Help! Pugachev's Cobra

[Noodles]

LMAO...

Please post a build thread. The planes you are modelling are very modern and will likely draw interest.

will do

 

[Vimana89]

I stumbled on this and remembered your interest in thrust vectoring. Here is a thread you might want to check out. https://forum.flitetest.com/index.p...ust-vectoring-of-prop-driven-airplanes.60488/

 

[GeoffS]

As far as I know, the Cobra is just a straight forward flare to a very high angle of attack. ...

Just for completeness, Pugachev's Cobra is a very specific maneuver.
It involves pitching up to beyond 180 degrees AOA and then pitching forward to normal AOA flight.

Wikipedia has a good short description and excellent diagram:

In aerobatics, Pugachev's Cobra (or Pugachev Cobra) is a dramatic and demanding maneuver in which an airplane flying at a moderate speed suddenly raises the nose momentarily to the vertical position and slightly beyond, before dropping it back to normal flight. It relies on powerful engine thrust to maintain approximately constant altitude through the entire move

By wikipedia user Henrickson, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5469045

 

[GeoffS]

Good video:

 

[Vimana89]

Just for completeness, Pugachev's Cobra is a very specific maneuver.
It involves pitching up to beyond 180 degrees AOA and then pitching forward to normal AOA flight.

Wikipedia has a good short description and excellent diagram:



View attachment 144529
By wikipedia user Henrickson, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5469045

Thanks for clarifying that. I saw a brief documentary clip or something where somebody defined it differently/only showed the plane going to about the third position on your picture. My little 3ch plane can do that all day, and even turn while keeping its nose just shy of straight 180. What you are showing is a bit more extreme and looks like a higher G maneuver. It could probably still be done with the 3ch planes, just a bit more difficult than standard high alpha stuff. Thrust vectoring will definitely help and may even be necessary to do that reliably. Alright, well now I know what a proper Cobra looks like and what defines it, and this is not the first time some narrator on an aviation related vid left me with the wrong term for something. I had called wing rock "Dutch Roll" for the longest time and that's something related but an over simplified way of putting it. I guess BS checking every new and unfamiliar term I hear the narrator use on a vid about planes is a policy I should adopt.

 

[Vimana89]

Anyway, the thread I linked about thrust vectoring is solid, and is by a member with much more experience in such things than I have. It can possibly help @Noodles in setting up TV on the proposed SU-27 or SU-33. I know nothing about gyro stabilizers and such either, but there's enough people here who use them to where if you have any questions about it somebody will be able to help.

 

[GeoffS]

I found a video of an R/C Su-30 jet doing the Cobra.

 

[buzzbomb]

Forgive me my ignorance. I am and will forever be a student of this great hobby that is RC flight. I just don't see the difficulty here. Fly forward. Nose up. Drop throttle. Nose down. Increase throttle. There's some obvious elevator work going on in there, but nothing that seems amazing.

Doing so without losing forward speed would take some finesse, but it doesn't seem that far out. What am I missing?

 

[cranialrectosis]

Forgive me my ignorance. I am and will forever be a student of this great hobby that is RC flight. I just don't see the difficulty here. Fly forward. Nose up. Drop throttle. Nose down. Increase throttle. There's some obvious elevator work going on in there, but nothing that seems amazing.

Doing so without losing forward speed would take some finesse, but it doesn't seem that far out. What am I missing?

The part where you stall out, faceplant into the deck and your plane goes home in a bag...

I think you would need to have a lot of thrust available to pull that maneuver off and I think that's what this stunt demonstrates.

I don't think you could pull it off with a model of a B-25. Then again, it could be fun to try.

 

[Vimana89]

The part where you stall out, faceplant into the deck and your plane goes home in a bag...

I think you would need to have a lot of thrust available to pull that maneuver off and I think that's what this stunt demonstrates.

I don't think you could pull it off with a model of a B-25. Then again, it could be fun to try.

Yeah that's why I said if he's going with props use a 3 or 4 blade. Less efficient, but lots of that spare thrust that helps on maneuvers like that. A thrust vectoring setup, or at least a prop in slot with some good wash over the control surfaces should do the trick. Still not the easiest maneuver to retain stability. I've done some ridiculous high alpha but not past 180 degrees.

 

[cranialrectosis]

180+ and back seems a bunch harder than those slow flips he was doing in the RC video. I wonder if the slow flips were botched attempts at the Cobra.

Sure is a purdy plane though. I'd love to see a foamie model of this doing similar.

 

[Vimana89]

@Noodles if you are still checking this, a lot of good responses. If there's a better option, somebody will correct me, but I think your most reliable and economic option for this would be to take the SU-27 or SU-33(probably this one IMO) design, and make it a prop-in-slot pusher with plenty of flow over the control surfaces. Add to your motor mount a simple thrust vectoring setup like the one in the thread I linked. The combo of the thrust vectoring and prop wash over your control surfaces will give you exactly what you need.
Use a three or four bladed prop appropriate to your motor that gives a lot of thrust to weight.

