Bell AV-22A Mamba

[kitnut617]

My last comment, what do you think the forward speed of your bird will be with the rotors in hover position (like a helicopter)?

https://www.youtube.com/watch?v=GIe3BOrcSVk

Good question ---- I would imagine the same as the V-22.  But from what I've seen (and read) of the V-22 (real life demo down in Yuma) I don't think the plan is to be in hover for very long (take-off and landing). I've read an article about when they introduced the AH-1 Cobra and it said that when on a mission they were never in the hover, always buzzing around the LZ keeping the oppositions heads down.  I surmise this would be the same for the AV-22.

[kitnut617]

Only just seen this Robert, I am really liking it. One for the Marines I think.
Chris

Cheers Chris, yes, it will be an USMC bird.  Although I have thought it might be good for another role ---- helicopter hunter   

[kerick]

The Marines need a bird like this as the V-22s are much faster than their Cobras and are always out running them. The Cobras have to launch much earlier than the V-22s to arrive at the LZ at the same time.

[kitnut617]

What my thinking was Kerick ---

This would be what the AH-1 was for the UH-1, a fast escort. The article I read about the AH-1 when it was introduced said the UH-1's were barely making a 100mph when on an insertion mission, while they were buzzing around at around 180mph. My thinking with the AV-22 is it has to outperform the V-22 by some margin.

I did some calculating on what the V-22 does at it's top speed which is advertised now as being 316mph. In it's present form I don't think it will go much faster even though there is a planned engine power upgrade. This is because the props blade tips are reaching supersonic speeds when you add the blade tip speed and forward speed together. This is why I went with a much reduced prop diameter on the AV-22 (one of the two main reasons for it, the other is that the ordnance has to be able to fire while the props are in full forward flight regime ), I worked out that if everything remains the same (like prop rpm) the AV-22 should be about 70mph faster than the V-22 if you add the prop blade tip speed of the AV-22 which at 30 ft diameter, the blade tips are travelling 70 mph slower. Then by using the advanced swept blade tips (BERP), you could probably add a few more mph to it. So the max forward speed would be around 400mph I'm thinking. Hovering max speed doesn't really come into it in my opinion because it's not supposed to operate operationally in that environment.

[kerick]

Nice engineering thought process. It's either the Mamba or the Marines buy the A-10s the Air Force is trying to get rid of.

[kitnut617]

Nice engineering thought process. It's either the Mamba or the Marines buy the A-10s the Air Force is trying to get rid of.

Thanks again kerick, as you might surmise, this AV-22 is the same size as an A-10. I was thinking it could also be an A-10 replacement ---- 

Speaking of guns, I'm a little dubious of the recoil effects of having a single GAU-8 that far from the centreline...

Harold, I would be the first to admit I don't know too much about the affects of firing the gun placed where I have it. But I've done a little bit of investigating on the subject and a couple of things spring out.  I measured where I have the gun placed to the centerline of the fuselage and it is 6'-0", which happens to be the same as where the M61 is in an F-15 wing root. I did a google search on both the GAU-8 and M61 and according to the article in wiki', the M61 has a higher muzzle velocity than the GAU-8 (not by much though) so I'm thinking there wouldn't be a lot different when being fired. I'm thinking that the explosive charge would have less of a bang in the GAU-8 to get the shell to have near the same velocity as the M61 due to the rear end of the shell having a bigger area. Of course this thinking maybe totally out to lunch ----

As I said to kerick, this AV-22 is the same size as an A-10 but I have changed a couple of things. I've used the V-22 fin/rudders which are about 1 1/2 times as big as the A-10's fin/rudder as you can see in this pic below. I have though made the length of the rudders longer than what you see there which are now nearly twice as long. Plus I extended the rear of the fuselage by 3'-0" (1/2" in 1/72). All things which would help keep the aircraft in line when firing the gun.

[kerick]

When the A-10 was designed the engineers were very careful about the placement of the gun barrels. The barrel that was actually firing was placed on the centerline to minimize the effect of the recoil. That's why the nose gear is offset, to clear the gun barrels. The story was that if the GAU-8 had enough ammunition the recoil could actually stop the forward motion of the aircraft. Mathematily possible but impossible for practical reasons. Let's say the muzzle velocity of two different guns are the same. If the one shell is larger than the other it will take more force to accelerate it down the gun barrel and get to that velocity. The larger shell will therefore produce more recoil. Now the GAU-8 ammunition was used in a gun pod that could be mounted under the wings of most fighter aircraft but it had shorter barrels and developed less muzzle velocity which led to less recoil. You could say your sponson mount is an adaptation of the gun pod. Small potatoes either way to me. Your Mamba is pretty cool and inspires me to build a version that's been on the back burner of my mind for a long time.

[Weaver]

When the A-10 was designed the engineers were very careful about the placement of the gun barrels. The barrel that was actually firing was placed on the centerline to minimize the effect of the recoil. That's why the nose gear is offset, to clear the gun barrels. The story was that if the GAU-8 had enough ammunition the recoil could actually stop the forward motion of the aircraft. Mathematily possible but impossible for practical reasons. Let's say the muzzle velocity of two different guns are the same. If the one shell is larger than the other it will take more force to accelerate it down the gun barrel and get to that velocity. The larger shell will therefore produce more recoil. Now the GAU-8 ammunition was used in a gun pod that could be mounted under the wings of most fighter aircraft but it had shorter barrels and developed less muzzle velocity which led to less recoil. You could say your sponson mount is an adaptation of the gun pod. Small potatoes either way to me. Your Mamba is pretty cool and inspires me to build a version that's been on the back burner of my mind for a long time.

The recoil energy is simply the kinetic energy of the projectile (and the gas behind it), which is proportional to their mass and their velocity. Since we're talking about auto-cannons here, the recoil force-over-time will also be proportional to the rate-of-fire, i.e. how many small backwards pushes do you get in a 3-second burst?

