Forum: F-16 Design & Construction

Simple question:

A significant portion/mass of the wingtip rail lies ahead of the wingtip, and when a missile is installed this extra mass only increases. This would have the effect of twisting each wing and reducing its incidence, especially in high-G maneuvers.

Is this effect large enough to create noticeable washout? Or is this just another nuance of the FBW system?

This could be a novel feature of the F-16, because washout can be useful in maneuvers to help the pilot (or computer) ride the stall more effectively. Washout also improves the lift distribution on the wing. It is, however, usually considered detrimental to cruise performance. But if the wingtip rails were designed to create washout the effect would almost disappear once the pilot unloaded the airframe.

Any thoughts?

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The Falcon's wing (and the Hornet's too) already has a washout, even without a missile on the rail.

If you pay close attention you can notice it in this photo:

RDAF F-16AM #E-609 left wingpylons details spotted at Aalborg AB on April 1st, 2004.

Salute!

We could see the nose of the 'winders dipping down when we pulled real hard at high speeds. Low speed knoife fights didn't exhibit this.

Get Roscoe here to explain everything.

Gums

There are two things that cause the missile/launcher to twist, inertia loads and airloads, The inertia load from positive g will of course cause nose down twist, but positive g also has positive airload (lift on the missile/launcher) which causes a nose up twist. The experience Gums had (nose down dip at high speeds, but not at low speeds) is explained by the angle of attack at the missile. In the high speed case, the AoA change is small in the turn, so the up airload is not enough to overcome the down inertia load and the missile nose dips. In the low speed case, the AoA change is greater, so the up airload is roughly equal to the down inertia load, so the missile nose doesn't dip. Rolling during the turn will also cause local g and local AoA changes, and the missile will be affected by those terms. Being so thin, the F-16 wing is quite flexible, especially in twist.

Missile twist is much more complex than my explanation, but those are the basics.

The FBW flight controls don't play a part in this characteristic, but the weapon system does. I'm certainly no expert on this topic and what I know about it is 30 years old, but at that time, the missile pitch and roll was measured in flight test and the results put into the weapon system, so that when the radar told the AIM-9 where to point its seeker, the missile pitch and roll corrections had to be included.

In a similar way, gun, HUD, and radar bore sight corrections are applied due to fuselage flexibility.

Gums wrote:

Salute!

We could see the nose of the 'winders dipping down when we pulled real hard at high speeds. Low speed knoife fights didn't exhibit this.

Get Roscoe here to explain everything.

Gums

You're funny...not sure what there is to explain. G's bend things...more Gs bend things more. (Hope I wasn't too technical...)

For the rest washout is when the tips have less incidence (angle relative to the fuselage) than the root. Everything being equal, it reduced the loads at the tip so bending moments would be reduced, thus reducing the spar size and therefore the weight. That said, on swept wings everything isn't equal as spanwise flow forces air toward the tips. How that affects the local AOA varies from plane to plane.

Salute!

TNX, Roscoe, we needed an aero-freak from the Test Pilot School to add credibility.

If all would look at the early bent wing jets, especially the USSR ones, there were "fences" on the swept wings to hinder the spanwise flow. The U.S. manufacturers, NASA, DARPA, etc fingured out how to avoid the "fences" by shaping the wing, so jets like the F-106 and such didn't have them. One technique was the "notch" you see on several of our early jets (pre-70's). The notch introduced high energy air that helped to cut down on the spanwise flow.

In the late 70's and 80's, folks like Rutan and others introduced the "winglets" at the end of the wing. This feature reduces spanwise flow, best I can postulate.

The A-37 had a lot better cruise numbers using the same wing as the T-37. It had those 90 gallon tip tanks, which helped reduce spanwise flow even tho the wing wasn't swept. Same for the T-33 versus the P-80.

I would bet that the Viper has better fuel consumption with 'winders on the tips than when they are not loaded.

Gums ...

Gums,
Right about the better fuel consumption with missiles. Higher aspect ratio is the reason.