You may even want to try an elevons setup like a lot of prop slot F-22's utilize, and not do ailerons on the wings. This puts your aileron controls more in line with your prop wash as well where wing ailerons would fall outside it. I'm not sure how much you will actually need a moving rudder with TV, but it couldn't hurt. I'm just unsure if it is worth the extra channel and weight(maybe someone else could answer that one).

 

[buzzbomb]

The part where you stall out, faceplant into the deck and your plane goes home in a bag...

I think you would need to have a lot of thrust available to pull that maneuver off and I think that's what this stunt demonstrates.

I don't think you could pull it off with a model of a B-25. Then again, it could be fun to try.

Ooohhhh. That's a job for...... "Wait! Look! what is that, up there in the sky? Why it's the Super Plane! It's the Mini Scout Biplane! I bet she can do it!"

When I watched the video and read the descriptions, that was kind of my thinking. Not doing it as a pilot sitting in a jet. Rather recreating it with my available RC aircraft. I thought "My MSBP can do that, no problem."

Go forward. Climb. Raise nose beyond 180 degrees, and attempt to come back down without changing altitude. Doesn't that sound like a fun challenge??!! I'll take it on. Anyone else? It's only foamboard, right?

 

[Chuppster]

@Mid7night's F-18 and F-16 both cobra just fine. Personally, I think all you need is a swept leading edge (for stability) and a ton of elevator.

 

[GeoffS]

@Mid7night's F-18 and F-16 both cobra just fine. Personally, I think all you need is a swept leading edge (for stability) and a ton of elevator.

I've seen a lot of F-16 and F-18 demonstrations, and I've never seen (or heard) that either plane can do the cobra maneuver.

The only US Air Force plane I know of that can pull it off is the F-22.

 

[GeoffS]

... I just don't see the difficulty here. Fly forward. Nose up. Drop throttle. Nose down. Increase throttle. There's some obvious elevator work going on in there, but nothing that seems amazing. ... What am I missing?

The big problem with the cobra maneuver is keeping the flow across both wings symmetric at the 180+ degree AOA.

Typically what happens at an AOA over about ~45 degrees is that the flow over the wings becomes increasingly unstable and one wing's lift decreases relative to the other (usually very quickly). This causes the aircraft to roll and "fall out of" the maneuver with most/all of the control surfaces ineffective (aka "depart controlled flight").

Even getting up to 45 degrees AOA takes some careful aerodynamic design. Probably the most common device on fighters these days is the Leading Edge Wing Root Extension" (LERX). The F-18 is one of the most obvious applications.
At very high AOA, the extremely high sweep-angle of the LERX generates a strong vortex that washes up over the top of the main-wing and maintains attached/stable flow.

 

[Chuppster]

I've seen a lot of F-16 and F-18 demonstrations, and I've never seen (or heard) that either plane can do a cobra maneuver.

The only US Air Force plane I know of that can pull it off is the F-22.

Well, Ben designed them so well that these scale models can pull them off just fine. I did break a wing on the F-18 doing one though. Make sure you aren't going too fast when you snap up...

Here's Ben's video. He Cobras about 26 seconds in.

 

[Vimana89]

The big problem with the cobra maneuver is keeping the flow across both wings symmetric at the 180+ degree AOA.

Typically what happens at an AOA over about ~45 degrees is that the flow over the wings becomes increasingly unstable and one wing's lift decreases relative to the other (usually very quickly). This causes the aircraft to roll and "fall out of" the maneuver with most/all of the control surfaces ineffective (aka "depart controlled flight").

Even getting up to 45 degrees AOA takes some careful aerodynamic design. Probably the most common device on fighters these days is the Leading Edge Wing Root Extension" (LERX). The F-18 is one of the most obvious applications.
At very high AOA, the extremely high sweep-angle of the LERX generates a strong vortex that washes up over the top of the main-wing and maintains attached/stable flow.

The symmetrical air flow thing is a big issue not just with the Cobra, but with all extreme high angle of attack maneuvers in order for the plane to keep its balance. The LERX thing a lot of fighters use is sort of akin to the whole wing itself on my little slender delta in terms of strong air flow vortices. Although mine has not done a proper cobra, it can fly around and turn at a really steep angle of attack, a bit shy of 180, because of the extreme symmetry of air flow and strong vortices. The less swept/more "conventional" types of wings definitely have more air flow issues at high angles of attack and need more engineering to get them to do a lot of drastic high alpha.

 

[Noodles]

Thanks for the help! will keep in mind of the tips and pointers thanks so much!

 

[GeoffS]

Well, Ben designed them so well...

I missed the possessive starting your earlier post...
... and didn't realize you were talking about R/C planes.

Very impressive performance!