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need to talk with an aeronautical engineer

RVbySDI

RVbySDI

Well Known Member
I have some specific questions concerning wing design associated with wing plan forms, w&b, lift ratios, wing size, cord size, etc. Are there any professional aeronautical engineers here who could answering some basic questions on these issues?

I know I can read general information on these things in a great number of books. As a matter of fact, I have already read several books on these topics but I am really looking for someone who might be able to answer some specific questions I have, not just general questions. If anyone out there would be willing to converse with me please reply to this thread or feel free to send me a private message.

Thanks.
 
Happy to!

Hi,

By all means, send me a PM with your questions, I'll do my best.

I've been at NASA Ames for 30 yrs, w/PhD in applied aerodynamics. I do wing design.
 
Please post what & when you can, I like reading this engineering stuffs! Just don't tell me my plane won't fly. ;)
 
Geico266 said:
Please post what & when you can, I like reading this engineering stuffs! Just don't tell me my plane won't fly. ;)
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I don't mind sharing my questions as long as the engineers who answer don't mind sharing their answers.

For anyone who is interested in my thoughts (yeah right, who is this bozo? :p):

This is not RV related Per Se, lets just say it is aviation related. I am interested in the wing dimension requirements (span, chord, plan form, etc.) for a small, single place, light weight aircraft. I would like to know how much weight any given wing of X dimension could lift and the associated stall characteristics, speed characteristics, and such of said wing.

I have these ideas rattling around in my head but do not have the appropriate engineering training to know whether my brain is full of **** or if I may have a legitimate idea.
 
Ironflight said:
Put a big enough engine on it Steve, and the wing size becomes irelivent....:)
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Well, interesting you say that. I am interested in a motor (as in electric) not an engine (as in internal combustion). So that leads me to other engineering type questions. How big does my motor have to be to give me the thrust I need for the size wing I need to lift the weight I need to lift and then to keep it aloft and motivating at an adequate speed to make it all worth the effort? So many questions.
 
Steve I believe there are many software packages out there that aren't to expensive that you could use for your development that would probably give you more accurate answers than anyone of us could provide.
 
RVbySDI said:
I am interested in the wing dimension requirements (span, chord, plan form, etc.) for a small, single place, light weight aircraft. I would like to know how much weight any given wing of X dimension could lift and the associated stall characteristics, speed characteristics, and such of said wing.
Click to expand...

RVbySDI said:
How big does my motor have to be to give me the thrust I need for the size wing I need to lift the weight I need to lift and then to keep it aloft and motivating at an adequate speed to make it all worth the effort? So many questions.
Click to expand...

Wing design is as much speed related as it is vehicle weight. The slower you are going the more wing Cl (lift coefficient) you need. Think sailplane or Cub. As pointed out, if you are going fast then small (or no) wing can work. Think F-104 or Gee-Bee. The wing loading (aircraft weight divided by wing area) will determine your ride quality. think cessna 170 vs Bonanza. The higher the wing loading the least likely it will bounce around in gusty conditions. Airfoil selection can help a small wing make up for the higher wing loading and perform well at lower speeds (think RV).

So you see there are lots of varibles that effect wing size. I just love the variations this has caused in the variety of airplane designs out there flying (and the ones on my drawing board)
 
Steve,

I'm affraid the answers to your questions depend on many things. Here are some simplified answers:

L = 1/2 x rho x V^2 x CL x S
rho is density
V is velocity (ft/s)
CL is a non-dimensional lift coefficient
S is wing area (sq ft)

If you want to know the required wing area, you can arrange the above equation for S. S = 2 x L / (rho x V^2 x CL)

Now let us consider a sea-level take-off at gross weight. Plug in gross weight for L, 0.002378 for sea level density, desired stall speed for V and the wings maximum lift coefficient for CL.

RV-6 example:
Gross Weight: 1,600 lb
Sea Level rho: 0.002378
Stall Speed: 80 ft/sec
CLmax: 1.85

S = 2 x (1,600) / [(0.002378) x (80^2) x (1.85)] = 113.65 square feet which is very close to the real wing area.

Stall characteristics have a lot to do with the airfoil. Some stall very abruptly and some stall progressively. You can get an idea of this by looking at the CL vs Alpha charts. It also has a lot to do with the washout (twist) that you put in the wing.

