shdw

Form drag is a type of parasite drag.


Since all types of parasite drag are a function of speed and the idea of an experiment is to isolate the variables as long as speed doesn't change all parasite drag will remain relatively constant. That being said, this experiment will directly show a link between AOA and induced drag, as you admit, and that is its purpose. A slight increase in parasite drag wouldn't matter, the point would be to realize that tilting your hand back increases AOA and induced drag, a pretty obvious recognition from this simple experiment.

Cubdriver

The term parasite is a catch-all term for many types of drag, and it only matters that you distinguish between them. A few sources will lump form drag with parasite drag, which it probably should be grouped with for simplicity, while others make it a separate item because it bears more resemblance to induced drag. Whichever definition you choose to use, be clear that form drag arises from changes in the pressure field surrounding an object in a viscous medium as it encounters an object in a fluid flow, and it is caused by the shape of an object itself, and it does not arise from circulation, skin friction, interference between surfaces or any other cause. It operates by and adheres to the Bernoulli Principle. In contrast, skin friction is drag created between air molecules and a given lifting surface by increasing the thickness of the boundary layer through introduction of lateral movement (swerving) of air molecules in close proximity to the surface.

shdw

Can you show how it resembles induced drag? I just spent the last hour on google and in my aerodynamics books and cannot find a source that doesn't link form/pressure drag to parasite drag. I even ran through some calculations done on the MD80 by MIT students who did not separate form drag from parasite/induced. All the sources I find show form drag is related to speed and a portion of parasite drag, not related to lift or induced drag.

I also looked for an individual formula for form and pressure drag (I realize they are the same but searched for each individually for thoroughness), coming up empty.

How does any of this disprove the use of ones hand out a window to demonstrate induced drags relationship to AOA? Assuming no drastic changes in speed are experienced during the experiment.

Cubdriver

Not trying to shake anything up here, shadow. Use your peer-reviewed textbooks for your ultimate understanding of these topics. These forums are for exploring ideas and I am none other than a humble servant of such topics. What I say does not supercede the current peer-reviewed scholarship available on the subject. I gladly defer to any peer-reviewed textbook copy you would like to bring forth.

shdw

Well the textbooks I looked in were Aerodynamics for Naval Aviators and Flight Theory for Pilots. The sites were a Stanford site, wikipedia, and various other no name sites. Nor am I, I just didn't see a source which referred to form drag as something other than parasite or a source that linked it to having similarities with induced drag. That is why I asked if you could post them up or explain its relationship so I can understand it.

Cubdriver

Here's one- Denker on Types of Drag.

Notice that he associates form drag with induced drag because it is a type of pressure drag, and with parasite drag, which of course is the more common grouping.

Also please note that Mr. Denker's website is not peer-reviewed. It is also free, available online and without a subscription, and he is a good writer and a top scientist. I use his site for these reasons alone. But as much fun Denker may be to read, his website does not constitute peer-reviewed scholarship. We need to bear in mind that peer-reviewed textbooks used by ABET-approved colleges and universities are the best source of information and represent the accepted standard for knowledge on these topics.

This brings up a few points that seem well placed here.

One is, there should be a free-wheeling place in the world for which to discuss topics like the ones we discuss here at APC. Such places should not be held to a very high level of scholarship. One should be able to post whatever erroneous or correct impressions they happen to carry on a given topic and feel free to do so, the only restriction being a self-imposed mandate for sincerity. I have noticed a tendency for users of the Flight Training and Technical forums to enter needless contests about sources, when the standard for scholarship should already be clear to such users. Doing so they run the risk of dragging the easy, free-wheeling mood of the discussion down while making it very difficult to share ideas. We want an atmosphere of mutual respect and we want a mood of sharing to prevail. If you want 100% correct, accurate, well-studied, approved, and complete sources on a subject you do not want to be conducting your research on an internet forum. That is not to negate the value of any informal, free-wheeling, sincere discussions had at APC or any other quality internet forum.

Second, one needs to acquire their initial understanding of involved scientific topics such as these at a recognized places of learning, such as an ABET-approved universities, FAA approved flight schools, and authorized government training institutions such as the US Military. To come to APC and expect to do anything other than speak casually about a topic is not a very good idea. I admit I have not always been a perfect example of an ideal internet user, but these are the thoughts I always keep in mind when using the Flight Training and Technical forums at APC.

shdw

A buddy of mine hooked me up with this if anyone is interested:




Cub: I am assuming this is the image you refer too:



You will notice Denker is not showing or claiming form drag to be a type of induced drag, it is its own separate entity. What he is showing is that form drag and induced drag are both drags that occur because of pressure. It is an interesting play on terms but careful how you read it.

Induced drag is agreeably a drag caused by pressure, after all lift is because of pressure. So one can conclude that if induced drag is a result of lift, and lift is a result of a difference in pressure, than induced drag is a result of pressure as well. Do notice though that form drag is still classified as parasite drag and noted that it as well is a result of pressure.



I would like to say, and I mean no disrespect, I question your points you make though and they somewhat deter me from what these forums as a place for accurate information. Your first point makes it seem it is ok to represent erroneous information and one who attempts to clarify it or correct it using sources is only deteriorating the purpose of APC forums. I find that a concerning representation of what should be an informative tool that users can come to for with questions, expecting to receive correct information.

I realize you don't want fights occurring on the forums with regards to representing ones ideas. But backing up information presented with sources, preferably multiple sources, and striving to find the truth, or at least a descent representation of it, should be the goal of any forum IMO.

While a forum shouldn't be a place to go for all your information on studying a subject, it should be a place where one can clarify and further understand a topic. The use of ideas from others, as well as providing sources to back up a claim, only act to further ones understanding of a topic and validate its truths. It shouldn't deter someone from posting with a question or their idea, wrong or right, but those ideas, if wrong, should be corrected.

Again I mean no disrespect with this post, just seems you are almost warranting false information and regarding sources as unnecessary just because this isn't a scholarly site. Seems to me your setting the bar kind of low.

Cubdriver

No disrespect taken, shadow. I appreciate your enthusiasm for truth. My whole point can be summarized as, "relax, this is just an internet forum". The rules we must not run afoul of are listed in the Terms of Service, but no one here is abusing them at all.

I am not sure what you want at this point as far as this discussion is concerned. Do you want some sort of reassurance that form drag is grouped always and by everyone in the academic community as parasite drag? That's not really true in an absolute sense, but if you want me to say that, then sure, it is usually grouped with parasite drag. Nice poster, by the way.

You worry me that you are even talking about using these forums for a primary source of information about aerodynamics. Discussion is fine, but the bar is low, and my earlier point was simply that you should go to textbooks not the internet for your education. I would say that to anyone.

Anyway, let's give it a rest because I think the original poster has lost interest in the original question at this point.

shdw

No, I was just trying to see the connection you were making. The relationship between form drag and induced drag, I wasn't sure if it goes deeper than they are both drags that occur because of pressure.

No I did not mean this. I have a degree in aviation and have been reading a lot since graduation about various aspects of flying, especially aero. I use these and other forums as a place for discussing topics, the only goal being to further my understanding by seeing if the points I make are agreeable to others. Edit: If they are not than hopefully I can find out where to go to do more research on the topic.

Than stop asking me questions!!! lol :sarcasm: I am done and I hope those answers satisfy the questions/concerns you have.