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Minimum Power Requirement

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Minimum Power Requirement

Old 03-01-2008, 02:21 PM
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kerklein2
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Default Minimum Power Requirement

I know this is kind of a difficult question because most here are concerned with a good performing aircraft. What I'm trying to figure out is what the absolute minimum power needed to fly a 10-15lb flying wing. I've estimated the stall speed using some calcs at around 17-20mph and doing some rough calculations, the power requirement for level flight came out to around .6 - 1.2 HP.

My question is:

1. Does anyone believe this?
2. Does anyone have a more accurate way to calculate this?

I'm trying to figure out what the smallest engine I can select would be just to barely fly.
Old 03-02-2008, 12:46 AM
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Default RE: Minimum Power Requirement

The weight is almost irrelevant, of more use will be the drag calculation at best L/D, then you can use an engine to give you that same amount in thrust. Trying to fly as slow as possible will need much more power than you will need to maintain flight.
Evan.
Old 03-02-2008, 01:59 AM
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wellss
 
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Default RE: Minimum Power Requirement

drag in turns is higher as well. Every time you turn, you will lose height, so you need to account for that too. In order to calculate induced drag, we would need the aspect ratio for the wing (span and area).
Old 03-02-2008, 09:44 AM
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Villa
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Default RE: Minimum Power Requirement

Hi kerklein2
I would love to read more details about your project.
Old 03-02-2008, 10:00 AM
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Default RE: Minimum Power Requirement

One thought in reading this is that it is sort of directly related to electric power. With glow the power output is near same from fire off to out of fuel. 'cept it gets slightly lighter as it consumes fuel.

However, the electric guys do not lose weight with each minute of flight, it stays relatively constant. The longer they fly, the less power is available to them too, for the rated output is at Maximum, on freshly charged batteries, not after five minutes of flying time when they are lower. Why they do not design and rate an A/C after five minutes of flight....

I would not say design for maximum thrust, but as stated the thrust available at five minutes in to a flight. When the electric guys are thinking of touching down soon, but horsing it into the air for a few more minutes.

Wm.
Old 03-02-2008, 11:28 AM
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Default RE: Minimum Power Requirement

I was the pilot for a heavy lift model in the SAE Aerochallenge. We flew a 33 or 35# gross weight model with a .61 engine. So I pretty much believe your estimates. In-flight drag may be even less than you calculated. The most challenging issue was thrust to overcome the mass on the takeoff roll; good old F=Ma. Once up to speed the model was relatively easy to fly. I'm the wrong kind of engineer to give advice on your methods of analysis.
Chuck
Old 03-02-2008, 05:59 PM
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kerklein2
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Default RE: Minimum Power Requirement

It's about an 82 inch span, and the wing area is about 1000in^2.

Villa: What exactly do you want to know?
Old 03-02-2008, 06:23 PM
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Villa
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Default RE: Minimum Power Requirement

What is the purpose of the project?
Old 03-02-2008, 07:09 PM
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Default RE: Minimum Power Requirement



I think he needs a picture, weight. Do you do any stunts or repeat Victory Rolls.
Maybe hang it in the air by prop thrust only. Fly inverted close to the ground, do touch and goes.
Take off from grass, how tall is it ?

What is the maximum wind speeds do you fly in ? That is not a easy answer for 99 % of us.

Give him a feel for what you have & what you want it to do.

Rich
Old 03-02-2008, 10:57 PM
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kerklein2
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Default RE: Minimum Power Requirement

No stunts, just level flight. It is just a flying wing, 30 degree sweep angle. It will most likely be hand launched.

Would like to be able to fly in high wings if possible.
Old 03-02-2008, 11:46 PM
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Default RE: Minimum Power Requirement

Why the expected high weight? There's no way a properly built 1000 sq inch wing of that size needs to weight that much. WIth even a little care it can be made to be around 4 lbs and fly with a .15 to .25 on it. Or is this a load carrying model?