Speed depends on aspect ratio, planform, airfoil, twist, skin friction, wingtips, interference drag, etc. There is no simple answer to this one.

In general, Power = Thrust x Velocity and in level cruise, the thrust is equal to drag. Let's say an airplane has 100 pounds of drag at a speed of 250ft/sec. The required power in this condition is 100 lb x 250 ft/sec = 25,000 lb ftxlb/sec...... or 45.5 hp or 33.9 kilowatts if you prefer. However, you must also account for speed control, motor and prop efficiencies.


-David
 
Guys, this is really interesting stuff. I love learning about this stuff
plehrke said:
Wing design is as much speed related as it is vehicle weight. The slower you are going the more wing Cl (lift coefficient) you need. Think sailplane or Cub. As pointed out, if you are going fast then small (or no) wing can work. Think F-104 or Gee-Bee. The wing loading (aircraft weight divided by wing area) will determine your ride quality. think cessna 170 vs Bonanza. The higher the wing loading the least likely it will bounce around in gusty conditions. Airfoil selection can help a small wing make up for the higher wing loading and perform well at lower speeds (think RV).

So you see there are lots of varibles that effect wing size. I just love the variations this has caused in the variety of airplane designs out there flying (and the ones on my drawing board)
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It sounds to me like my design ideas are going to have to be altered in a great way. I guess in my dream design I am trying to have my cake and eat it too.

For anyone still interested in talking with me concerning my pie in the sky dreams here is some of my basic flight characteristics I keep thinking about.

First the flight characteristics:

  1. I want an aircraft that stalls as slow as possible (I was thinking 20-30 MPH)
  2. I want an aircraft that flies as fast as possible (no top end speed here but thinking about a sleek efficient design)
  3. I want an agile highly maneuverable flying craft with no surprises
  4. I would like a respectable endurance (although I know the batteries will limit this to a great degree. See number 2 below. )

Now the design characteristics (here is where my thoughts start leaking outside the box):

  1. I want as small a wingspan as possible to meet the required flight characteristics.
  2. I would like it to be powered by an electric motor with appropriate batteries housed on the CG and the motor and prop aft in a pusher configuration.
  3. I want it as light as possible.
  4. I am interested in a canard wing design but not committed to it. The reason for a canard wing is the thinking that I can keep the stall speed down with the canard.
  5. I am really interested in having as little vertical component for the aircraft as possible. I really don't want a big vertical tail sticking up behind the wing unless I just have to do so. I really do not want a long empenage (sp?) sticking out back behind the wing.
  6. I am thinking about a non-traditional fuselage. In fact, I am really not thinking about a traditional fuselage at all. I really am thinking about a sleek enclosed "space" on top of the wing for a pilot to lie prone/supine that will decrease the overall vertical area of the airplane (does anyone remember the movie "Flight of the Phoenix"? I prefer the original version with Jimmy Stewart.)
I guess what I am thinking about is an aircraft that is basically just a wing with as little other structure attached as possible. So, a flying wing. I know there have been flying wing designs for almost as long as there have been airplanes. I know there have been major control issues with them but I am wondering if it is possible to design something close to a flying wing perhaps with some compromises as needed.

Ok, that is some basic insight into my screwy brain. Am I nuts? Am I looking at this all wrong? Am I needing to give up my design career before I ever start? What say you?
 
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Norman CYYJ said:
Do you want to be able to ride in this thing or are you making a small model plane?
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Yes, I am looking to ride in this thing. At this time it is a speck of an idea in my head. I have no illusions that it is even possible. That is why I am asking for opinions from those of you with engineering experience dealing with aircraft design.
 
RVbySDI said:
Yes, I am looking to ride in this thing. At this time it is a speck of an idea in my head. I have no illusions that it is even possible. That is why I am asking for opinions from those of you with engineering experience dealing with aircraft design.
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For something as radical as an electric powered flying wing, I would make a model first. Make it large, 1/4 - 1/3 scale. That way you will see if it will fly, and how it flies, etc etc, and you can do all the adjustment and changes you want 10x faster and 10x cheaper.

