They goofed on the F-15 - RC Groups
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Dec 03, 2016, 11:09 PM
Zen in the art of foam
djacob7's Avatar
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They goofed on the F-15


I was told (so you can blame someone else for this nonsense) that the horizontal stabilizer should never be below the wing level because the downwash from the wing will hit the stabilizer from the top as seen in the pic.
I'm building an F-15, and would like to know if McDonnell Douglas knows what they're doing.
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Dec 03, 2016, 11:11 PM
I don't want to "Switch Now"
pmackenzie's Avatar
They optimized it for inverted flying of course
Dec 03, 2016, 11:23 PM
Zen in the art of foam
djacob7's Avatar
Then I'll have to practice inverted flying a bit more :-)
Dec 03, 2016, 11:53 PM
Registered User
Problem solved.
Just need to sort out the landing gear location.
Dec 04, 2016, 01:24 AM
Zen in the art of foam
djacob7's Avatar
Haha!
(Oops, that might have been a tragedy)
Dec 04, 2016, 05:00 AM
B for Bruce
BMatthews's Avatar
Every high wing airplane out there has the wing above the tail. So what‘s that rule again?
Dec 04, 2016, 06:52 AM
Registered User
Photographers and editors like to twist their photos to show planes going seemingly level w.r.t. the fuselage, but we know that in reality (ever walk up the aisle towards the front of an airliner in flight?) that the body is ALWAYS "nose-high" in level flight.

Here is a more accurate and realistic pic showing that the stab is well clear of the wing wake




... as confirmation, note that the wingtip is at a very slight AoA w.r.t. the line of flight (the airflow will asymptotically merge with the level onflow some distance outboard of the tip) and that the stab is maybe at a micro negative AoA, all of which indicating stable level flight
Last edited by xlcrlee; Dec 04, 2016 at 07:08 AM.
Dec 04, 2016, 09:52 AM
Registered User
Quote:
Originally Posted by BMatthews
Every high wing airplane out there has the wing above the tail. So what‘s that rule again?
well, lots of 'em, anyway ....




Dec 04, 2016, 01:25 PM
Zen in the art of foam
djacob7's Avatar
New rule for stabilizer location: Best place for the stabilizer is anywhere it's convenient. No need to consider wing downwash.
Dec 04, 2016, 01:43 PM
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ShoeDLG's Avatar
Quote:
Originally Posted by xlcrlee
we know that in reality (ever walk up the aisle towards the front of an airliner in flight?) that the body is ALWAYS "nose-high" in level flight.
Not ALWAYS. If you maintain level flight while extending flaps, you frequently find yourself in a nose-low attitude in level flight. Some airplanes have a distinctly nose-low attitude in steady level flight with their flaps deployed (see the P-3 in the attachment).

Quote:
Originally Posted by xlcrlee
Here is a more accurate and realistic pic showing that the stab is well clear of the wing wake.
What information in the photo indicates where the wing wake is?

Quote:
Originally Posted by xlcrlee
... as confirmation, note that the wingtip is at a very slight AoA w.r.t. the line of flight (the airflow will asymptotically merge with the level onflow some distance outboard of the tip) and that the stab is maybe at a micro negative AoA, all of which indicating stable level flight
You cannot infer the longitudinal stability of the aircraft from the stab incidence (see T-38 in the attachment).
Last edited by ShoeDLG; Dec 04, 2016 at 01:50 PM.
Dec 04, 2016, 02:00 PM
Registered User
the F-15 stab is NOT in the wing wake

Dec 04, 2016, 02:06 PM
Registered User
Quote:
Originally Posted by ShoeDLG
Not ALWAYS. If you maintain level flight while extending flaps, you frequently find yourself in a nose-low attitude in level flight. Some airplanes have a distinctly nose-low attitude in steady level flight with their flaps deployed (see the P-3 in the attachment).
normally landings are not considered level flight
Quote:
Originally Posted by ShoeDLG
You cannot infer the longitudinal stability of the aircraft from the stab incidence (see T-38 in the attachment).
exactly, why I wrote "maybe"
Dec 04, 2016, 02:10 PM
I don't want to "Switch Now"
pmackenzie's Avatar
Quote:
Originally Posted by xlcrlee
the F-15 stab is NOT in the wing wake

That is at high AOA, so the stab is below the "wake".

Stab position on a plane like the F-15 might come down to trans-sonic or supersonic considerations., or it might just be that where it is is the only place they could put it.
Dec 04, 2016, 02:27 PM
Registered User
notice that in general, the airflow past a wing is fairly close to following the rear part of mean camber line, also generally regardless of aircraft AoA



Wind Tunnel Smoke Flow over Aircraft Wing (1 min 38 sec)



it does not require a broad sweep of the imagination to figure out that this true of the F-15, as clearly noted in the tip vortices shown previously above and this video
Dec 04, 2016, 02:43 PM
AndyKunz's Avatar
Is that red light on the top of the fin on the wrong side? Looks like the green is on the left side.

Andy


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