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January 26, 1999
QuestChat with Rabi Mehta

Senior Research Scientist
NASA Ames Research Center, Moffett Field, CA



[ Sandy/NASAChatHost - 6 - 09:56:05 ]
Today's QuestChat with Senior Research Scientist Rabi Mehta will begin in about an hour. Be sure to read Rabi's very interesting bio before the chat begins: http://quest.arc.nasa.gov/aero/team/rabi.html. Do you know the title of Rabi's new book??? And, have you ever played his favorite sport???

[ Sandy/NASAChatHost - 7 - 09:57:07 ]
RE: [Deb-Mrs.Regal/YpsilantiCOPE] Ypsilanti COPE is standing by ready to begin today's chat.
Good morning Deb! We will begin in about 5 minutes... Stay tuned :-)

[ Sandy/NASAChatHost - 11 - 10:05:04 ]
Everyone: Welcome to today's chat with Rabi. He is here and ready to take your questions.

[ RabiMehta/ARC - 12 - 10:05:27 ]
Hi everybody -- I am here and ready to chat with all of you.

[ RabiMehta/ARC - 14 - 10:07:14 ]
RE: [Sandy/NASAChatHost] Hi Brice. The chat hasn't begun yet. Hold on about 5 minutes and then we'll begin. Thanks!
There was an article in the Scientific American Magazine (November 17 1997 issue) --- get a hold of that and then you can send me e-mail if you have more questions.

[ RabiMehta/ARC - 16 - 10:07:56 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] How long have you worked for NASA?
I have been here at NASA AMes for 17 years --- I studied in England and came to work here.

[ RabiMehta/ARC - 18 - 10:08:43 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] What did they find out about the Wright brother's plane when they tested it in the wind tunnel?
It has not been tested in the wind tunnel yet, but it is going in very soon, so stay tuned.

[ RabiMehta/ARC - 19 - 10:09:33 ]
RE: [Chris-ChrisWee/WTDOmoderator] Hi Rabi, I was wondering where I could find the articles on the rules for building wind tunnels you mentioned in your biography.
I just sent out details of a very basic article -- if you need something more advanced, please send me e-mail.

[ RabiMehta/ARC - 20 - 10:10:53 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] What have you found out about tennis balls in relation to aerodynamics?
Our main task was to show how lift or side forced is generated on a spinning tennis ball --- we did a lot of flow visualization studies in a wind tunnel. We found that the "fuzz" on the ball has a significant effect.

[ RabiMehta/ARC - 22 - 10:13:20 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] What are some of the new designs in spaceships?
The latest idea is to replace the Space Shuttle with what is known as a "Reuseable Launch Vehicle (RLV)" --- this will be used for transportation to the space stations.

[ RabiMehta/ARC - 27 - 10:16:59 ]
RE: [Dan-Mr.Kelly/TrinitySchool] Dr. Mehta, How critical is the consideration of tunnel usage when in the design phase of a wind tunnel? Specifically, are results from scale model testing easy to apply to the full scale object?
Great question Dan. Wind tunnel useage is critical in the design stages --- computers alone cannot be used to design aircraft. A lot of the initial testing is done in wind tunnels to determine aerodynamic loads and performance characteristics. The application to full-scale flight is always a problem, but the big companies (e.g. Boeing) have enough experience with this to be able to do it adequately.

[ RabiMehta/ARC - 31 - 10:18:22 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] How much weight was saved from not painting the shuttle?
I am not sure. Of course, painting the shuttle became a nonissue once the heat protection tiles had to be put on.

[ RabiMehta/ARC - 33 - 10:19:44 ]
RE: [Chris-ChrisWee/WTDOmoderator] Here's a hard one for you...what is the relavence of matching Reynold's #'s when testing full scale models or wing sections in a small-scale tunnel (i.e. CO2 racers, sections of model airplane wings, or a balsa wood plane like one in the Right Flyer Project)?
Reynolds number determines the flow behavior (it is not the only parameter, but one of the basic ones) -- so whenever possible, the Reynolds number is matched in the wind tunnel test.