Enthusiast Joined: Aug 12, 2007 - 05:33 PM Posts: 20

johnwill wrote:

Gums,
Right about the better fuel consumption with missiles. Higher aspect ratio is the reason.

Is the fuelflow alot lower with missiles like AIM-120 at 1&9?? Does it influence the max speed too??

Winglets only affect the airflow that would be leaving the wing at that point anyway, in the form of the wing-tip vortices. They function to reduce some induced drag and also slightly increase aspect ratio. They obviously create some parasite drag, but it's a positive trade-off.

I'm not sure that everything that can be put on a wingtip provides a positive trade-off. I've got my Viper book sitting right here but I'm too lazy to look at it. But, only my guess, I suspect the wingtip missile rails with or w.o. the AIM's add to the total drag number without an overriding positive affect on the vortices. There may be some certain range of AOA's where they do, I don't know. Just off the top of my head, If I had the choice to go X-C with or without the captive, I'd go without. Winglets are specifically shaped for this.

I've flown the B727 with and without winglets, and there is quite a difference in cruise fuel flow, as well as a pronounced tendency to float in the flare.

I have flown the T-33 without tips and the cruise numbers are much better than with them. Tips on the T-33 were high parasite drag, but the increased amount of fuel allowed you to go further at the higher fuel flows, just like any external tanks.

Another good discussion.

regards, OL

Outlaw, since you are knowledgable about the T-33, have you ever heard of any comparison with the P-80? As you know, the P-80 had underwing tip tanks, while the T-bird tanks were mounted like the F-16 Sidewinders. I have heard the T-bird was faster, so I'm wondering if you or others have any knowledge of that. I know the T-bird was 3 ft longer, and that could have helped too.

There must have been a reason to move the tanks on the T-bird, maybe that was it. Ground clearance could have been a problem too.

I’m old, but not old enough to know that much about the P-80. The tips on the T-33 are larger & longer than the P-80 tips. So I think you’re right about ground clearance. I believe even the under mounted tips reduced the induced drag at slower speeds/higher AOA’s. You probably still lost out on the higher speed end due to parasite drag. If they were jettisonable, that tells you a lot (I know, weight’s involved too).

The bigger tips were put on the T-33 to make up for some of the internal fuel lost to make room for the instructor. Just guessing, but it is possible that in the landing attitude if they had been mounted under the wingtip you would have marginal ground clearance for the aft end of the tank. Add in a notoriously slow off idle centrifugal flow engine and a student who over-rotates in a sink, and…..”Sir, what’s that scraping noise?”

We only had the tips off for one or two cold weeks in December, I don’t even remember why (tank overhaul?), but the performance on the high end/cruise was definitely better and I don’t recall any noticeable difference on landings. Flights were shorter.

What I do remember is dragging the unwieldy seat pack parachute to the airplane and back, the fuel counter that you had to set because the only true fuel level measure was in the “feed” tank and I think it only turned on a red light when you got to it. I also remember the landing gear handle on the floor on the left side of the seat, which was pulled “up” to put the gear “down”.

BTW, a guy named Whitcomb invented the winglet.

from the "has-been" side of town, regards OL

Salute!

JR007 has flown the thing recently and may be able to help.

I had forgotten about the gear handle - didn't it have a button on the top of it, a golfball with button in middle??

Same for the "totalizer" fuel doofer. Sheesh, I'm having flashbacks.

We only had one tip drop when I was at Craig. Seemed like we had two handles connected to the release mechanism by cables. Not sure why the guy had to punch them off, as I don;t think it was a c.g. problem. Could have been only one fed and roll would have been a problem when slow.

Golfball with a button in the middle is a good description. "Jiggle-check".

Didn't fly 'em as a student, only as a target. Got a really short course checkout from an ex-RCAF instructor. He even let me spin it a few times. "Canadian recovery".

I think you're right. T-33 tips were jettisonable for reasons that had nothing necessarily to do with high end performance.

overcome with nostalgia, regards OL

(BTW, Craig & Laredo both still had 'em when I was a student.)

Forum: F-16 Design & Construction

Washout Induced by Wingtip Rails?