IMO electric powered flight (real airplane not RC) is all by itself a huge challenge. Continuous 100+ HP from a battery powered electric motor (with battery and controllers) is not something you can pick off the shelf, it doesn't exist yet, except for in some of those new exotic luxury electric sport cars that cost 1M $ or more.
 
SvingenB said:
For something as radical as an electric powered flying wing, I would make a model first. Make it large, 1/4 - 1/3 scale. That way you will see if it will fly, and how it flies, etc etc, and you can do all the adjustment and changes you want 10x faster and 10x cheaper.

IMO electric powered flight (real airplane not RC) is all by itself a huge challenge. Continuous 100+ HP from a battery powered electric motor (with battery and controllers) is not something you can pick off the shelf, it doesn't exist yet, except for in some of those new exotic luxury electric sport cars that cost 1M $ or more.
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A model is a great idea. If I get to that point I will definitely start there.

I am really not thinking on the lines of 100+ HP. Of course HP requirements opens up a new discussion. My desire for a design that has low drag and is very light is driven by a desire to have a smaller (lighter) electric motor that will provide the necessary thrust. So, 100+ HP motor is probably overkill. I am not sure how powerful a motor I would have to have but I was at Oshkosh last year and looked very long and hard at the 50+ HP electric motors on display at the yuneec electric aircraft display. These are more in line with my thoughts.
 
rich said:
From what you are describing it may look something like this.
Make it electric.

http://samsonmotorworks.com/
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I saw these guys at Osh last year also. I like their concept but just not sure if it will iron out to a practical solution. I bet whatever happens with them it will not be cheap.

My thoughts were something that I could build myself fairly cheaply. Not sure this one would fit that requirement.
 
David Shelton said:
Steve,

I'm affraid the answers to your questions depend on many things. Here are some simplified answers:

L = 1/2 x rho x V^2 x CL x S
rho is density
V is velocity (ft/s)
CL is a non-dimensional lift coefficient
S is wing area (sq ft)

If you want to know the required wing area, you can arrange the above equation for S. S = 2 x L / (rho x V^2 x CL)

Now let us consider a sea-level take-off at gross weight. Plug in gross weight for L, 0.002378 for sea level density, desired stall speed for V and the wings maximum lift coefficient for CL.

RV-6 example:
Gross Weight: 1,600 lb
Sea Level rho: 0.002378
Stall Speed: 80 ft/sec
CLmax: 1.85

S = 2 x (1,600) / [(0.002378) x (80^2) x (1.85)] = 113.65 square feet which is very close to the real wing area.

Stall characteristics have a lot to do with the airfoil. Some stall very abruptly and some stall progressively. You can get an idea of this by looking at the CL vs Alpha charts. It also has a lot to do with the washout (twist) that you put in the wing.

Speed depends on aspect ratio, planform, airfoil, twist, skin friction, wingtips, interference drag, etc. There is no simple answer to this one.

In general, Power = Thrust x Velocity and in level cruise, the thrust is equal to drag. Let's say an airplane has 100 pounds of drag at a speed of 250ft/sec. The required power in this condition is 100 lb x 250 ft/sec = 25,000 lb ftxlb/sec...... or 45.5 hp or 33.9 kilowatts if you prefer. However, you must also account for speed control, motor and prop efficiencies.


-David
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Thanks for the information David. I am going to spend some time going over your information and see how much of it I can assimilate. From the comments I have heard so far it looks like I need to rethink my wing design to allow for a high aspect ratio shape. Perhaps my desires for slow stall speed, light weight and high end speed and maneuverability all in one package may have to be reevaluated.
 
RVbySDI said:
A model is a great idea. If I get to that point I will definitely start there.

I am really not thinking on the lines of 100+ HP. Of course HP requirements opens up a new discussion. My desire for a design that has low drag and is very light is driven by a desire to have a smaller (lighter) electric motor that will provide the necessary thrust. So, 100+ HP motor is probably overkill. I am not sure how powerful a motor I would have to have but I was at Oshkosh last year and looked very long and hard at the 50+ HP electric motors on display at the yuneec electric aircraft display. These are more in line with my thoughts.
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Nice airplane. Looking at the numbers, 40 kW motor, 66.6 V, 30 Ah battery, the duration at full power is only about 3 minutes. To get 1 h endurance, the motor can only draw 2.7 HP at average. 2.7 HP is probably more than enough to keep a very light (motor) glider like the yuneec afloat at best sink rate, but is a long way from a practical solution in a GA airplane.
 