[ RabiMehta/ARC - 34 - 10:20:43 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] What experiments would you conduct on the International Space Station if you were assigned to go there?
I am not a Space expert, but I would guess that most experiments would have to do with living in a zero gravity environment.

[ RabiMehta/ARC - 35 - 10:22:03 ]
RE: [BlakeDoughty-MrCampbell/Heritage] How long until computer wind tunnel models make actual wind tunnels obsolete? What is this kind of technology lacking right now?
We are a long way away from replacing wind tunnels with computers --- we still cannot compute the whole flowfield around an aircraft with sufficient accuracy and so wind tunnels are vital.

[ RabiMehta/ARC - 38 - 10:23:39 ]
RE: [MsMetcalf-Ms.Metcalf/yokayoschoolukiahca] Please tell my fifth graders about schooling in England, and what you miss about the east coast of Africa.
I was in boarding high school --- not much fun, but University life was similar to that here. The thing I miss most about Africa is our super home and all the friends I had --- also vacations along the coast.

[ RabiMehta/ARC - 39 - 10:25:22 ]
RE: [MrCampbell-MrCampbell/Heritage] I have a question: What is a "googlie" and how is it thrown?
I am assuming you know something about cricket! A googly is a delivery which is thrown with an "off-break" action, but it turns the other way --- away from a righthander -- hope this makes sense.

[ RabiMehta/ARC - 43 - 10:27:33 ]
RE: [Jordan-MMrs.Choate/ViennaGrade] Can you put a person in a wind tunnel? How fast can the wind go with a person in a wind tunnel?
Good question. Putting people in wind tunnels in not generally a good idea (unless you really dislike them!!). I have seen a very good application --- downhill skiers practicing in wind tunnels -- they adjust their attitude to minimize drag. To get a good idea of what you can withstand, hold your hand out of a car window (only if safe!).

[ RabiMehta/ARC - 45 - 10:28:36 ]
RE: [Dean-Mr.Richard/homeschool] What other balls have you worked with the aerodynamics of?
I have done extensive testing of cricket balls (in England) and I have also looked at the aerodynamics of baseballs, golf balls and tennis balls.

[ RabiMehta/ARC - 49 - 10:30:13 ]
RE: [Chris-ChrisWee/WTDOmoderator] Yet another wind tunnel question...when designing a tunnel, after you have decided the dimensions of the test section, what do you do to find the best dimensions for the contraction cone and the diffuser section?
Once you have picked the test section size, the contraction ratio should be about 7-9, so that gives you the inlet dimension -- the diffuser is really to recover energy and about a 5 degree total angle is used.

[ RabiMehta/ARC - 50 - 10:31:20 ]
RE: [Nick-Mrs.Regal/YpsilantiCOPE] Can you tell us more about why a basketball is designed like it is?
As with most sportsballs, I am sure there is a lot of history attached to it. The "ribs" though help to grip the ball and also help to generate aerodynamic lift when it is spinning.

[ RabiMehta/ARC - 52 - 10:32:22 ]
RE: [BlakeDoughty-MrCampbell/Heritage] Can things be damaged while being tested in the wind tunnel?
Sure can, Blake. Models often lose parts and there is generally a cacther screen in the tunnel to recover them.

[ RabiMehta/ARC - 53 - 10:33:25 ]
RE: [BlakeDoughty-MrCampbell/Heritage] Have live animals ever been tested in a wind tunnel? Which ones are the most aerodynamic?
The only "animals" I know of are insects --- there are several projects on insect aerodynamics that i know of -- insects are smart and we need to learn from them.

[ RabiMehta/ARC - 55 - 10:35:13 ]
RE: [MrCampbell-MrCampbell/Heritage] Could you give more detail on that--how does fingering make a difference? How is it like a curve ball--and how different? For that matter, how does fingering make a difference when throwing any ball? How can what you did to the ball suddenly make it swerve or break some 60 feet away?
I am not sure what you mean by "fingering", but if it is the use of fingers, then you want to generate as much spin as possible (on baseballs) -- the idea of a ball suddenly breaking towards the end of the flight is more of a myth.

