Ye Olde Rocket Forum

Go Back   Ye Olde Rocket Forum > Work Bench > Building Techniques
User Name
Password
Auctions Register FAQ Members List Calendar Today's Posts Search Mark Forums Read


Reply
 
Thread Tools Search this Thread Display Modes
  #31  
Old 06-29-2006, 09:05 PM
sandman's Avatar
sandman sandman is offline
Custom Cone, Kit & Decal Maker
 
Join Date: Mar 2005
Location: Way North of Detroit
Posts: 5,048
Default

I wasn't goint to take the pads off.

I just wasn't going to put them on for the first flight.

I wanted to fly it without the pads just to see if it made any difference.

I can always glue them on later.
__________________
"I'm a sandman. I've never killed anyone. I terminate runners when their time is up." Logan from "Logan's Run"

http://sandmandecals.com/
Reply With Quote
  #32  
Old 06-30-2006, 12:26 PM
tbzep's Avatar
tbzep tbzep is offline
Dazed and Confused
 
Join Date: Mar 2005
Location: TN
Posts: 11,610
Default

The landing pads would cause stability if the airstream hits them at the same angle....but it never does because the pads are tilted.

Since the angle of the pads changes as the angle of attack changes, the problem is magnified. When the rocket tilts, the pads that need to have the most drag, will be angled more vertically and have less drag. The pads that need to have the least drag, will be tilted more horizontally, causing more drag and making the Lander tilt even more. To add self correcting stability for vertical flight, the pads would need to be tilted opposite the way they are on the model.
Reply With Quote
  #33  
Old 06-30-2006, 02:16 PM
Tweener's Avatar
Tweener Tweener is offline
Out To Launch
 
Join Date: Mar 2005
Location: Ottawa, Ohio
Posts: 281
Default

Quote:
Originally Posted by tbzep
The landing pads would cause stability if the airstream hits them at the same angle....but it never does because the pads are tilted.

Since the angle of the pads changes as the angle of attack changes, the problem is magnified. When the rocket tilts, the pads that need to have the most drag, will be angled more vertically and have less drag. The pads that need to have the least drag, will be tilted more horizontally, causing more drag and making the Lander tilt even more. To add self correcting stability for vertical flight, the pads would need to be tilted opposite the way they are on the model.
Interesting point. You may have just discovered the reason a ML could suddenly "go unstable" after many flawless launches. Since the pads are the first thing to hit the ground, one of the pads or legs may get knocked out of alignment.
__________________
John Lance Ladd
Yoyodyne Propulsion Systems
Grovers Mill, NJ
Established 1938
lladd.netau.net
Reply With Quote
  #34  
Old 06-30-2006, 02:39 PM
marslndr marslndr is offline
Craftsman
 
Join Date: Mar 2005
Location: Boston
Posts: 168
Default

An interesting idea, but I don't think so. The pads are very sturdy, it would take alot to get one out of alignment. I think any wack that could move a pad would break the leg.
Only the very bottom of each pad touches the ground.

I think Carl found the problem with the leg wooble.

Mark
Reply With Quote
  #35  
Old 06-30-2006, 02:49 PM
dwmzmm's Avatar
dwmzmm dwmzmm is offline
Master Modeler
 
Join Date: Apr 2005
Location: Katy, TX
Posts: 2,345
Default

Quote:
Originally Posted by Tweener
Interesting point. You may have just discovered the reason a ML could suddenly "go unstable" after many flawless launches. Since the pads are the first thing to hit the ground, one of the pads or legs may get knocked out of alignment.


I still believe my Estes ML went unstable for a reason that's a mystery (to me) to this day.
The "simple" adding of a little nose weight (with an additional screweye) solved the problem.
Since it was built "like a tank," I can't think what components within the model could have
shifted after all those successful flights to change the CG/CP relationship. Here's a pic of
that ML in my display case (pic taken last year; the ML itself is 35 years old):

http://www.nasarocketclub.com/galle...17/FH000024_001

I have a roll of masking tape underneath the ML to help support the landing legs while on
display...
__________________
Dave, NAR # 21853 SR.
Reply With Quote
  #36  
Old 06-30-2006, 02:56 PM
tbzep's Avatar
tbzep tbzep is offline
Dazed and Confused
 
Join Date: Mar 2005
Location: TN
Posts: 11,610
Default

I'm not talking about getting out of alignment. I'm talking about the angle of the pads causing a varied amount of drag and vectoring.

The first picture shows the pads on my Alien Explorer, which are angled similarly to the Mars Lander.



When the rocket tips off to the left, for whatever reason, the pad on the left becomes fully horizontal and much more draggy than the other, which is now tilted at roughly 45 degrees. The 45 degree angle pad acts like a canted fin. Both pads work together to tilt the rocket even more.



If the pads were tilted the opposite direction, the effect after tipoff would be the opposite. The left pad is almost vertical, giving the least amount of drag, and it's also not canted as far, so it doesn't want to change the rocket's direction as much. The right pad is almost horizontal, creating extra drag on that side, so the rocket wants to tilt back to the vertical.



Of course, the ML has smaller pads, but it's also a much shorter rocket and outside forces will do more to it.

Last edited by tbzep : 07-03-2006 at 09:34 PM.
Reply With Quote
  #37  
Old 06-30-2006, 03:18 PM
marslndr marslndr is offline
Craftsman
 
Join Date: Mar 2005
Location: Boston
Posts: 168
Default

Oh, I understand now.

I suppose, It would have a small effect but the lander is marginally stable anyway.
Reply With Quote
  #38  
Old 06-30-2006, 03:25 PM
tbzep's Avatar
tbzep tbzep is offline
Dazed and Confused
 
Join Date: Mar 2005
Location: TN
Posts: 11,610
Default

Quote:
Originally Posted by marslndr
Oh, I understand now.

I suppose, It would have a small effect but the lander is marginally stable anyway.


The less stable the rocket is, the more effect these forces would have. Aircraft and watercraft have used this technique to create straight line stability by using wedge shaped airfoils or turning double finlets like the last picture, not like the first, so it definately can have more than a marginal effect.

Build two clone Alien Explorers, one with pads and one without. You will see a huge difference in vertical stability after multiple flights of each.
Reply With Quote
  #39  
Old 07-01-2006, 09:25 AM
Tweener's Avatar
Tweener Tweener is offline
Out To Launch
 
Join Date: Mar 2005
Location: Ottawa, Ohio
Posts: 281
Default

Quote:
Originally Posted by tbzep
I'm not talking about getting out of alignment. I'm talking about the angle of the pads causing a varied amount of drag and vectoring.
Yes, I understood that from the beginning. But if the alignment and width distribution of the legs (fins) is critical as in the marginally stabe ML, a small change in the angle of one (perhaps a difference in tension of one spring or rubber band) may throw it off a little. That small deviance might then be magnified by the phenomenon you describe.
__________________
John Lance Ladd
Yoyodyne Propulsion Systems
Grovers Mill, NJ
Established 1938
lladd.netau.net
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump



All times are GMT -5. The time now is 06:55 PM.


Powered by: vBulletin Version 3.0.7
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Ye Olde Rocket Shoppe © 1998-2024