Here are some general relationships.

1. Low speed is directly dependent upon wing loading. The more wing loading, the faster it'll be, if there's enough power to push it.

2. Power required is closely dependent upon span loading. The longer the wingspan for the weight, the less power is required.

3. Climb rate is based on the excess power available and the weight of the aircraft. More power and lighter weight gives a faster climb rate.

4. Generally, longer wingspans are heavier than short ones.

5. A canard configuration often has a low maximum lift coefficient for the whole aircraft, because of the hazard of stalling the back wing.

6. Flying wings often have a low maximum lift coefficient due to the lack of control power.

This will give you a start on the various trade-offs a designer faces.

David Paule
 
David Paule said:
Here are some general relationships.

1. Low speed is directly dependent upon wing loading. The more wing loading, the faster it'll be, if there's enough power to push it.

2. Power required is closely dependent upon span loading. The longer the wingspan for the weight, the less power is required.

3. Climb rate is based on the excess power available and the weight of the aircraft. More power and lighter weight gives a faster climb rate.

4. Generally, longer wingspans are heavier than short ones.

5. A canard configuration often has a low maximum lift coefficient for the whole aircraft, because of the hazard of stalling the back wing.

6. Flying wings often have a low maximum lift coefficient due to the lack of control power.

This will give you a start on the various trade-offs a designer faces.

David Paule
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I really like this list. It really puts a lot of what I have been thinking about into perspective.

Concerning point # 5: Could you explain further the idea of "low maximum lift coefficient"? I have been thinking a canard could help keep the stall speed down but are you saying here the stall speed will only be helped by a canard up to a point? That there will be a specific weight loading and then a specific speed at which the main wing will stall and will not be recoverable?
 
Ideally, when the canard stalls, it drops, and because it's ahead of the center of gravity, the airplane's angle of attack is reduced.

If the main wing stalls, the airplane pitches up. Since this is an unstable, divergent event, it's avoided by proper design.

Since the main wing can't be allowed to stall, it can't reach its maximum lift coefficient. Because it's usually a high percentage of the total aircraft lift surface, the total maximum lift coefficient for the whole airplane is generally lower than for the traditional wind-first configuration.
 
Looked a bit closer at the Yuneec. The battery is only 30Ah, but it has 6 or 10 of them, so that total capacity is 180 or 300 Ah, and this certainly improves things. However, the controller can only put out 400 A, and at 66.6V this will be max 26.6 kW (36 HP).

With 6 battery packs, endurance at full power (36 HP) is 27 minutes.
With 10 battery packs, the endurance is 45 minutes at full power.
 
plehrke said:
Not a bad wing design but there are actually other things to consider in the wing. One is the span is 45 ft. Not real practical for the average hanger.
I do think the fuselage looks pretty good (from and engineering point of view) as drag is all about wetted area. Looks like they did everything they could to keep it low.
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I agree with you concerning the length of the wing. I like this airplane a lot, except for the fact it would never be able to fit into my hangar.

The wetted area is exactly what started my thinking about design and the reason for beginning this thread. I am interested in an electric powered airplane that basically has as little wetted area as possible in order to eliminate drag. To that end I have been thinking about the elimination of as much vertical surface in the fuselage area as possible. My thoughts have been toward the idea of just having a shallow cockpit for the pilot on top of the wing structure that could eliminate a great deal of drag and hopefully weight that is associated with a cabin area normally designed for carrying a human.
 
wing span is more important than wetted area

Everyone focuses their attention on wetted area, because their intuition can sort of guide them to the benefits of smaller, more streamlined shapes in terms of parasite drag.

However, induced drag is equally, and sometimes MORE important. Most lay people barely have any concept of induced drag. At take-off and landing speeds, it amounts to about 85% or more of the total drag. At best range, best rate-of-climb speed, it is half the total drag.

This is the reason that the Yuneec has a 45-ft wingspan. If you want low power required, which you need for electric, then you need lots of wing span.