[ RabiMehta/ARC - 58 - 10:35:55 ]
RE: [Deb-Mrs.Regal/YpsilantiCOPE] Speaking of damage to objects in the wind tunnel, we are wondering whether or not the pressure senditive paint technique could be used on the Wright Flyer to measure airflow?
The pressure paint could be used on the Wright Flyer, although the surface may present some problems.

[ RabiMehta/ARC - 61 - 10:37:30 ]
RE: [Jimmy-Mrs.Regal/YpsilantiCOPE] Why are footballs shaped as they are? Are there other balls shaped as other than spheres?
Hi Jimmy. Yes, most balls are round, except the American football which I think probably evolved from the rugby ball which has similar shape --as does the one used in Australian football.

[ RabiMehta/ARC - 62 - 10:38:20 ]
RE: [Lonnie-Mrs.Regal/YpsilantiCOPE] What changes were made in baseballs to make the balls "live balls?"
The common trick is to scuff the ball using sandpaper and another is to apply spit (to throw the knuckleball).

[ RabiMehta/ARC - 65 - 10:39:37 ]
RE: [BlakeDoughty-MrCampbell/Heritage] Is it true bumblebees, aroedynamically speaking, should not be capable of flight?
I was not aware of this --- bumblebees have an unaerodynamic body, but the wings probably make up for this.

[ RabiMehta/ARC - 66 - 10:40:33 ]
RE: [Autumn-MMrs.Choate/ViennaGrade] What does a wind tunnel look like?
Go to one of the NASA sights -- the one for Ames has a wind tunnel page -- should be easy too find, type "NASA Ames" on your search engine

[ RabiMehta/ARC - 68 - 10:41:37 ]
RE: [Dane-Mrs.Regal/YpsilantiCOPE] What kinds of changes, improvements, or new ideas have you suggested for the sports balls you've worked on?
I have designed a novel cricket ball which can be used in practices -- it swings easily. I have also helped with golf ball and club designs.

[ RabiMehta/ARC - 70 - 10:43:05 ]
RE: [Deb-Mrs.Regal/YpsilantiCOPE] Whether you are working with sports balls or other objects, how do you approach a research problem or question? Please describe the steps in your process.
Normally, you would identify what you are trying to determine and why -- from there you plan out the project based on availibility of resources.

[ RabiMehta/ARC - 71 - 10:43:57 ]
RE: [Britany-MMrs.Choate/ViennaGrade] Have you ever experimented with a kosh ball in a wind tunnel?
No Brittany, I have not. Koshballs are not really designed for their aerodynamics --- they are fun balls which are kind to your hands.

[ RabiMehta/ARC - 75 - 10:45:42 ]
RE: [Chris-ChrisWee/WTDOmoderator] Rabi, in the test section, isn't the pressure higher? If so, how do we compensate for changes in drag due to higher pressure in the test section? Does the higher pressure affect drag or lift of the model?
The static pressure in the wind tunnel is often adjusted to match the reynolds number we talked about earlier. The dynamic pressure (flow speed) is what determines the drag and that is matched through the reynolds number.

[ RabiMehta/ARC - 78 - 10:46:47 ]
RE: [Dane-Mrs.Regal/YpsilantiCOPE] What exactly is a CCD camera?
CCD stands for "Charge-Coupled Device" -- it is the chip in your home video camera that takes the light intensity levels and converts them to a signal.

[ Sandy/NASAChatHost - 81 - 10:47:43 ]
RE: [MrCampbell-MrCampbell/Heritage] The Heritage Physics class would like to thank you for your time, we enjoyed the opportunity to chat with you, and we look forward to finding out about the results of the Flyer's wind tunnel tests! Thanks!
Thanks so much for joining us today! You asked some very good questions! Please join us again soon!