For windy flying you will find that you need more power to maintain flight. The low level turbulence can play havoc on the model and a touch of extra power is all that helps to maintain altitude. I've never studied or thought about why this is the case but I suspect it's got to do with the sudden mini stalls due to parts or all fo the wing being subjected to various velocity variations and some of those will be high drag stalls that may last for just a moment and then you've got normal airflow again. Either way it takes a bit more power to hold a climb or fly level in turbulent conditions.

If it is a load carrier then you can forget about hand launching. There just is no way to throw a flying wing at enough airpseed to be at the minimum from a hand launch.

When you're calculating the lift and thus the speed you need to be aware that a flying wing is limited to lower lift coefficients. In the case of the plank style wings this is due to the airfoil that has a lot of reflex in the camber line to be self stabilizing. IN the case of a swept flying wing this is because the tips are operating at a much reduced lift coefficient that can often be near 0 while the center is lifting its heart out. Again this is due to the need for stability. But it means that less of the wing is actually there to lift the load as some of it is flying at greatly reduced lift values because it's being used to stabilize the pitch moment during flight.

All of this means that you need to alter your calculations for minimum flying speed. My own rule of thumb is that for swept flying wings I figure that only 2/3's of the wing area is actually "lifting" and the other third is there for stability purposes. For plank style wings you can just use the airfoil's maximum lift coefficient. Typically they are down around 0.6 to 0.8 which is a lot lower than many regular airfoils.
Old 03-04-2008, 12:17 PM
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kerklein2
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Default RE: Minimum Power Requirement

Somehow my response got deleted.

It is a load carrying model.

I don't understand why the lift coefficient would be less near the tip. It is still at the same angle relative to the flight direction, and the airfoil is still the same shape.

Why wouldn't you be able to hand launch it? I think you could throw 10lbs at 20-30 mph.
Old 03-04-2008, 02:06 PM
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Default RE: Minimum Power Requirement

A lot of posts got dumped in the site maintenance from yesterday. I lost a few myself.

The tips on a swept flying wing are highly washed out so that they can provide stability to the design. So they are not flying at the same angle by a long shot. Because of that washout angle they are flying at a much lower lift coefficient compared to the center part that is more forward. As a result you cannot use the whole wing when determining your flying and stall speeds. The tips are just not pulling the same load as the rest.

If you try to avoid that and make the wing with no washout then it'll be unstable in pitch unless you reflex up the elevons to a high degree like Zagi's and other stuff like that does. But that's a highly inefficient way to get your stability as well and it still pushes those airfoils into a non lifting mode since the angled up surfaces act like spoilerons ALL THE TIME while providing the needed stabilzing forces. At least with the washout angle they still help out a little.

Wander over to www.b2streamlines.com for some tech articles on designing flying wings. Lots of good info there. In particular check out the Panknin swept wing washout tool that runs in Excel or using the spreadsheet in Open Office (a free download that opens and runs Microsoft Office Word and Excel files www.openoffice.org )

And you will NOT be able to throw that size and weight of model at that speed unless you're facing a 15 to 25 mph headwind..... unless you can morph into the Incredable Hulk. Sorry But if you want to try it then buy a 10 lb bag of sugar or get a 10 lb hunk of 2x10 and try throwing it and see how far it goes. It won't be far and it won't be fast. With a good headwind you MAY get away with it but is far more likely just a matter of time until you don't have enough headwind or you don't throw it quite hard enough and it stalls and flips over for a cartwheel crash. Mind you with a good hand hold and at 5 lbs it'll fly a lot slower so no problem. It's only when loaded up that you should not be trying to hand luanch this thing.

Definetly plan on some landing gear even if it's just a dolly that the wing flys out of when up to speed. Be sure to include a streamer on the dolly so you can find it later. It's amazing where they can get to.
Old 03-04-2008, 04:41 PM
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Default RE: Minimum Power Requirement


ORIGINAL: kerklein2

Why wouldn't you be able to hand launch it? I think you could throw 10lbs at 20-30 mph.
Flying wings are extremely hard to hand launch if you have to impart extra speed to them with the launch for three reasons.

There is usually nothing to hold onto but worse, there is no fuselage to help you judge the AOA of the wing, something extremely critical to launch success. They pitch up or down and over in a heartbeat, something to consider if yours has a spinning prop on it.