Even at high speed cruise at high altitude, induced drag is still important.
At very high speeds and low altitude, like racing RV's, there is still a trade-off when cutting the wing span down. You are reducing wetted area (good) but raising the induced drag. At some point, there is a cross-over where cutting more wingspan off will hurt more than help. This becomes even more important for pylon racing where turning causes higher lift coefficients and higher induced drag. This is why most WWII fighters have aspect ratios of around 6-ish. The way to make a faster RV is to reduce wing area without reducing wing span.
 
Did you know,

the Chinese company that designed and built the Yuneec is a model aircraft company. R/C designs don't scale to full size well but the electrics and gliders I fly all have high aspect thin airfoils. I think the Yuneec follows that design to enable it to have the performance it does on the power is makes. Motor technology is very good now, we need one more leap in battery technology (double energy density) for electic full size aircraft to be practical (fast charging would also help). For those of you who are not involved in R/C aircraft, electric planes of most sizes are more powerful than their IC counterparts. Most of the electric powered planes I fly have well over a 1/1 power to weight ratio.
 
scsmith said:
Everyone focuses their attention on wetted area, because their intuition can sort of guide them to the benefits of smaller, more streamlined shapes in terms of parasite drag.

However, induced drag is equally, and sometimes MORE important. Most lay people barely have any concept of induced drag. At take-off and landing speeds, it amounts to about 85% or more of the total drag. At best range, best rate-of-climb speed, it is half the total drag.

This is the reason that the Yuneec has a 45-ft wingspan. If you want low power required, which you need for electric, then you need lots of wing span.

Even at high speed cruise at high altitude, induced drag is still important.
At very high speeds and low altitude, like racing RV's, there is still a trade-off when cutting the wing span down. You are reducing wetted area (good) but raising the induced drag. At some point, there is a cross-over where cutting more wingspan off will hurt more than help. This becomes even more important for pylon racing where turning causes higher lift coefficients and higher induced drag. This is why most WWII fighters have aspect ratios of around 6-ish. The way to make a faster RV is to reduce wing area without reducing wing span.
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Induced drag all depends on where you are operating on the drag polar. For the RV we cruise way above L/Dmax. According to the CAFE report on the RV-6A min drag occurs at 106mph and where we cruise the induced drag is 25% of total drag. We fly above the most aerodyanmically efficient speeds because we can as we have lots of thrust and therefore we fly at where 75% power equals drag. I would not think increasing aspect ratio to reduce induced drag is the benefit you realize with the idea of reducing wing chord. It is the reduction in skin friction drag (I think the Rocket guys agree since they actually reduce aspect ratio by cutting span and maintaining wing chord to get more speed). The reason higher aspect ratio has benefit for pylon racing is they are spend 60% of there time in a 3-6 G turn. That increases induced drag by at least 3-6 times and therefore makes iduced drag a major player. The real perameter for range to optimize in 1 g flight has velocity times L/D which I believe is the Carson number. For the RV that is at ~140mph where Cdi is 1/3 total drag.

One way to reduce induced drag on an RV is to reduce the lift required, meaning reduce weight. People forget that weight effects drag.

For an electric airplane (which is more like a sailplane I would think) you are spending more flight time near L/Dmax and therefore you want to move the drag polar to match better your cruise condition due to the limited power. So I would agree that working induced drag could be just as improtant as skin friction.
 
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plehrke said:
...One way to reduce induced drag on an RV is to reduce the lift required, meaning reduce weight. People forget that weight effects drag...
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I am no engineer but have a bit of practical experience messing around with experimental airplanes.

One item that certainly reduces induced drag is laminar flow when it can be achieved. It would make a difference with an RV as with any airplane. (the P-51 was so designed but probably did not hold laminar flow for long after intering service due to wear, tear and dirt)

When flying into rain with the LEZ, laminar flow tripped and the airplane would go into a descent that required an increase in AOA to maintain flight. When coming out of the rain and the surface dried, laminar flow was again present and the airplane would begin to climb until retrimmed nose down.

