Helicopters: How They Work


The AHIP Program (OH-58D)

When the U.S. Army buys a new aircraft type, they usually purchase an airframe from the lowest bidder on the given project. With the AHIP (Army Helicopter Improvement Program) the U.S. Army showed that they no longer wanted to buy aircraft based on lowest price. They instead came up with an innovative program that has now allowed them to not only purchase the aircraft that they want, but also get the performance that they need. The aircraft that they were able to purchase performed well above the expected standards, and the Army started a trend in purchasing that has now allowed them to buy other types of aircraft that they needed like the D model of the CH-47 Chinook.

How did they do this? Well first, you must understand "why" they did it. 58D-HoverThe Army needed a scout aircraft that would replace the aging fleet of OH-58 A and C model helicopters. It was not that the A and C models were bad aircraft, but they were older technology, and the modern battlefield needed a newer scout aircraft with capabilities far above those of the older OH-58 airframes. The Bell Helicopter company developed a new combat scout aircraft on their own with a mast mounted sighting system (MMS), and called it the OH-58D. What they did was to take an older A model OH-58 and totally refitted it with a new larger engine, heavier duty transmission, and a 4 bladed rotorsystem which was much quieter than the older two bladed system, and topped it with the MMS (Among other subtle changes). By filling the back seat area with complex electronics, the OH-58D became the scout aircraft that the Army wanted, and so desperatly needed.

If the Army were to put the "New Scout Project" up for bid, there would have been costly research programs, time consuming evaluation periods, and other companies would have to attempt to create something to compare with the OH-58D at a lower cost. That alone would have taken a long time for the other companies to scramble to design, build, and test the new aircraft. The resultant time period would have made the aircraft chosen almost obsolete before it could ever see combat. Also, there was a good chance that the Army would not get the OH-58D as it was. They would have had to eliminate some of the performance or capabilities in difference to lowering the production cost to be competitive with other manufacturers. Sometimes, the protections that we have imposed upon ourselves to save the Government money can work against us. So they just decided to "Improve" the existing fleet of OH-58A and C models to become OH-58D's (I have since been told that only "A" models were "Improved"). This helped the Army to bypass the limits of bidding wars, and allowed them to get the aircraft that they wanted. Thus the Army Helicopter Improvement Program of AHIP was born.


The OH-58D has some really incredible capabilities. Now the first thing everyone asks is, "Is it faster than the older models?" The answer to that is; Not really. Maximum air speed in helicopters is secondary to other more practical capabilities. The actual VNE (Velocity Never Exceed) or maximum speed of the aircraft did not change much from that of the OH-58Dolder OH-58 series of aircraft. The new aircraft may have gained 4 knots of maximun airspeed with the doors on, but with the doors off, it originally kept the 100 KIAS (Knots indicated airspeed) limitation that the older OH-58's had (I hear that has also changed). The limitation for doors off flight was imposed on OH-58C models because of the flat plate windscreens that they had. At faster speeds, a vacuume would be developed behind the flat windscreen when flown with the doors off, and it would be sucked into the aircraft and it's pilots. The D model has a different bubble style windscreen like those of the older A models, but the C model inspired limitation was initially imposed on it in the interest of safety.

So, what are the new capabilities that this new airframe has that the older models did not have? First of all, the OH-58D has the MMS system that we briefly mentioned before. With this new sighting system, the entire aircraft can remain masked behind trees, hills, or other obstructions and can completely view the battlefield. The MMS acts like a periscope in that it towers above the highest point of the aircraft, and allows the pilots to view what is going on without exposing the aircraft to small arms fire, missles, or other potentially hazardous situations. The pilots have two CRT screens on the instrument panel which act as aircraft instrumentation, as well as viewing screens for target aquisition and laser designation. Through the MMS, they can see a target in 1 of 3 different viewing modes: standard video, magnified video, or thermal imaging. They can aquire the target by setting a "capture" icon on the screen, and point a laser at the target. The laser can perform many tasks, but the main function of the laser is to "paint" a laser signature on a target for laser guided munitions to follow. Once a target is painted, the laser guided munitions will follow the laser to the target and completely destroy it. One of the special capabilities of the OH-58D's laser designator is to allow for "offset lasing". What that means is that the target can be aquired and a false target can be aquired right next to the real target. When the laser guided munition is fired from an Apache helicopter, a A-10 warthog airplane, or another ground based weapons system, the munition will follow the laser toward the false target. At the last few seconds, the laser is switched to the primary target, and the primary target is now painted and is then destroyed by the weapon fired. What this does is limits the use of laser tracking warning devices that tell a target that it is being painted by a laser. If the operators of target, (like a tank) are aware that they are being lased, then they have the opportunity to take evasive measures to reduce the possibility of being destroyed. They may move behind cover, or depart the area. The offset lasing system only allows them a very short period of time to realize that they are being lased and allows no time to take evasive measures. If you have ever watched a capabilities exercise put on by the Army, you will know that there is nothing more accurate than laser guided munitions. They always go right for the laser spot on the target, and they are extremely destructive. Development of newer self propelled howitzers (Like the Paladin project) have made the OH-58D much more useful on the modern battlefield.

