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challenger_II

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Fisher County, Tx. USA
On the subject of horsepower:

"Horsepower" is a slippery subject. One wants to consider "Thrust", more than calculated horsepower.
An example: the Curtiss OX-5 was rated at 90hp. The Continental C-90 was rated the same. If One mounted a C-90 in a Curtiss Jenny, it would not fly.

More closely related to your project, let's compare the larger Mosler 40hp, and a Rotax 447 with a 2:1 reduction drive. Both are "rated" at 40hp. The Mosler turns a 54x24 prop at about 3200rpm. The 447/2:1 combination turns a 54x30 at 3250rpm. There will be a significant difference in thrust generated. You begin to see why "horsepower" is a less than perfect means of calculating speed/load-carrying capabilities.

Hope this is of assistance.

A quick re-calc to see if I could make a more sensible HP calculation:
 

Sockmonkey

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Flint, Mi, USA
On the subject of horsepower:

"Horsepower" is a slippery subject. One wants to consider "Thrust", more than calculated horsepower.
An example: the Curtiss OX-5 was rated at 90hp. The Continental C-90 was rated the same. If One mounted a C-90 in a Curtiss Jenny, it would not fly.

More closely related to your project, let's compare the larger Mosler 40hp, and a Rotax 447 with a 2:1 reduction drive. Both are "rated" at 40hp. The Mosler turns a 54x24 prop at about 3200rpm. The 447/2:1 combination turns a 54x30 at 3250rpm. There will be a significant difference in thrust generated. You begin to see why "horsepower" is a less than perfect means of calculating speed/load-carrying capabilities.

Hope this is of assistance.
Even then it's still uncertain as big slower-moving prop giving X amount of thrust will do better for takeoff than a smaller faster prop giving the same thrust.
 

BJC

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Even then it's still uncertain as big slower-moving prop giving X amount of thrust will do better for takeoff than a smaller faster prop giving the same thrust.
Are you saying that two airplanes with equal thrust will accelerate differently just because of a difference in prop diameter and RPM? Gee whiz, Mr. Wizzard! I thought that thrust was thrust.


BJC
 

Tiger Tim

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Take two: starting from a side elevation. I’ve traced one out as reference and one as a light outline to build up on.
283D46AC-C0C7-428B-8AD6-D28ED4192960.jpeg

Looks like a five foot root chord is about right, later I’ll see where that leaves me for a wing planform but for now I’ve laid out where the main fuselage tube would go (and limited it to 12’). I’ve also put the seat about nine inches ahead of where it was on the Piper Skycyle since a UL’s pilot makes up such a large fraction of its takeoff weight. Putting the pilot right on the CG should allow a wide range of pilot weights.
EED779F2-BF57-4162-94CA-FFB2E5888436.jpeg

From here I should be able to sort out the tail surface sizes next, working backwards from required tail volume.
 

Tiger Tim

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Looking for a quick sanity check. Based on the following formulae I get a vertical and horizontal tail surface area of 16.7 and 23.2 square feet, respectively.
3BE3867F-3DAC-4428-89BB-6F3AA6113D84.jpeg

That’s a vertical tail area of 12.8% of the wing area and a horizontal area of 17.8%. Compared to my model experience the stabilizer and elevator are on the small side while the fin and rudder seem quite large. I don’t think either of those numbers is too far out though but would like a second opinion.
 

wanttobuild

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A very nice way to use the forum here on HBA.
I do appreciate you taking the time to publicly document your process. I hope you don't mind and tag along without comment and learn.
 

cluttonfred

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What is your reference? I am used to seeing tails described in terms of volume - distance from CG x area of surface. That adjusts for a big surface on a short fuselage or a small surface on a long fuselage (within reason).
 

Tiger Tim

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What is your reference?
At this point I’m working out of Beaujon’s book, which is more or less an extremely simplified approach to UL design. Maybe too simple. I’ll have to look for a proper tail volume formula and see how it compares.

This hiccup is a good thing though, it’s a chance to either learn why the recommended fin/rudder area is enormous or it will help prove or disprove the book.
 

bifft

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Utah
Enjoying this thread, have spent a lot of time doing similar calcs.

Just looking at your tails sizes as you list them so far, calculated your tail volumes per How big the tail.

For your sizes so far as I understand them:

Sw=130
Sv=16.7
Sh=23.2
L=12
MAC=6.5

From this, calculate a span of 20 (Sw/MAC)
Can then calculate the volume coefficients.

Vh=23.2*12/(130*6.5)=0.33 - if you look at the formula you posted it is just running this calculation the other way to get an area based on a Vh of 0.33. 0.33 strikes me as kinda low, might lead to a limited CG range.

Vv=16.7*12/(130*20)=0.077 Which is above average for Vv. The calculation you posted before is odd, using the gross weight (presumably in lbs). The volume coefficients will work out in any measurement system (the lengths divide out giving a dimensionless number), but the calculation you have is lbs*ft/ft^2 = lbs/ft or a wing loading.
 

Tiger Tim

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Vertical tail area is based on wing span, not weight.
I didn’t recall using weight for this either. I wonder what the intention was when it was put in Beaujon’s formula, maybe it originally had wing loading (for some reason) but after a bunch of simplification gross weight was left?

Either way, I’m going to go in another direction with it.
 
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Tiger Tim

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On the subject of horsepower...
I couldn’t agree more with your whole post and thanks for bringing it up.

At this early stage of design I chose to go with the FAA’s guidance on maximum horsepower, which may or may not even be accurate, as a sort of go/no-go reality check. At some later point I plan to determine an approximate minimum thrust for level flight then actual engine selection can have some power level between the two, depending on desired climb performance... I think.
 

challenger_II

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I might suggest calculating minimum thrust needed for a safe rate of climb in warmish, humid conditions, at 150% of gross weight, and wing loading. Start with the hardest conditions, first, and you may not find yourself in a dangerously underpowered machine at the worst possible moment. Then, minimum power for a safe level cruise will fall into place of its own accord.
 
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