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

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by J.L. Frusha, Apr 30, 2019.

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1. May 26, 2019

### poormansairforce

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Hardly, I was going to show you why you need the area the others suggested while using something you apparently trust. You say you want to learn but it not always apparent to us.

If everything we say is snarky then there is no way for us to communicate anything you need in order to learn. Thats exactly why most of these kinds of threads get a "rtfm" response. We get tired of being told we are a mean bunch!

Yes, 5.66 ft x 26 ft span gives 147 s.ft.

If you taper the wings then the span needs to increase.

2. May 26, 2019

### J.L. Frusha

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I don't trust it, I found it. there's a difference and, no, I don't know how to use it either.

Yes, I KNOW the span increases, which is wtf I ASKED for HELP calculating it.

rtfm... Not being snarky and mean at all, right?

Maybe if folks didn't come across as snarky and mean, they wouldn't be TOLD they are snarky and mean.

3. May 26, 2019

### BJC

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J. L.

I like your enthusiasm.

Please understand that answering technical questions takes time and effort, and is a real PITA even for extremely knowledgable people. Asking succinct questions, then asking follow up questions to ensure your understanding will result in a beneficial exchange of information. Being argumentative only results in the knowledgable people opting not to reply and the snipers taking shots to spur you on.

Note: I tried to send this via private conversation, but got a message that I am not allowed to start a conversation with you.

BJC

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4. May 26, 2019

### poormansairforce

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Its hard to communicate anything with words across the net and no one is going to spend hours helping to design a plane for you. You really need to read some books on the subject to get yourself grounded in it.

You need the lift coefficient of whatever airfoil you choose to use and that will tell you the wing area needed and the wing plan form will decide the span needed and the Reynolds number at different points of the tapered wing may suggest different airfoils at different locations. Again, books will explain it much better than we can over a forum.

Last edited: May 27, 2019
5. May 26, 2019

### poormansairforce

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One other point:
Its easier if you try to solve it and show us your work so we can just check it.

6. May 26, 2019

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At 500 gross and using 3.4 lb/sf the area needed is 147

The area divided by the mean chord is the span 147/3.8 = 38.7 ft. This is the span for your tapered wing.

7. May 26, 2019

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So we have established that the plane has a span of 38 feet and a length of 14 feet. Does anyone else see the problem?

8. May 26, 2019

### Tiger Tim

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9. May 27, 2019

### J.L. Frusha

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No idea why, I'll look at my settings, see if I can find anything.

Thank you and, yes, getting answers then asking more questions helps a lot. What I've tried to do is take answers, learn from them and show applied learning.

Nope, that's for a rectangular wing.So, since it's tapered between the 2, yet still 147 sq ft.

So, it's between 25 ft 9 1/2 inches for a rectangular wing at the root chord of 5 ft 8 1/2 in
and your 38 ft 8 3/8 inches... Closer to 32 ft 3 inches Still a bit far out of reason, so ditch the factory tapered wint and go with the shorter 25 ft 9 1/2 in with the chord of 5 ft 8 1/2 inches.

Thank you, that helped and it simplifies construction...

10. May 27, 2019

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You gave the mean chord as 3.8 ft....did you mean something else (tip chord?). Do you know what "mean chord" is?

Anyway, a rectangular wing should work for making legal stall speed at 500 gross wt. But, with only a 13.5 ft. fuselage tail volume may be a problem. You will need good sized horizontal and vertical surfaces.

11. May 27, 2019

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Tiger Tim........I appreciate your interest and I will think about sharing information at some point....just not now.

Not pertaining to you Tiger Tim :
But for the time being...this Forum in general has taken a turn for the worse and is being steered by a few particular members down a path that I dont particularly care to take.
The pics were posted to prove a point..."Dreamers" have a place in this world and without their struggles and innovations the human race would still be huddled in caves and be terrified of the things that go thump in the dark.
The Wright brothers were fricking idiots in the beginning with their designs but that situation turned out pretty well in the end.
Cut the man some slack and actually try to be helpful instead of discouraging him.
I have 1 rule for my guys at work....don't ever come to me with a problem unless you have a solution to fix said problem !
Point out issues to the man but also try to have a solution to the issue you just pointed out.
Pretty easy to do and no reason it should be anything but civil.

