Calculatiuon of CD

[raptor22]

I have been able to find the equations for induced drag coefficients, but i am at a loss to find a way to calculate parasitic and profile drag coefficients without a windtunnel and (like most people) I do not exactly have access to one.

Thanks,
Alex

[vintage1]

You and the rest of the aerospace industry.

That's why they use wind tinnels and enormously expensive powerful computers.
There is no simple answer.

[Ollie]

If you want to understand more, I recommend Model Aircraft Aerodynamics by Martin Simons. The explainations are in plain English. Most of the formulas are contained in the Appendices where you can dig them out if you need to do a calculation. The text is well illustrated.

[raptor22]

You and the rest of the aerospace industry.

That's why they use wind tinnels and enormously expensive powerful computers.
There is no simple answer.

I had thought it was just something "you don't do", but I noticed that in post 22 of the thread "airfoil nerds I need your help" that Ollie calculated these coeffecients. So, there must be a way to calculate it without either windtunnels or multi-processor supercomputers (assuming Ollie doesn't have all of this at home....).

So, I fugured I's ask for the equations. Its worth an attempt. I even have managed to get the CDp equations but would still be forced to approzimate CF from a table of "most light planes should have a CF of...".

Ollie: I understand aerodynamics pretty well (especially for a 16 year old, as I'm not still spounting what my 6th grade science teacher said). I would prefer (I don't have much cash) not to buy any more books. So, I do research on the internet. etc.

--Alex

[vintage1]

I think - and Olie will I hope corect me if I am wrong - that profile drag calcualtions come in 4 sorts

(i) Look at it, take the cross sectional area, work out how streamlined it is, multiply by the last number you thought of when you tested it in the wind tunnel, and use that. I.e. 1920's style.

(ii) As above, but using numbers someone else thought of when THEY tested it in a wind tunnel.

(iii) As above, but adding in every rivet and blob strut and wire using yet another 'fudge factor' on those

(iv) Buy a Cray, feed it with the shape in question, the Newtonian equations plus the material properties of the air, and sit back and wait a month for the answer.

Simple shapes can be guesstimated quite well, but complex ones are very very hard, especially if there are turbulent flow patches.

Wasn't there an apocryphal story of the prototype spitfires thatwere flush riveted, and they stuck blobs of clay on the rivets one by one notig the performance loss on each one to work out whow many really needed to be flush, and how many cheaper simpler round heads they could get away with?

[Ollie]

vintage1 Eng. plus of Ci of tails.

(20 min. of aphasia)

[Sparky Paul]

...
(iv) Buy a Cray, feed it with the shape in question, the Newtonian equations plus the material properties of the air, and sit back and wait a month for the answer.
...

You left out... "build it,fly it, and add another in a long line of "correction constants" to make the flight test stuff agree with the computer stuff.
.
'nother anecdote.. The C-119 was certified by... "It's an update of the C-82, which flew OK. therefore it will also fly OK".

[Ollie]

vintage1:
Simple shapes can be guesstimated quite well
OK

Ollie:
Tail
OK

Sparky Paul:
You left out... "build it,fly it, and add another in a long line of "correction constants" to make the flight test stuff agree with the computer stuff.
OK

raptor22:

Think Engr.

[vintage1]

Simple shapes can be guesstimated quite well
That's what I said Ollie...viz...
Look at it, take the cross sectional area, work out how streamlined it is, multiply by the last number you thought of when you tested it in the wind tunnel, and use that. I.e. 1920's style.

:D

But I though that paragraph would be more amusing than saying 'guesstimate' :D

And definitely more amusing than calling me aphasic......:(

If that is what you were doing...your post was rather cryptic...or were you merely being egregious?

[Ollie]

Ollie is asphasic !!! It is 3 Mos.

[vintage1]

Now I am completely stumped Ollie.

All I can do is echo the error message on an old compiler - from 'Brain Damaged Software' as it happens....teh BDS 'C' Compiler.

'Completely confused in parse phase: Giving up'

:D :D

:D

[Ollie]

Jun 14, 2004, 02:48 PM
#40

Ollie
Registered User
Join Date: Apr 2002
Location: Punta Gorda, FL
Posts: 2,498


I have liked the hobby of model airplanes for 66 years. I had a stroke 3 weeks ago and so will not be active in the hobby for a while. I feel much better now than I did 3 weeks ago. As a result of the stroke, I can no longer express my ideas in writing. Physically, I feel fine. Ollie / Helen

[vintage1]

Damn and blast, Ollie/Helen. That is bad news for those of us who rely on you for definitive answers.

I hope you recover enough functionality to at least enjoy some aspects of the hobby.

[Sparky Paul]

Jun 14, 2004, 02:48 PM
..
I have liked the hobby of model airplanes for 66 years. I had a stroke 3 weeks ago and so will not be active in the hobby for a while. I feel much better now than I did 3 weeks ago. As a result of the stroke, I can no longer express my ideas in writing. Physically, I feel fine. Ollie / Helen
.
Hey, you have to get over this! We depend on you! :D

[Ollie]

Ci=Ct+Cf+Cg+....
tail
fus.
gear
+
?
+
?

raptor22: think Engr. (math + Engr.)

[vintage1]

I could follow that Ollie! Lord knows what struggle you must have made to type it. We're rooting for you.

[Ollie]

Biplane.
~36ft. span wing
Ad sign had a ~6ft tall x ~60ft tow sign.
This, big, big sign has Cl part of all Cl.

Math of big, big ad sign tow of Cl ???

Odd of part many of Cl parts.