My next RV project will include an effort to achieve laminar flow - when and if that ever happens. My present airplane is a bit over the hill in such an effort. :)
 
David Paule said:
Since the main wing can't be allowed to stall, it can't reach its maximum lift coefficient. Because it's usually a high percentage of the total aircraft lift surface, the total maximum lift coefficient for the whole airplane is generally lower than for the traditional wind-first configuration.
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This is technically true, but only relevant if you're talking about the small percentage of the mission when you may operate at or near maximum lift... Usually taking off or landing. The Long-EZ and Vari-EZ designs are "limited" by this as well, but it doesn't prevent them from being extremely efficient aircraft. There's a Vari-EZ at one of my local airports that routinely cruises just over 200mph TAS on an O-200. He needs a long, smooth runway to take off and land, though... I don't know what his stall speed is.

Like everything else in aircraft design, it's a trade-off... :)
 
SvingenB said:
IMO electric powered flight (real airplane not RC) is all by itself a huge challenge. Continuous 100+ HP from a battery powered electric motor (with battery and controllers) is not something you can pick off the shelf, it doesn't exist yet, except for in some of those new exotic luxury electric sport cars that cost 1M $ or more.
Click to expand...


Nah, you are a bit off on your money figure there- the Tesla's are around 100,000, although they have one that will be 55,000$ starting 2011. Pure electric, and the $100,000 models are plenty fast.

http://www.teslamotors.com/buy/buyshowroom.php

edit- the roadster sport model has 288 horsepower and a range of 230 miles, so that is maybe just under 4 hours of driving time? I don't know what the horsepower rating is required to sustain 60 mph so i don't know the typical cruise hp.
 
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Danny7 said:
Nah, you are a bit off on your money figure there- the Tesla's are around 100,000, although they have one that will be 55,000$ starting 2011. Pure electric, and the $100,000 models are plenty fast.

http://www.teslamotors.com/buy/buyshowroom.php

edit- the roadster sport model has 288 horsepower and a range of 230 miles, so that is maybe just under 4 hours of driving time? I don't know what the horsepower rating is required to sustain 60 mph so i don't know the typical cruise hp.
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I might add that in the auto industry electric propulsion is poised to finally really take off soon.

Even Chevrolet, who has drug their heels kicking and screaming into the electric power revolution, is getting ready to release the Volt for commercial sale this coming fall.

I returned from last year's Oshkosh touting the fact that the aviation world is poised to take off on the electric revolution as well. I think it is going to happen and it is going to happen in the next 5 to 10 years, or less. Most of us are still insulated from the technicalities of how it all works but do a little research and you will be surprised at what you will find out there in terms of the changes that are soon to happen.

It is true that there will not be an electric RV anytime soon, if ever. But when you look at other types of flying electric power will prove to be a very good option for many reasons.
 
David-aviator said:
One item that certainly reduces induced drag is laminar flow when it can be achieved. It would make a difference with an RV as with any airplane. (the P-51 was so designed but probably did not hold laminar flow for long after intering service due to wear, tear and dirt)

When flying into rain with the LEZ, laminar flow tripped and the airplane would go into a descent that required an increase in AOA to maintain flight. When coming out of the rain and the surface dried, laminar flow was again present and the airplane would begin to climb until retrimmed nose down.
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A couple things. Laminar flow would improve skin friction drag and not induced drag. Also, true passive laminar flow is currently not practical (or maybe achievable) on aircraft. Until it is perfected on a commercial airliner, where you would get the biggest payoff and the best chance, it will not be perfected for GA use. Laminar flow requires a very specific design point and will not give benefits if you stray off the design point. Many may claim but the proof is if it is actually in service and working.
 
I have seen the laminar drag bucket in tests of an old T-33 that had been in everyday service in the Air Force for at least 17 years.

Granted, a military T-33 is built more robustly than most General Aviation aircraft, but the approach is effective, reliable and durable, when designed into the airplane.
 
what Plehrke said is not quite true

Two different things that PLEHRKE said are not quite true.

First: On airplanes with high cruising speed, yes it is true that at low altitude, induced drag becomes fairly insignificant. However, when you cruise at high altitude, even at high speed, induced drag becomes more important again. To make an RV (3,4,6,7,8,10) a really efficient cruising airplane, it could use a little more aspect ratio.
And this is why the RV-9 is a nicer long-range cruising airplane.