In the meantime.....I'll just keep doing what I do and not worry about those that say I can't do it.
Ask me how I know it can be done......lmao

And as a side note.....I do some of my best designing on the "shitter"....gives a man time to put life in perspective.
Maybe some of our members are a tad bit "backed up" and could benefit from some alone time.......

Kevin

12. May 27, 2019

### pictsidhe

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I often go thump in the dark, promptly followed by an expletive about my toe.

I am also a dreamer as is anyone who designs their own aircraft, or even ultralight! Many here were deeply skeptical about my Hurricane (some still are), but I did some maths to see if it was feasible. Lots of people here respect numbers. you know where you are with numbers, to 3 significant figures. The numbers said yes, go for it. Some (okay, a lot ) of my ideas have turned out to be duds, but I have managed to determine most of those by maths or experiment. I am slowly making progress. I'm feeling increasingly optimistic about the project, despite having few fully designed parts. I have ballparked most of it several times (redesigns of basic parts that wouldn't work) and have a much clearer idea how to proceed. I'm getting close to doing the details. They are like ballparking, only in far more detail. Ballparking saves a lot of time. My undercarriage required a different root rib truss. Aeroelasticity considerations needed a completely different wing spar scheme, which then affected the centre section truss. I've lost count of all the changes to the centre truss . Major knock on changes to parts that I'm glad I hadn't designed in detail...

Dreaming? well, I spent a few hours struggling with my control surface hinges the other night. My wing and empennage are going to be much simpler and lighter if I let them bend somewhat, but that causes a problem with the control surfaces wanting to centre. My gut knew that there is a fairly simple way to hinge them so that the control surfaces wouldn't self centre when the wing or tail bent under high g, but it just wouldn't come to mind. I went to sleep. I woke up with my simple non-centreing hinges

Apparently genius is 1% inspiration, 99% perspiration. I am no genius, I need about 99.99% perspiration...

Designing a plane from scratch is hard. Designing a monocoque 103 is really hard. It does need good maths skills. While a P-51UL could never be as cool as Hurricane Mk103 (sorry, couldn't resist), I like to think that I have some idea of how one could be built as well as just how much maths is needed to do it reasonably well. It's about 4000 miles away from a Hurricane, not a million . There is no way it could be done half well by guesswork unless you happen to be a professional, skilled aeronautical engineer who has designed many aircraft already and have a feel for it. There are other people here who have also put some sweat and maths into designing planes.

There are many people here with considerable engineering acumen, I have learnt a lot from them.

13. May 27, 2019

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Pictsidhe........
There is nothing wrong with being a "Dreamer" at all.
If it wasn't for "A Dream" we would have never tried to fly,go to the moon,work for equality in this country,have life saving break through medical procedures...etc.
Everything worth obtaining in life starts out as a "Dream" and progresses from there.
It befuddles me to figure out why a particular member on the forum would make a statement to the effect that he is tired of the "Dreamers" starting threads and trying to ask for advice because I thought that was the whole intention of this forum...for like minded people to come together and discuss a topic that they share a common interest in.

As far as your design goes.....educate yourself,plan what you need to do and then do it.
It's alright not to get it right the first go around.....hell,even the major aircraft companies screw up and they have **** near unlimited data,material and manpower at their disposal.
One common mistake that I constantly see people make when they design aircraft is that they rely wholeheartedly on the numbers which puts them into a loop that they can't escape.
One number says it wont work so they change something to fix it....now another number says it wont work so they change something to fix it....now another number says it wont work.....on and on and on....etc.
Yes,calculations are a good thing to have... but are they the absolute gospel when designing something ? HELL NO!
Give a mathematician and an aerodynamisist a big ass computer,pencils,paper,slide rules,scales and etc.....they will give you 100 reasons why a bumblebee can't fly....but yet the little fellows do it quite well every day.
Sometimes the numbers just wont add up no matter what you do...and they dont have to.
Look at other aircraft and see what previous designers have done that worked.....then incorporate their R&D into your design.
I bet the numbers didnt add up across the board for them either.........but they were still successful.