One way of dealing with the effects of an off-centre gun is to have an automatic rudder input linked to the firing circuit, so when the gun's trying to push the nose one way, the rudder(s) are trying to push it the other. IIRC, the F-15 has such a system. On the Mamba, I'd imagine you could also get the same effect in forward flight by differential rotor pitch control between the two rotors: it wouldn't take much to make a difference, because the rotors are so big and powerful (in effect the rotor opposite the gun becomes an airbrake) and this would prevent the rudder from getting 'saturated'. In hovering flight, you'd have to apply differential cyclic control to get the same effect, i.e. the equivalent of giving the rudder pedal a kick. Since, presumably, the aircraft is fly-by-wire, none of this should be difficult to implement: the issues would be a) making sure that the software applies just the right amount of correction in all flight (or at least in all permissable firing) conditions, and b) the net effect of the firing plus the corrections on the aircraft, i.e. slowing it down.

[kerick]

I had never heard of control inputs countering off center recoil effects. Learn something new every day.

[kitnut617]

There was a discussion on SPF the last couple of days about some control that's similar. The thread was showing a B-2 dropping two MOP's in quick succession. The aircraft doesn't appear to move as it drops 15,000lb not once but twice. A post queried it and I added a post saying it didn't suddenly gain altitude when the bombs were dropped. One of the replies explains that there's some automatic flight control system that adjusts the aircraft as the weight suddenly leaves the aircraft to keep it on an even keel etc.

[scooter]

Nice engineering thought process. It's either the Mamba or the Marines buy the A-10s the Air Force is trying to get rid of.

Thanks again kerick, as you might surmise, this AV-22 is the same size as an A-10. I was thinking it could also be an A-10 replacement ---- 

Speaking of guns, I'm a little dubious of the recoil effects of having a single GAU-8 that far from the centreline...

Harold, I would be the first to admit I don't know too much about the affects of firing the gun placed where I have it. But I've done a little bit of investigating on the subject and a couple of things spring out.  I measured where I have the gun placed to the centerline of the fuselage and it is 6'-0", which happens to be the same as where the M61 is in an F-15 wing root. I did a google search on both the GAU-8 and M61 and according to the article in wiki', the M61 has a higher muzzle velocity than the GAU-8 (not by much though) so I'm thinking there wouldn't be a lot different when being fired. I'm thinking that the explosive charge would have less of a bang in the GAU-8 to get the shell to have near the same velocity as the M61 due to the rear end of the shell having a bigger area. Of course this thinking maybe totally out to lunch ----

Just one thing to think about, mainly for the backstory.  The GAU-13/GPU-5/GEPOD sucks.  Apparently, when they were used in actual combat, the pilots got one burst and blew the boresight.  I'm going to (hopefully safely) assume the GAU-8 is just as rigidly mounted as on the 'Hog

[kitnut617]

Nice engineering thought process. It's either the Mamba or the Marines buy the A-10s the Air Force is trying to get rid of.

Thanks again kerick, as you might surmise, this AV-22 is the same size as an A-10. I was thinking it could also be an A-10 replacement ---- 

Speaking of guns, I'm a little dubious of the recoil effects of having a single GAU-8 that far from the centreline...

Harold, I would be the first to admit I don't know too much about the affects of firing the gun placed where I have it. But I've done a little bit of investigating on the subject and a couple of things spring out.  I measured where I have the gun placed to the centerline of the fuselage and it is 6'-0", which happens to be the same as where the M61 is in an F-15 wing root. I did a google search on both the GAU-8 and M61 and according to the article in wiki', the M61 has a higher muzzle velocity than the GAU-8 (not by much though) so I'm thinking there wouldn't be a lot different when being fired. I'm thinking that the explosive charge would have less of a bang in the GAU-8 to get the shell to have near the same velocity as the M61 due to the rear end of the shell having a bigger area. Of course this thinking maybe totally out to lunch ----

Just one thing to think about, mainly for the backstory.  The GAU-13/GPU-5/GEPOD sucks.  Apparently, when they were used in actual combat, the pilots got one burst and blew the boresight.  I'm going to (hopefully safely) assume the GAU-8 is just as rigidly mounted as on the 'Hog

Hey Scoot' ---- Yeah I've read that about all the gun pods when used and I had wondered what was the point of it. But after seeing the GAU-8 photos and how it's bolted to the airframe, was one of the reasons I put it in the sponson. The sponson is divided into top and bottom with a solid structure/floor in between the two halves which is built as part of the two spars which runs through the stub wings and fuselage from one sponson to the other , the gun is firmly bolted down to this structure. In reply #28 you can see how I've built the sponsons.

https://en.wikipedia.org/wiki/GAU-8_Avenger

Yup! I read that too although I should have read all of it. But a lot of my thinking comes from reading that and about the M61 in a similar article.

[kitnut617]

OK, refueling probe Mk.3 (3 stage cylinders)

Retracted:



Extended:





After spending quite a few hours looking at photos of refueling probes on helicopters and a fair few videos, I don't think this is in the realms of impossibilities, bordering on ridiculous  ----  but possible.

[kitnut617]

Out of interest, I measured the distance of another V-22 kit I have from the leading edge of the wing root next to the fuselage to the tip of the nose and it came out as 18'-0". The measurement to the probe tip end of the refueling probe that comes in the kit (Italeri) measured at 24'-0".  Doing the same with my AV-22 the wing root to nose tip measurement is 23'-6", the probe when extended is another 6'-0" further out.

[Weaver]

That seems about right. Most of the slack I've seen in hoses in refuelling videos produces 'loops' that are smaller in size than the new probe-tip-to-blade distance.