Video: Dr. Pete Gall's 'Designing light aircraft' presentation, Airventure 2011

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Monty

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The siren's call of the NLF wing. I started down this path for the same reasons on my little airplane. Problem is the spar winds up being coincident with my posterior if I place it in the thickest part of the wing. So either I move the spar under my knees, and accept that it will be way too heavy because the depth is not sufficient, or I increase the frontal area an unacceptable amount by sitting on top of the spar. Then factor in that to get NLF performance the wing will have to be composite or wood. How long will the finish required last in service? For what? For my particular case, the high aspect ratio wing is heavier, and the CG range unacceptably small. Faster means higher gust loads, higher Vne, more weight, more flutter issues. My airplane is for fun, not to set performance records. I enjoy flying. Do I really need to get there 15 min earlier? Wouldn't I rather fly the additional time and enjoy the view? I am not at all surprised that we mostly still use 23XX type airfoils. That's what I decided to do. They are very forgiving, for the pilot, builder, and designer.

A corporate transportation machine is a different matter, but even for the Bonanza, I don't see the improvement as being all that great. Most flights are much shorter than the original range of the aircraft. How much difference does that 30mph make in time saved during a typical flight? How much more must the airplane cost to get that small amount of performance gain? I'm not arguing that what he said isn't true. It is. I'm just thinking the costs are typically not worth it. Thanks for posting this, I wish all the technical forums were streamed on the web. I also wish they wouldn't schedule them during the air show so you could hear what the speakers are saying......
 
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BBerson

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Thanks for the video, the sound was clear enough and much better than my little Flip camera.

It was interesting the contrast from Pete Gall's talk and John Roncz and Barnaby Wainfan. Each had a different explanation for induced drag. Roncz and Wainfan said only span is important, where Gall did not mention span.

My takeaway is this: it depends on the aircraft purpose and the airspeed. It is not so simple as just "span-loading".
For example, the Blanik glider has large area increasing flaps to reduce induced drag at very slow thermalling speeds needed to stay in a thermals small core. High speed cruise is a completely different situation for induced drag.

As for Gall's plan to increase wing-loading, this will work, but remember these large flaps and high wing-loading will also increase the unpowered sink rate dramatically. A successful emergency landing is less likely with high wing-loading. Triple slotted flaps will work well with power approaches, not so good for power off gliding. Something to think about for a single engine experimental airplane.
BB
 

karoliina.t.salminen

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My airplane is for fun, not to set performance records. I enjoy flying. Do I really need to get there 15 min earlier? Wouldn't I rather fly the additional time and enjoy the view?
...
Most flights are much shorter than the original range of the aircraft. How much difference does that 30mph make in time saved during a typical flight?
Thanks for kind comments about me sharing the video :).

Off-topic, this topic was for the video rather than speculation for efficiency, but despite
I am just a messenger with providing you this video, I feel that I would like to comment on this your statement that looks to me not understanding the point of this: Your reasoning is quite typical for someone living in USA (paradise of general aviation - to go faster you can put a bigger engine and no matter it consumes fuel more, it is cheap) which does not work for the rest of the world. And thus it is failing in the basic point of efficiency, even when thinking efficiency from purely economical standpoint. I am still here in Europe as I haven't had a chance so far to move to the other side of the Pond even if that would make my aviation hobby much more easier. But still, even in USA, I believe there would be need for machines capable flying from coast to coast with a small fuel bill, in one single day instead of two days that it takes with our Diamond. On our 2009 flight, it had a significant role on how many days we were able to enjoy Airventure. Missed the time window for Oshkosh landing and had to come next day :/. The plane just was too slow and there were too many stops prior to Oshkosh.

It makes a lot of sense to have 30 mph more speed because that also means you can fly the same speed (this 30 mph lesser speed) with a lot less power. For required power, this amount of reduction of drag, makes already a big difference. It is all about fuel consumption. If your gasoline happens to cost 18 dollars per gallon _and_ you can't refuel on many airfields.

How would you consider this: you have to carry gasoline along inside the cockpit, on the back seat, to refill the tanks from canisters.
Would you do that and would you think it is practical and nice? What if you refill the tanks and you have to pay 700 dollars.
And you can fly one flight with 700 dollars. How many 700 dollars you have for fuel cost alone per month? I don't have that many extra
700 dollars per month (and by the time I would end up flying with a design of mine, it might no longer be 700 dollars, it could as well be 1400 dollars, and it could be that there would be only 1 airport left where you could fill the tanks on a country that is over 1000 kilometers long (meaning that round trip to the other end will require enormous range if you can't refuel unless of course you have your back seat reserved for fuel canisters (not nice, IMHO))).

I like flying too, that's why I am flying the DA40 with around 50% power and leaning it aggressively to consume only about 6.3 gallons per hour. At low altitude, I can get about the same range, but not the same air time by using 65% power and leaning again aggressively, that is around 7 gallons per hour.