Second: On natural (passive) laminar flow - there are plenty of airplanes out flying around that achieve a significant amount of natural laminar flow. Lancair, Cirrus, etc. Even Glasair to a lesser extent. Arguably a well-kept, clean Mooney or Piper. All the hot formula one racers. The argument about commercial airplanes overlooks a very important fact: Wing sweep greater than about 20 degrees effectively eliminates the possibility of significant natural laminar flow, because the sweep changes the transition mechanism from T-S waves to crossflow instability. With very careful design, this can be mitigated some, and is an area of active research. But I promise you that natural laminar flow will be (already is) common on GA airplanes long before it is a factor in commercial transport design. IF the traveling public would like really "green" airplanes and is willing to slow down to Mach 0.70, then we could design natural laminar flow wings that would make HUGE fuel savings for commercial airlines. But the market still wants to go Mach 0.8. So you get 30 degrees of wing sweep, or a little less.
 
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Danny7 said:
Nah, you are a bit off on your money figure there- the Tesla's are around 100,000, although they have one that will be 55,000$ starting 2011. Pure electric, and the $100,000 models are plenty fast.

http://www.teslamotors.com/buy/buyshowroom.php

edit- the roadster sport model has 288 horsepower and a range of 230 miles, so that is maybe just under 4 hours of driving time? I don't know what the horsepower rating is required to sustain 60 mph so i don't know the typical cruise hp.
Click to expand...

I thought the Tesla was more pricey. The battery pack has 53 kWh of available energy, so if it last for 4 hours it delivers about 20 hp on average. The battery weigh 450 kg. To deliver 100 hp on average for 4 hours, the battery will weigh more than 2 tonnes. A steam engine will do better than that.

So even though electric power starts to become practical for cars, a revolution in battery technology is needed to make electric propulsion practical for flight. It can deliver lots of power for short duration of time without too much weight penalty, so for self launching gliders it probably will be OK, as well as some other niche products I guess..
 
I think we are in agreement but missing each others point

I think we are talking about operations at different parts of the drag vs speed plot per my intial posting. I am talking about reducing drag for max speed and you are talking about reducing drag for max efficiency.
scsmith said:
On airplanes with high cruising speed, yes it is true that at low altitude, induced drag becomes fairly insignificant. However, when you cruise at high altitude, even at high speed, induced drag becomes more important again.
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I agree that as you go higher induced drag will increase in importance in the overall drag but for the altitudes that an RV flies it will not become higher then form/skin friction drag in overall magnitude. For an RV cruising at 175 mph (where form/skin friction drag is 400% of induced drag) and flying at higher altitudes, induced drag will increase ~35% more then form/skin friction drag from sea level to 10,000 ft (and 87% more at 20,000 ft) but that still makes it a smaller percent of overall drag than form/skin friction for an RV at cruise. This is due to induced drag varies with altitude by 1/(rho)**2 where it varies for form/skin friction drag by 1/rho. If the RVs flew at or above 30,000 ft then induced drag would be again over 50% of total drag. You can see how this answer would change if not operating as far to the right on the drag/speed plot (see page 4 of the RV-6A CAFE report).

A couple of caveats. You will notice I am now using the term form/skin friction drag. Since we are "getting technical" I thought I needed to improve my accuracy of terminology. I do not know how much of the RV zero lift drag is skin friction and how much is form and interference drag. Also, since we are talking percents and not absolutes, you may get larger percent improvements in induced drag for minor changes in the design then you would for skin friction drag. Your example of reducing wing chord without changing span may reduce induced drag 3 times more then it reduces skin friction drag (depending on speed and altitude of interest) and therefore a major player. In my day job we call these the sensitivities or getting the most "bang of the buck". This is why most of the RV drag reduction work is on form (where I would also book cooling drag) or interference drag. For a flying RV the only ways to work induced drag is weight reduction and wing tip mods.

scsmith said:
To make an RV (3,4,6,7,8,10) a really efficient cruising airplane, it could use a little more aspect ratio.
And this is why the RV-9 is a nicer long-range cruising airplane.
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Totally agree. See my explanation at the beginning of this post.