The laser can also be used to determine range, elevation, and azimuth of a target. By figuring the position, elevation, and azimuth of the aircraft itself, the direction, elevation, and reflective response time of the laser can give valuable targeting information to the pilots. CRT DisplayBy aquiring and adjusting this information, the pilot can relay that information to artillery positions to fire on the target. This elimates the need for the test firing and "bracketing" that artillery positions used to have to do to land artillery on a target. The old way to get artillery on a target was to give the artillery position a target in distance, azimuth and elevation as estimated by the scout pilot. Then the artillery position would have to fire rounds individually, and the scout pilot would have to call adjustments to the artillery until they could shoot on target. This would leave the scout aircraft exposed for long periods and it would allow the target to move when fired upon. By using the laser to gather this information, the exposure of the aircraft is minimized, the information is much more accurate, and the artillery can fire for effect right away. The target does not get the opportunity to move, and the artillery has to use less ammunition to get the job done. Another thing the laser can do is to aquire several targets at the same time. While only one can be painted with the laser at a time, the ability to select several targets at once allows for quick movement between targets. Once the last target is destroyed, the system can shift to paint the next one while the scout observer finds still another target for the system to lock on to.

1. Laser designator.

2. Stabilized platform.

3. TV camera.

4. Boresight assembly.

5. Thermal imaging sensor.

6. Multiplexer electronics.

7. Composite post.

8. Heat exchanger.

Thermally stable.

Reduces image vibration.

Magnifying video sensor.

30 seconds in flight.

120 element module.

TV/FLIR correlation.

Kevlar composite based.

Glycol cooled.


The CRT screens that I described before serve several purposes. Not only are they for video viewing, target designation, and aircraft instrumentation, but they are also capable for being used for navigation (INS). The new "R" model aircraft manuals refer to it as "EGI/INS" (Embedded Global Positioning System/Inertial Navigation System). EGI now updates the INS, unless EGI isn�t functioning, then manual updates must be performed over surveyed waypoints to correct for gyroscopic drift in the INS. By inserting lattitude and longitude information, and initializing the aircraft position, the navigation system will tell the pilot where he is if GPS information is not available. On the older models, before the GPS was available, the pilot had to periodically update positional information manually over known waypoints to keep the system accurate, and the navigation system could help him to accurately navigate at low levels. Now EGI will make those updates if available. It now gives even more accurate positional information critical to the adjustment of artillery as well.

Click on this image for a larger view of the OH-58D cockpit.

Another major feature of the OH-58D is the SINCGARS radios installed in the cockpit. SINCGARS radios are a channel hopping, digital FM frequency radio that must be time coordinated with other SINCGARS radios to be able to work. They are coordinated by giving all communicating units the same preset system time. The system time is exact to the second, and is not reflective of actual date or time. Once they are set to the same time, they hop frequency in sync with one another, and transmit data in code. The radios change channel automatically every few seconds, so they can not be locked onto and descrambled. Even if another SINCGARS radio is being used to listen in, it can not decode the messages without being initialized to the exact time code of the other units involved. It creates an unbreakable code for very secure transmissions. By setting up the radios together, a scout aircraft can send coded firing information over the radio in total security. The firing positions receive the data, and fire on the unsuspecting targets. Anyone else listening will only hear a series of high pitched squeals that mean absolutly nothing to them (Like computer modem sounds). SINCGARS radios can also be used in non-scrambled mode to allow communications with other units using standard FM radios on the battlefield.


The controls of the OH-58D were designed to allow for a lighter pilot workload. The controls of the aerial observer side of the cockpit can be stowed and locked in place so that the aerial observer does not have to worry about hitting them and causing control problems for the pilot. The controls can be unlocked in flight, and used in the case of pilot emergency. The pilots collective has a headstock on the end of it with a series of controls. He can operate exterior lights, manipulate engine controls, and switch between any of his 5 different radios without taking his hand off of the collective.

CRT screens have the controls built around the perimeter of each screen allowing the pilot to change operating modes, initialize and update the navigation system, operate targeting systems, monitor the engine and systems status, and change viewing modes without having to divert his attention from looking straight ahead. He can easily transition from inside the cockpit to outside the cockpit and back inside with only a minor change of glance. This keeps him from having to move his head around extensively while flying the aircraft. Also, one screen can be doing one set of functions, and the other screen can be set for other tasks. This allows both occupants to be performing separate functions and monitoring different mission requirements at the same time.


I hope that you have enjoyed this rather quick look at the awesome capabilities of the OH-58D. I have had 15 hours of co-pilot time in the aircraft, and I worked very closely with the Aerial Observation Branch at Fort Sill while they trained the aerial observers there between 1988 and 1993. I have had the opportunity to see up close why this aircraft is such a great asset to the U.S. Army. Fairly recent developments within the Army have made it so the OH-58D is now armed with external weapons, and renamed the "Kiowa Warrior". The future may bring even more developments for this awesome weapons platform.


Special thanks go out to 2LT Cameron Keogh for corrections and updates to this page.



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