With low power it takes longer before I need to purchase more fuel. Unfortunately touch and go landings will eat a lot of fuel, on Sunday, we did an experiment: arrived to one airfield, only 5.7 gallons used (about the contents of 1 5 gallon fuel can overfilled). Two touch and go landings, one little circle around the small village and then landing. 9 gallons used, oops that is almost two 5 gallon canisters already. Over 3 gallons went to the touch & go practice. Uneconomical to keep flying currency up, much more economical to fly straight and level and long distances.

Do you fly your plane with 50% power and are you running always at least leaned at peak, including starting from 1000 ft AGL. We do it here because it is very much waste of money to not do that. Good news is that engine never gets too hot, not even nearly, CHTs are always very low on all cylinders. And it does not get shock cooling either because it is quite cool when starting the descent. When we were flying this plane in USA, it was so great to fill the tanks, costed almost nothing. And there were even nice service on FBOs no additional cost. In here, we have no FBOs, landing fees, navigation fees, parking fees, airport security that makes it hard to get to your private plane because the airports are open only few dozen of minutes per day and other times everything is closed and when it is not closed it is "Hello where are your tickets, and what flight number? Do you have any liquids?".

If one wants to have meaningful market for plane nowadays outside USA, it would be quite wise to make it as efficient as possible, making it as fast as possible with as small engine as possible. Therefore it will certainly be 30 mph faster by tuning aerodynamics rather than having enough horse power to go 30 mph faster. And going 30 mph faster by increasing horse power would require increasing the horse power by a lot.

And indeed, the legs are much shorter than the original range of the aircraft. Even in our Diamond DA40. That's because there needs to be reserve. On some legs there needs to be reserve to fly back to the departure airport because there are no other alternate airports. You can see some stats from the length of the legs with our Diamond from our 2009 flight here:

http://gc.kls2.com/cgi-bin/gc?PATH=kcrg-kjax-krmg-1h0-ktop-khys-kpub-kfmn-kigm-kwjf-kpao-keko-kcpr-ksfd-kosh-kcak-ksfz-kpqi-cyzv-cywk-cykl-cyvp+-+CYFB+-CYXP-+BGSF-bgkk+-birk+-bieg+-ekvg+-enbr+-ekah+-+efma+-efhf &RANGE=&PATH-COLOR=red&PATH-UNITS=nm&PATH-MINIMUM=&SPEED-GROUND=&SPEED-UNITS=kts&RANGE-STYLE=best&RANGE-COLOR=navy&MAP-STYLE=



We would have preferred much longer legs, and it would have been more economical, but unfortunately we were limited to much shorter legs due to safety margin and also the slow speed made it difficult to deal with weather since spending a night in middle of nowhere meant 600 dollar hotel expenses per night and sometimes more than one night waiting for the weather.

To stay marginally on-topic: My next video (John Roncz's presentation 1) is coming soon. Compressor is baking it as we speak. We had a interesting discussion about span, induced drag, downwash etc. with Pete Gall (and Todd) after the presentation. The common understanding actually was in line with John Roncz and Barnaby Wainfan, not contradicting them.

What comes to my own concept, what I have discussed on other posts, my ultimate goal is so high efficiency that it makes traveling large distances practical with private plane. In other words, one I can fly from Europe to Oshkosh without talking to my bank manager. I think some others might find such concept also useful. What comes to surface quality and how long it lasts, on top of composite skin, a two component polyurethane paint lasts for long and it also gives a smooth finish which can be polished to very high gloss with sanding and polishing compounds. It can be painted with no pin holes with my technique that I have been prototyping with building some molds and plugs. I can tell about that sometime in a separate topic.
 

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Monty

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Karoliina,

We are all subject to our opinions being colored by our particular circumstance.

You misunderstand my comments. My particular airplane is a single seat aircraft of less than 40hp. The fuel burn is quite small...insignificant compared to other cost in fact (2-3 gal/hr). Your mission is entirely different. When I travel, I fly in a Cessna 172 and I understand your objections. I too would rather have an efficient cross country machine for travel. Still, most of the time my flight is less than 300 miles/leg and below 10Kft. Fuel burn is equivalent to a typical SUV. So in my country for a typical trip, not too bad, especially since I have an auto fuel STC. As others have pointed out, power off performance with slotted flaps extended is rather scary. I suggest you look into the MU2's safety record. The MU2 is the prime example of taking this design concept to the limit. If flown properly the MU2 does provide rather impressive performance, but it is demanding of its pilots.

For what you want, I think the Bonanza is a poor starting point. For the kind of legs and mission you desire, a pressurized aircraft is the way to go. You need to fly high, just like the airlines. A Piper Malibu would be a better starting point.