Kevin

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14. May 27, 2019

### J.L. Frusha

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Tail surfaces are larger than those of the Ultracruiser. I don't know how to do the calculations, but I can do a visual comparison.

P-51 Mustang has a tapered wing. Many model aircaft use a simpler rectangular wing on the original style fuselage for multiple reasons, ease of construction being one.

According to the way FAR Part 103 describes finding the mean chord for a tapered wing, it is the chord at half the span of one wing, midway between the root chord and tip chord.

Calculating a wingspan using one chord produces numbers for a rectangular wing, which is why I was asking for help calculating for the Mustang wing using the root and mean chords, since it has a uniform taper.

I've already deviated from scale, considerably, and was at least trying to maintain the tapered wing. That doesn't appear to work, in this case.

Combination of Cartoon Scale and Standoff Scale..., as long as it works and is relatively safe, is fine with me. Not trying to kill myself, or anyone else.

Note - I said "relatively safe"... I fully understand that life is not 'safe'. I've broken ribs, getting kicked by a mule, broken my collar-bone in a motorcycle accident, hit the windshield before seatblets were mandatory... I also passed the final Army PT test with a herniated disk and fractured Tibia, but I am fully aware that I am taking my own life in my hands, when at the controls of a vehicle.

15. May 27, 2019

### J.L. Frusha

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Best guess, using ecalc and comparinsin with Marske Monarch

16. May 27, 2019

### pictsidhe

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When the numbers don't tally with reality, you've done them wrong. I like to calculate with two different methods. An accurate one and a cruder sanity check. If they aren't in the same ballpark, I need to work out why. Often, the sanity check is software. Sometimes, it's easier to derive an empirical relationship than a mathematical one. But that takes experiment.

17. May 27, 2019

### pictsidhe

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That looks like it has a horizontal tail volume of somewhere around 0.25 That's very low.
A full size Hurricane has 0.37, and I'm trying to determine if I can get away with that...
Cue opinions on tail volume. The lowest figure that I found was 0.35 for an early Cub.

101sqft of wing area is going to need good flaps. You may do it with a thick wing (also good structurally), split flaps plus flaperons. Plain flaps as the full size p-51 won't get you there.

Last edited: May 27, 2019
18. May 27, 2019

### mcrae0104

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I just looked through AC 103-7 and I do see how you got the idea that mean chord is at mid-span.

The term mean chord is misused in the advisory circular. Keep in mind they're using it in the context of measuring camber:

The MAC is not simply the average of all of the chord lengths. For a tapered wing, that would be at the center of the length of the semispan. MAC is the centriod of the semispan, and on a tapered wing that is inboard of the midpoint because more area is inboard. Dan Raymer gives a nice way of understanding MAC in Simplified Aircraft Design for Homebuilders:
You really owe it to yourself to get a copy of this book. It is not aimed at ultralights per se but it covers many topics and concepts you will need to understand in the course of designing your plane.

There is also a geometric way to find MAC for a given wing (google for more info).

Now, all that being said, I don't think MAC matters (strictly speaking) for now. Here is an equation that will be helpful, expressed two different ways:

$$\text {span} = \sqrt{\text {aspect ratio x area}}$$
$$\text {aspect ratio}={{\text {span}^2} \over {\text {area}}}$$

You can find the area needed (based on the AC103-7 chart; it will be more than 100 ft^2). You can find the aspect ratio of a full-size P-51 by measuring the span & area. Now that you have the area and desired aspect ratio, you can find the resulting span.

Last edited: May 27, 2019
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19. May 27, 2019

### J.L. Frusha

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I'm specifically looking at Fowler Flaps. They can be relatively simple to hinge and operate.
http://www.tspeer.com/Fisher/C506F.htm

Thank you, I'm guessing, for their purposes, it's 'close enough for government work' and probably how they expect most people to calculate it, to comply. Oversimplification usually complicates things, instead of making useful contributions.

I'll look for the book at the end of the week... Payday...

I've changed things a bit, by moving the chord at the fuselage fairing to the root, stretching to match length at the fuselage, and stretching to meet the span. I'll see what recalculating the A/R with the formula comes up with, as well. FAR 103 does specify that real-world testing and certification is an acceptable alternative. Thoughts on the Fowler Flaps above..?

20. May 27, 2019