[ RabiMehta/ARC - 82 - 10:48:00 ]
RE: [Nick-Mrs.Regal/YpsilantiCOPE] you started working with cricket balls. In your biography yoou mentioned that cricket is like softball. How do you play cricket? What size is a cricket ball?
Cricket is a little bit like baseball --- there is not enough time or space here to go into cricket, but do a web search and you will find a bunch of articles on cricket.

[ Sandy/NASAChatHost - 83 - 10:48:22 ]
RE: [Chris-ChrisWee/WTDOmoderator] I have to leave, I will e-mail you later Rabi, thanks for your help!
Goodbye Chris. Thanks for joining us today!

[ RabiMehta/ARC - 84 - 10:48:51 ]
RE: [Joey-MrCampbell/Heritage] For balls such as baseballs or golfballs, does the tightness of winding of the core, have a large effect on its flight capability?
Not on its flight quality, unless the mass changes, but it affects the "bounce" property significantly.

[ Sandy/NASAChatHost - 85 - 10:49:59 ]
EVERYONE: There are about 10 minutes left in our chat with Rabi today. Please fill out a short chat survey when this chat is over. It will help the Quest Team figure out if we're doing a good job for you or not. The survey can be found at-- http://quest.arc.nasa.gov/qchats/qchat-surveys Thanks :-)

[ RabiMehta/ARC - 86 - 10:50:06 ]
RE: [BlakeDoughty-MrCampbell/Heritage] Have you ever used live chickens to test engines in a wind tunnel?
We have not out here, but I have heard of such testing -- the problem being bird ingestion at airports -- chickens are cheap and readily available!!

[ RabiMehta/ARC - 88 - 10:50:28 ]
RE: [MrCampbell-MrCampbell/Heritage] The Heritage Physics class would like to thank you for your time, we enjoyed the opportunity to chat with you, and we look forward to finding out about the results of the Flyer's wind tunnel tests! Thanks!
Thanks for participating -- hope you had fun.

[ RabiMehta/ARC - 90 - 10:52:26 ]
RE: [Nick-Mrs.Regal/YpsilantiCOPE] What is fun about your job? What has been your biggest success?
The fun in research work is being able to do something new all the time and the fact that you do not know the answer. My biggest success is probably in guiding some Ph.D. students through graduate school.

[ Deb-Mrs.Regal/YpsilantiCOPE - 91 - 10:53:50 ]
Ypsilanti COPE is signing off. Thank you so much for your time and for sharing your expertise.

[ RabiMehta/ARC - 92 - 10:54:00 ]
RE: [Jimmy-Mrs.Regal/YpsilantiCOPE] What do you hope to accomplish before you retire and when do you think that will be?
Jimmy, I am some ways from retiring. I am now in a managerial position and I am hoping that I can change the future somewhat for aeronautics research at NASA.

[ Sandy/NASAChatHost - 93 - 10:54:51 ]
RE: [Deb-Mrs.Regal/YpsilantiCOPE] Ypsilanti COPE is signing off. Thank you so much for your time and for sharing your expertise.
Goodbye Ypsilanti COPE! You did your homework and asked some really good questions! Please join us again for another chat (and fill out the chat survey!) Thanks :-)

[ RabiMehta/ARC - 94 - 10:55:38 ]
RE: [Deb-Mrs.Regal/YpsilantiCOPE] Ypsilanti COPE is signing off. Thank you so much for your time and for sharing your expertise.
Thanks for participating -- hope it inspired the kids.

[ Sandy/NASAChatHost - 95 - 10:59:04 ]
EVERYONE: It's time for Rabi to get back to his real job, so we'll say good bye for now. Thank you very much for all of your interesting questions :-) We hope you can join us again very soon. Please fill out the chat survey when you can...

[ RabiMehta/ARC - 96 - 10:59:38 ]
Bye kids, I had a great time answering your questions. Keep working hard and stay in school!

 
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