Trying to run and throw a wing that's got a spinning prop on it is going to be very dangerous.
Old 03-04-2008, 05:07 PM
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kerklein2
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Default RE: Minimum Power Requirement

The wings have been designed for us and they do not have any washout (twist right?). I have been told that they have been well designed to take care of a lot of the problems that flying wings normally encounter. I am personally designing the center section/fuselage and it wont be hard to design in something to hold on to when throwing it. You really don't think you could throw 10 lbs at 20mph with a couple steps? What about a water balloon launcher type of launch?

Oh, and I still don't have an answer to my original question.
Old 03-04-2008, 08:18 PM
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Default RE: Minimum Power Requirement

If the wings are a swept style and they do not have any washout then I sure hope they are using a self stabilizing airfoil. But by using such an airfoil you're shooting your self in the foot since on a swept planform you can push the more forward part of the wing to higher lift coefficients and the higher lift is actually part of the stabilzing force setup. And yes, when I talk washout I'm meaning twist. Although for a flying wing "twist" is tecnhically more accurate since the term "washout" is something done on regular airplane wings to reduce tip stalling rather than make the wing more stable. It's the same thing but the name indicates why the twist is added. Clear as mud?

And I still say that when it's loaded that you're going to be lucky to get it to 10 mph let alone 20. And consider too that there's still that issue about the "effective" wing area and thus the effective wing loading and the resulting flying speed.

Without seeing the planform and airfoil choice it's hard to say much more than that. But either way you can't input 1000 sq inches operatintg at high lift coefficients and get a true picture of the real world situation here. Flying wings need to give up something in the lift department in some manner in order to be stable in pitch. So realistically your minimum flying speed will be higher than you have calculated. Again if you could show us the numbers you used we can indicate if you're being unrealistic in some area.

A power launcher is full of risks and how it works would depend a lot on the use of guide rails or other factors. I still say that if you really don't want landing gear on the model then the best way to take off is to use a big wire frame takeoff dolly. That way there's no surprises for the pilot like you're bound to get from a handlaunch of this large a model.
Old 03-04-2008, 08:30 PM
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Default RE: Minimum Power Requirement


ORIGINAL: kerklein2

You really don't think you could throw 10 lbs at 20mph with a couple steps? What about a water balloon launcher type of launch?

Oh, and I still don't have an answer to my original question.
You're not getting the point.

Anything can be thrown like a ball. Launching a wing with exactly the correct AOA is an entirely different thing. I've launched gliders for years and years and years. Off of cliffs, straining against powerful winches, gently on test glides, every way you can think of. A running launch is no problem for me either. But EVERY flying wing I've ever handled has been extremely sensitive. And I've never seen one launched by a newbie the first try. Never.

You're not throwing a water balloon.

A water balloon doesn't have a critical AOA that has to be held very, very closely. Flying wings have a strong tendency to somersault when pushed at the wrong AOA. And you're not going to have a bit of help from a long easily observed fuselage to visually judge the AOA. And a water balloon doesn't have a propeller doing 10,000rpm about 6 inches from your "throwing" hand.

You are NOT throwing just any old thing. You're going to be trying to accelerate something at a very specific attitude without influencing it's stability, which is something it has very little of.
Old 03-04-2008, 08:34 PM
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Default RE: Minimum Power Requirement

And btw, most running launches of powered models do not include much "pushing" to accelerate the model. Because almost every push winds up adversely affecting the AOA you try to establish by feel as you're running with the model held high.
Old 03-04-2008, 09:50 PM
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kerklein2
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Default RE: Minimum Power Requirement

I guess I really should say that the finished product will be flown by autopilot, although initial testing will be done in manual mode at a reduced weight.

Currently it is running a .71 OS 4 stroke modified to run gasoline. I am just wondering if we can go any smaller.



That is my calc sheet. All the non-bolded variables were just to help my calculate the wing area (I suck at geometry). I wish I could upload the document so you could see the formulas, but it is all pretty standard.

