The following article concerns itself not only about DG gliders even if we give examples from our own gliders.
In all types of aircraft, we hear unusual sounds from rumbling to whistling or vibration at specific speeds. The reasons for these things are hard to diagnose from a distance by telephone or even if they are brought to the factory and a test flight is made often the phenomenon doesn’t show itself at the factory!
It is certainly helpful to list the different possible reasons because then the pilot can probably find the cause himself when he knows where he should look.
Here is the Funny Sound Checklist:
Whistling sounds in the cockpit are the easiest to locate because one can put his hand on it during flight.
- Does the canopy closing gap or the sliding window whistle?
- Sometimes the window is open a very small amount which can’t be seen. Press on the various possible locations and see if that changes the tone of the whistle.
- Does the ventilation system make a noise?
Sometimes a limp leaf can stick in the input and vibrate. Or maybe the ventilation valve is not closing properly.Is the canopy seal defective?
- Look at the open canopy rim.
Is the rubber seal missing in places or is it clamped? Are there clearly visible gaps between the fuselage and canopy?
B. Gap seals
- Has the gap tape between the fuselage and wings come loose?
You can see that when the end of the tape hangs free and is frazzled.
- Is the tape on the motor compartment loose?
When this happens there is a terrible vibration which disappears at high speed. This usually means that all the tape has come off and now it’s quiet!
- When the buzzing occurs only at high speed with the flaps set at -10 or -14 degrees then it may be the lower Mylar seal is vibrating. That should not occur with any glider that has an inner Teflon seal for the flaperons, the way that has been standard in our gliders for years.
- Is there buzzing from the Mylar tapes on the wings?
- The ballast outlet under the wing should be covered. If it isn’t it whistles. The cover will last a long time if there is no water ballast system. In such a case, the cover should be taped over. Otherwise it can be closed securely with a bit of Teflon foil. In order to prevent this from rumbling or buzzing, the area surrounding the closure should be well waxed. When the wax has been all washed away, the foil begins to vibrate and tears itself up. Is it frazzled on the end? Then new wax is needed before the next flight.
- Does the ballast outlet rumbles or whistle?
Often they have been loosened by something and vibrate. The tape should not be too long because the stretch too much and loosen when the doors are opened. Renew old tape and trim tape that is too long!
- Is the tape on the motor doors in order?
- The rudder gap should be sealed on both sides from top to bottom. If something is loose there it will rumble.
- Are the seals on the rudder still in order?
By the way, if you put the seal on yourself, be careful that the surface to taped to is sparkling clean and free of oil. The best thing to clean it with is acetone and then be sure not to touch it with your fingers!
- Do the wheel doors close properly?
You can check that in the hangar. Maybe the bungee in the wheel well is old and stretched? Or maybe the doors are crooked after a gear up landing?
- Does the steerable tail wheel vibrate?
If it does, then something is wrong with the attachment. Push hard on both rudder pedals. Does the noise change? If so, it could be the rudder cable attachments and should be inspected by a certified mechanic.
- Is the tail wheel axle loose? That only happens with tail wheels that do not have ball bearings. Lift the tail onto the tail dolly and try to wiggle the tail wheel. Does it have play? Then it can spin in the air and vibrate.
- Is there a vibration coming from the fixed tail wheel?
The reason for vibration given above is the main reason for vibration. Check the axle bolt for looseness. There should be no axial play. The bolt should be just tight enough such that there is no axial friction on the wheel.
If the tail wheel vertical axis is tilted, then the bearing must be replaced by a mechanic. Tail wheels that turn in the air are common causes for vibration. If the tail wheel bearings have no noticeable defect, try adding some friction to the system to prevent rotation in flight.
Unmount the wheel and attach some foam with double sticky tape to the wheel housing. Use a thickness that rubs slightly on the wheel. Does that cure the problem?
- Extract and retract the landing gear during flight.
When extracting the gear you of course will hear some wind noises. But does your strange noise change?
D. Rudder, Ailerons, Flaps
- Is the mounting of the rudder loose?
Does the noise change when you move the rudder or when you hold the pedals in position with both feet?
- Does your noise change if you move the aileron or flaps?
- Probably the reason is the Mylar-Sealing.
E. Engine doors
- Do the engine compartment doors make a noise?
This may be caused by a gap at the front edge of the doors. Stick some foam (e.g. draught excluder) under the front edge of the engine compartment doors. Did this get rid of the noise?
These are the most likely causes of vibrations and odd noises known to us.
If you have other experience from which other pilots might learn, please share them with us
– friedel weber –
translated by David Noyes, Ohio