Minimum Power Requirement  
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Minimum Power Requirement - 3/1/2008 7:21:33 PM   
kerklein2


 

Posts: 84
Joined: 7/30/2003
From: Austin, TX, USA
Status: offline 		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.
       Post #: 1

RE: Minimum Power Requirement - 3/2/2008 5:46:47 AM   
pimmnz


 

Posts: 522
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: offline 		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.

(in reply to kerklein2)
       Post #: 2

RE: Minimum Power Requirement - 3/2/2008 6:59:24 AM   
wellss


 

Posts: 65
Joined: 6/14/2002
From: St. Catharines, ON, CANADA
Status: offline 		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).

(in reply to pimmnz)
       Post #: 3

RE: Minimum Power Requirement - 3/2/2008 2:44:11 PM   
Villa



Posts: 873
Joined: 8/12/2003
From: Wilson, NC,
Status: offline 		Hi kerklein2
I would love to read more details about your project.

(in reply to wellss)
       Post #: 4

RE: Minimum Power Requirement - 3/2/2008 3:00:48 PM   
CoosBayLumber


 

Posts: 2997
Joined: 1/20/2002
From: San Bernardino Calif
Status: offline 		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.


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(in reply to Villa)
       Post #: 5

RE: Minimum Power Requirement - 3/2/2008 4:28:15 PM   
R8893


 

Posts: 406
Joined: 10/19/2002
From: Cincinnati, OH,
Status: offline 		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

(in reply to CoosBayLumber)
       Post #: 6

RE: Minimum Power Requirement - 3/2/2008 10:59:41 PM   
kerklein2


 

Posts: 84
Joined: 7/30/2003
From: Austin, TX, USA
Status: offline 		It's about an 82 inch span, and the wing area is about 1000in^2.

Villa: What exactly do you want to know?

(in reply to R8893)
       Post #: 7

RE: Minimum Power Requirement - 3/2/2008 11:23:15 PM   
Villa



Posts: 873
Joined: 8/12/2003
From: Wilson, NC,
Status: offline 		What is the purpose of the project?

(in reply to kerklein2)
       Post #: 8

RE: Minimum Power Requirement - 3/3/2008 12:09:49 AM   
cyclops2


 

Posts: 1831
Joined: 12/23/2005
From: Frenchtown, NJ, USA
Status: offline

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

(in reply to Villa)
       Post #: 9

RE: Minimum Power Requirement - 3/3/2008 3:57:20 AM   
kerklein2


 

Posts: 84
Joined: 7/30/2003
From: Austin, TX, USA
Status: offline 		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.

(in reply to cyclops2)
       Post #: 10

RE: Minimum Power Requirement - 3/3/2008 4:46:16 AM   
BMatthews



Posts: 8951
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		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.


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Free Flighters go that extra mile........

(in reply to kerklein2)
       Post #: 11

RE: Minimum Power Requirement - 3/4/2008 5:17:53 PM   
kerklein2


 

Posts: 84
Joined: 7/30/2003
From: Austin, TX, USA
Status: offline 		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.

(in reply to BMatthews)
       Post #: 12

RE: Minimum Power Requirement - 3/4/2008 7:06:24 PM   
BMatthews



Posts: 8951
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		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.


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to kerklein2)
       Post #: 13

RE: Minimum Power Requirement - 3/4/2008 9:41:06 PM   
da Rock



Posts: 6696
Joined: 10/11/2005
From: western, NC, USA
Status: online
quote:

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.

(in reply to kerklein2)
       Post #: 14

RE: Minimum Power Requirement - 3/4/2008 10:07:42 PM   
kerklein2


 

Posts: 84
Joined: 7/30/2003
From: Austin, TX, USA
Status: offline 		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.

< Message edited by kerklein2 -- 3/4/2008 10:08:08 PM >

(in reply to da Rock)
       Post #: 15

RE: Minimum Power Requirement - 3/5/2008 1:18:44 AM