I followed the formulas in this document to calculate the stall speed:

[link=http://www.google.com/url?sa=t&ct=res&cd=2&url=http%3A%2F%2Fwww.glide.net.au%2Fon-the-wing3%2F156-Jochen-Haas%2FHaas-software-Windows%2FDesign-Procedure.DOC&ei=VQjOR_37BKnSpgTLzP2XCw&usg=AFQjCNG-Vb9p_fO1WDY0FLYsxm1e0HC26A&sig2=JYsWWmqlrL28UtinvNVNlw]Link[/link]

We really cannot have landing gear. Hand launch or water balloon launch is the only thing we can do.
Old 03-04-2008, 10:36 PM
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Default RE: Minimum Power Requirement

I'm not on the cutting edge of anything. What is a water balloon launch?
Old 03-04-2008, 11:34 PM
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Default RE: Minimum Power Requirement

The wing planform looks fine. But with that much sweep you need some twist in the wing or something that takes the place of the usual twist. Such as making the tip airfoils lifting but cambered upward instead of down so that the tips can provide the stability needed. If the wing is truly flat with no twist and uses the same airfoil from root to tip then bad things are going to happen since there is no provision at all for stabilizing the model in pitch.

Which brings up the idea of equivalent wing area. Or more accurately determining the localized lift coefficients of each wing segment from root to tip and using calculus to determin what the overall wing lift coefficient will be. And I can assure you that it will not be equaly to 1.0. The center section area may be lifting at 1 or even more depending on the chosen airfoil. However there's no way the tips can lift the same amount if the model is flying in a stable manner. Be it from twist (which you aren't using according to the wing guy) a graduated shift to a downward lifting airfoil at the tip or the use of strongly reflexed elevons the tips will be much reduced for lift coefficient and the overall average lift coefficient will be more like .6 to .7 or maybe as much as .8 if the center section is using a strong lifting section with a max Cl of up around 1.5.

All of which is going to raise the minimum airspeed.

You need to go talk to the wing guy to find out more about this miracle wing or to understand where the communication breakdown has taken place. Something is just not right here.

And what is wrong with a takeoff dolly that you roll away with and then lift out of it to leave it on the ground while the model flys up? It's either that or some form of launch rail. Also since you're using an engine you need to be careful how you get the model up to speed. Sharp accelerative loads like javelin throws or heavy bungee launch rails (I'm assuming that a huge slingshot like deal is what your water balloon launcher is) can accelerate the model so fast that the engine starves for fuel and quits. The bungee powered balloon launcher is the way to go but to give the model room to get up to speed before it looses support I'd suggest a bungee powered launcher rail that is about 10 feet long. And plan on a speed at the end of the rail to be up around 30 to 35 mph just so you're safely flying when it leaves the support of the rails.
Old 03-05-2008, 12:47 AM
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kerklein2
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Default RE: Minimum Power Requirement

I cannot confirm that it is the same airfoil from root to tip. I do know that it is a flat bottom airfoil along its length. I also know that it is a proven design at lower weights which has flown on numerous models. I also know that it is not a conventional airfoil and you cannot find it in any listing of airfoils out there.

That lift coefficient was just a number I threw in there since I have no idea what it actually is.

As far as the launch platform, the issue is weight. We have a weight budget and are already over it. A dolly or rail launch would be included in the weight.
Old 03-05-2008, 01:21 PM
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Default RE: Minimum Power Requirement

A proven airfoil on flying wings or on conventional designs? I've already suggested that your team needs to double check on the theory of stability for swept flying wings based on what I've seen in this thread so I'll leave it at that.

All the best of luck with your tasking and test flying.
Old 03-07-2008, 07:15 AM
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Default RE: Minimum Power Requirement

It's proven on flying wings. Numerous models have been built and flown.

I still would like to get back to the original question.
Old 03-07-2008, 09:39 AM
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Default RE: Minimum Power Requirement

Answer to your original question.

Minimum power to fly has nothing to do with stall speed. It is where the sum of the induced and parasite drag is the lowest. Check out chapter 2 to at least chapter 12 of what ever book you have.

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