scsmith said:
On natural (passive) laminar flow - there are plenty of airplanes out flying around that achieve a significant amount of natural laminar flow. Lancair, Cirrus, etc. Even Glasair to a lesser extent. Arguably a well-kept, clean Mooney or Piper. All the hot formula one racers. The argument about commercial airplanes overlooks a very important fact: Wing sweep greater than about 20 degrees effectively eliminates the possibility of significant natural laminar flow, because the sweep changes the transition mechanism from T-S waves to crossflow instability. With very careful design, this can be mitigated some, and is an area of active research. But I promise you that natural laminar flow will be (already is) common on GA airplanes long before it is a factor in commercial transport design. IF the traveling public would like really "green" airplanes and is willing to slow down to Mach 0.70, then we could design natural laminar flow wings that would make HUGE fuel savings for commercial airlines. But the market still wants to go Mach 0.8. So you get 30 degrees of wing sweep, or a little less.
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I do not really want to get into a laminar or non-laminar discussion as that is like a primer or nose wheel discussion. Like you point out with wing sweep and speeds, laminar flow is more then a 2D airfoil and smoothness issue. My professional experience (fighter aircraft) makes me very leary of laminar flow. That maybe due to the challanges we have had with active laminar flow on military planes.
 
Hard numbers

For the RV8:

Altitude: 12000 msl
Speed: 170 ktas
Weight: 1700 lb
Wing area: 116 ft^2
Wing span: 24 ft
Aspect ratio: 4.96
CL: .198
Planform correction factor, to account for non-elliptic planform: .052

CD_i = .00265
Induced drag = 23 lb

Neglecting planform influence, the NACA23012 will have a form drag coefficient of about .0065 for a Cl of .20. So, the total wing parasite drag will be 55 pounds, based on area and Q.

This shows the relationship I believe Steve Smith was pointing out: the induced drag in this real world example is almost 42% of the total wing drag, which is 78 pounds.

*********************

Now, lets attack wetted area by chopping the span down:

Span: 21.8 ft (F1 Rocket span)
Area: 104 ft^2
Aspect Ratio: 4.57
CL: .218
Planform correction factor: .045

All other factors remain unchanged.

Cd_i = .00346
Induced drag = 27 lb

The 23012 is operating at a slightly higher lift coefficient but still at the same approximate drag coefficient of .0065. So, the new parasite drag value is 50 pounds.

For the chopped down wing, induced drag is 53% of total drag, which is now 77 pounds. We saved only 1 pound of drag by chopping the wing down, but gave up a lot on the low end of the speed envelope.

Whats more, the short wing penalizes flying at higher weights more than the long wing does. Since induced drag is tied to the *square* of CL, adding a pound to the short wing airplane extracts a much higher induced drag hit than it would on the long wing airplane.

I realize this has been a quick and dirty analysis using the well known empirical relationship for induced drag, but for comparative purposes, its quite good.

Edit: With 135HP at the crank (75% of 180), the Hartzell F7666 propeller will produce 211 pounds of thrust. So the drag numbers above show us the wing is responsible for more than a THIRD of the total drag on the airplane, making it the biggest single source of drag we have.
 
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Laminar

Laminar airfoils generally aren't found on airliners and fighters because of their high Reynolds Numbers. However, an electric airplane will have a substantially smaller Reynolds Number which makes laminar flow much easier to maintain. In the world of sailplanes, nearly every design since the 1970's has used laminar flow airfoils successfully.

If anyone is interested, the CAFE Foundation will be holding the 4th annual Electric Aircraft Symposium later this month. You can visit their website: www.cafefoundation.org
 
This is really fascinating stuff guys!

I want to let everyone know how much I appreciate all the posts here. I started this thread with some specific ideas in mind and with all the posts I have learned a great deal. In fact, I have learned so much that I realize I was biting off much more than I have knowledge to digest.

I am not sure how long a life this thread will have but I truly am learning tons every time someone posts. I hope to keep learning more.

Bill, your post was very informative. I like the comparisons you did with shorter wings and amount of drag reduction.

I thank Steve Smith especially for enlightening me. Steve, I hope we can continue exploring these concepts further in addition to this thread as I find your explanations very fascinating.

Thanks everyone, please keep posting your thoughts on induced/parasite drag, weight, thrust, HP needs, etc. I am learning a lot from each one of you.
 
49clipper

As long as there are some aero engineers online, could I pose a question? It concerns my trim tab being "up" (nose down trim) in flight at cruise. 29.5 degrees worth. Mine seems to be the only one, and Scott at Vans says don't worry about it, thats normal. Hmmm.
 
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