With current technology, it will never be inexpensive to fly this mission. You can pay for fuel, or you can pay for a more efficient airframe/engine. Where the tradeoff falls is unfortunately more a function of local tax policy than anything else.

Monty
 

karoliina.t.salminen

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Continuing a bit OT:

Monty: Slotted flaps extended, scary glide perf? Not really, at least in the case of the planes I have flown with slotted flaps; Cirrus SR20, SR22 and Dynaero MCR01 ULC. There is some drag when the flaps are out, but by pointing the nose more towards the ground fixes the problem. With my experience, slotted flaps are not any worse than plain flaps. In a clean aircraft in contrary, they are quite necessary to increase the drag enough for ability to land the aircraft in a meaningful distance.

I don't know how MU2 would be any optimized. It has full span flaps if I am not mistaken, but why should flaps be full span if it only increases maximum lift coefficient by 7% due to the lift at the tip is zero regardless if there flap or not and the lift near to the tip is low regardless of the lap due to the leakiness of the wing. Especially because MU2 has a very leaky wing, it has same wing span as Diamond DA40, but much higher weight, much longer chord (i.e. it has low aspect ratio and high span loading and high wing loading). The performance of the MU2 is not that impressive considering the enormous power it has. Stall speed is also so high that it would be quite hot to land alone because of the stall speed. No wonder that the stall speed limit for single engine aircraft is 61 knots, it is a good choice for safety. And oh, looking closer, it has rubber boots. That's a killer for the undisturbed airflow. I would not say that MU2 is at all even to the direction of being a example of highly efficient aircraft, in fact it is about the opposite I am thinking. I have not crunched the numbers, did not check the Clmax on landing config on it, but does not really look like so special concept to me.

Piper Malibu is rather poor for my use as well, that's why I am thinking about aircraft design. Such production aircraft which fits to the category I am talking about does not simply exist. Only some drag queens exist on that category where I am looking at. Think of it with the common terms: A large pressurized motorglider which flies at minimal power and carries massive amount of fuel along that has enormous endurance and enormous range and and is able to fly at high altitude with very low thrust available. Such aircraft simply does not exist these days elsewhere than in Mojave.
 

Monty

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Slotted flaps alone are not the problem, only in combination with very high wing loading.

Another aircraft that comes to mind is the White Lightning. Still probably not as efficient as what you are after.

From what you describe your problem is more jurisdictional, and government related, than technical. I can sympathize. We unfortunately seem headed in the same direction.

I don't like filling up after doing touch and goes either. Even over here when 100LL is above 5/gal the cost is a consideration, at least for me. Fortunately I have an autofuel stc, and there is 91 octane auto fuel on the field for around $4.30/gal. Still my fun flying airplane is designed around a small engine to keep fuel consumption down.

I also fly lean of peak, and usually throttle back to the bottom of the green arc. If it's a hot day, and airspace or terrain requires a rapid climb I keep it rich to help cool the engine. If I can climb slowly in a cruise climb, I lean even in climb. I usually climb up to 6500-10Kft for cruise, and pick the altitude based on the most favorable winds. The difference in speed between bottom of the green arc and the top of the green arc in cruise for my airplane is only about 10-15kts. The difference in fuel burn is about 4 gal/hr. Typically I burn 7-8 gal/hr. I usually try to time my descent so that 500ft/min gets me over the field at pattern altitude. I just leave cruise power in all the way down to prevent shock cooling. I can't always do this, if it's too rough and I would be above Va I have to throttle back, or start descending earlier.

Even here in the land of "cheap" gas, some of us try to burn less of it. If my plane is full of people the seat miles per gallon is actually quite good. Especially when you look at travel times and money saved on motel rooms/meals.

Monty
 

BBerson

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Thanks for kind comments about me sharing the video :).


To stay marginally on-topic: My next video (John Roncz's presentation 1) is coming soon. Compressor is baking it as we speak. We had a interesting discussion about span, induced drag, downwash etc. with Pete Gall (and Todd) after the presentation. The common understanding actually was in line with John Roncz and Barnaby Wainfan, not contradicting them.
Dr. Gall said at 8:40 of his video: " Induced drag is a function of aspect ratio."

John Roncz on his video at 17:10 said: " Aspect ratio has nothing to do with induced drag"

A black and white contradiction, I would say.

Thanks for the video's (instant replay feature is very useful for understanding).
BB
 

Monty

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I wouldn't hold Dr. Gall's feet to the fire over that statement. You can't use the same level of scrutiny when watching a talk that you would when reading a technical paper. He may have meant span and just said aspect ratio without really thinking about it.
 

BBerson

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I wouldn't hold Dr. Gall's feet to the fire over that statement. You can't use the same level of scrutiny when watching a talk that you would when reading a technical paper. He may have meant span and just said aspect ratio without really thinking about it.
Did you watch the video ?
He said aspect ratio at least ten times while pointing to the screen that also had aspect ratio in the formula.
I am not holding anyone to the fire, just stating what is on the video, see it for yourself.
There is an obvious disagreement here about the induced drag formula itself. They each had a different formula on the screen.


