps as pictsidhe mentioned), thrust line, etc. Billski has posted extensively on longitudinal stability. Search for his stuff as an introduction. If it doesn't make sense, you can learn a little about it from the Simplified book or a lot about it from Airplane Performance, Stability, and Control. Online RC calculators, maybe not so much.

If Mr. Spencer is around, he might be willing to comment some on the design process of his own plane, which might be instructive here. I think you would find a lot to enjoy if you read his thread. In the first post he noted:

Rockiedog2 said:
I had the spars, lift struts and gear leg professionally analyzed. The rest of the airframe is pure eyeball engineering and TLAR. This is my 6th build so I do have a fair feel for what will probably work for me altho I would stop short of recommending my stuff to anybody.

#### pictsidhe

##### Well-Known Member
Have look at the Rate my flaps thread for an idea of what is involved in the aero design of slotted or fowler flaps.

#### Lendo

##### Well-Known Member
J.L. Well I can't argue with any statements made here, I've been trying to make it simple with providing formula - as you need them.

Yes! The Lever Arm is 25% Wing MAC to 25% HT MAC to find the MAC.
Using a drawing or better (more accurate) if in CAD.

1. Add the tip length each side to the root length, (root length is that which is the wing extended into the center of the fuselage.)
2. Add the root length to each side of the tip length.
3. Draw a line from one side of the now extended root to the opposite side of the now extended tip.
4. Draw an opposite line on the other side.
5. Where the two lines cross is the MAC. ( Mean Aerodynamic Chord.)
6. Do the same to the HT.
7. The distance between the two is your lever Arm.

This is especially the case for tapered and or Swept wings.

Naturally there is a formula for this, however this is the easiest way and I use it in CAD.
If you need the formula for a Spread sheet, I can provide that.

For basic preliminary deign I use 30% Chord for Flaps, to 60% of the available wing semi-span.
I use 25% chord for ailerons to 40% of the available semi-span.
I use 40% chord for Elevator and Rudder. (plenty of control there).
Rudder should extend below the HT to allow for 30% to 50% of Rudder are to maintain control in the event of a Spin, where the VT is blanked by the HT from 30° to 60°

Your getting there, don't feel intimidated.
Everything is easy, when you know it!

George

#### J.L. Frusha

##### Well-Known Member
3-way comparison:
Ultracruiser
shortened 65% P-51 caricature
65% P-51

Now, the caricature I've been trying to work at, to cut weight, has the same wing and tailplane as the 65% scale P-51, both of which have significantly greater tailplane area than the Ultracruiser.

#### BJC

##### Well-Known Member
J.L.:

Remember that it is the tail volume that matters, not just the area. Also recall that side area forward of the CG is destabilizing, including the propeller.

BJC

#### Lendo

##### Well-Known Member
J.L.
Interesting thing about tail area, with a longer Lever Arm, the weight of the tail decreases the tail area and therefore the weight, and that decreased tail are is more than the extra weight of the extended fuselage, so it a 'win. win', decreases weight overall ( smaller tail area (both HT and VT) and reduced wing area due to reduced load on the wing by the smaller HT.),

BTW how is that Excel Spread Sheet (SS) going, if you have Excel on your computer, I can do some very basic calculations for you, which you can manipulate. My early SS were taken from Roncz's papers published in Sport Aviation 1990, supported by Several very good texts books. This is very basic stuff and doesn't go into the more complex areas of Longitudinal Stability etc.

George

#### pictsidhe

##### Well-Known Member
I'm waiting to see the monocoque stress analysis myself.

#### Lendo

##### Well-Known Member
J.L.
Just looking at your design. Here's an interesting bit of detail, which doesn't necessarily affect you, but interesting none the less. With tapered and Swept wings, the leading leading edge sweep angle can add wing area to the design. Up to 20% sweep at 25% Chord the difference is minimal and can be ignored, over that the area needs to increase and increases exponentially. So when you see aircraft with swept wing and swept Ht and VT, think of the drag need with those extra areas and extra weight, just to look like it goes fast.

And yes there's a formula for that as well, but I don't see a problem with your proposals in design.

Naturally over the speed of sound there's no option.
George

#### Lendo

##### Well-Known Member
Oops, Anyone spot that error, that 20% should have been 20°.
Sorry!
George

#### J.L. Frusha

##### Well-Known Member
J.L.
Interesting thing about tail area, with a longer Lever Arm, the weight of the tail decreases the tail area and therefore the weight, and that decreased tail are is more than the extra weight of the extended fuselage, so it a 'win. win', decreases weight overall ( smaller tail area (both HT and VT) and reduced wing area due to reduced load on the wing by the smaller HT.),

BTW how is that Excel Spread Sheet (SS) going, if you have Excel on your computer, I can do some very basic calculations for you, which you can manipulate. My early SS were taken from Roncz's papers published in Sport Aviation 1990, supported by Several very good texts books. This is very basic stuff and doesn't go into the more complex areas of Longitudinal Stability etc.

George

Using Open Office and I found a couple of spreadsheets I can almost use. Almost meaning I don't know enough to fill in enough information and don't know enough to calculate the missing information, but I'm plugging away at it, as time allows. In the middle of a project relating to a nonprofit I am trying to start, at the moment.