For the record, I think they have both got it wrong or incomplete (wing area must be considered directly rather than aspect ratio, in my opinion). I just looked up induced drag on wikipedia and found another formula that does include wing area.Lift-induced drag - Wikipedia, the free encyclopedia

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autoreply

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There is an obvious disagreement here about the induced drag formula itself. They each had a different formula on the screen.
Not really, both are correct, question is, which one is more practical to use. Span loading there is a pretty useless concept. If I tell you your span loading is 100 lbs/feet, you don't know anything about the design we discuss and it could be either a Starfighter or a B52 we are discussing...
If I tell you it's aspect ratio is 10, you know a lot more. You will roughly know best L/D, climb speeds, general flight envelope. In fact for your performance envelope, wing loading is irrelevant, since your envelope expands with the square root of your wing loading, though performance is obviously not.
 

Himat

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Not really, both are correct, question is, which one is more practical to use. Span loading there is a pretty useless concept. If I tell you your span loading is 100 lbs/feet, you don't know anything about the design we discuss and it could be either a Starfighter or a B52 we are discussing...
I do agree that far, but an aspect ratio of 10 dont tell that much as long as you don't now the wing loading and reynolds number. It could be a 30gram free flight model or a bizjet. Even between a heavy "hot" composite and a light tube and fabric airplane there would be large differences in minimum sink even if best L/D was quite similar.
 

Monty

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Did you watch the video ?
He said aspect ratio at least ten times while pointing to the screen that also had aspect ratio in the formula.
I am not holding anyone to the fire, just stating what is on the video, see it for yourself.
There is an obvious disagreement here about the induced drag formula itself. They each had a different formula on the screen.
As much as I could. It kept freezing up on me. So I may have missed that segment.

My guess is it depends on how you derive things. Rocz is using the theoretical argument from momentum theory. Which is correct. Assume you have a majic thread that deflects the air the required amount and is infinitely stiff....sure it's mathematically correct, but to actually acomplish that it will have look more like a wing and have an area, and since span and area are used to define aspect ratio, you can use aspect ratio to derive the same thing. So they are both right depending on your point of view as Auto said.
 

BBerson

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Roncz said he would reject any paper from a professor that includes aspect ratio and he also rejects the formula on the NASA website:Induced Drag Coefficient Then he said he doesn't care about wing area at all.

I guess you guys don't see the contrast, I give up.

BB
 

WonderousMountain

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If that formula were absolutely correct we could ignore a goodly portion of the tests done on airfoils. AR is a convenient and relatively effective way to measure induced drag, but doesn't replace other techniques. A better way is to look at total drag all the time. It's more difficult, but good practice for anyone serious about aircraft performance. All great innovators and many good teachers tackle a problem from unorthodox positions. There's a similar debate going on with Aerodynamic center which NASA tends to think ought to be measured from the 1/4 chord point but in fact is different at every flight speed and incidence angle. Personally I find measuring from the Center of mass more effective. Two (or more) well informed intelligent people will refuse the others explanation while using the same equations....:gig: Are we not lucky to have so much to choose from even in our so called "hard sciences"?
 

autoreply

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I do agree that far, but an aspect ratio of 10 dont tell that much as long as you don't now the wing loading and reynolds number. It could be a 30gram free flight model or a bizjet. Even between a heavy "hot" composite and a light tube and fabric airplane there would be large differences in minimum sink even if best L/D was quite similar.
Stop thinking in numbers and start thinking in dimensionless factors, since that's the only way to get a good idea which factors (aspect ratio) influence other outcomes. Otherwise you'll be stuck in numbers like span loading that in the end are meaningless without much more context.
 

Jan Carlsson

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This from other tread,

Re: reducing induced drag?
induced drag is realy written Di = (W/b)^2 / q / Π / e there is other formulas using Λ lambda (aspect ratio) But Λ realy only change the cL' (lift curve angle)
cL' =0,0965 / (1+1,76*c/b)

Meaning span load is vital.

This is what Erik Bratt says in his book, that many designers see after have looked long enough that the "old" classic formulas CL^2 / Π /e / Λ *S * q means that induced drag is related to AR. but it is not so!

Jan
 
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autoreply

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I'll let Auto speak for himself if he sees fit.
I couldn't possibly have said it any clearer as:
I suggest there was once this elephant......
People who proclaim wing loading, span loading, aspect ratio to be "not the case", who simply reject it, or who say one or the other is irrelevant/unimportant simply don't see the whole picture and might instead spend their time on understanding that big picture. All 3 explanations are correct, just different perspectives of the same phenomena...
 
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